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Selection of thermal management system for modular battery packs of electric vehicles: A review of existing and emerging technologies

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Li-ion battery cells are temperature sensitive devices. Their performance and cycle life are compromised under extreme ambient environment. Efficient regulation of cell temperature is, therefore, a pre-requisite for safe and reliable battery operation. In addition, modularity-in-design of battery packs is required to offset high manufacturing costs of electric vehicles (EVs). However, modularity of battery packs is restricted by flexibility of traditionally used battery thermal management systems. For example, scalability of liquid cooled battery packs is limited by plumbing or piping and the auxiliary equipment used in the system. An alternative thermal management system is, therefore, required for modular EV battery packs. In this paper, state-of-the-art developed to control battery temperature near a pre-specified state is qualitatively reviewed with the intent to identify potential candidate for implementation in a modular architecture. Some of the novel techniques that provide high-scalability in addition to appreciable cost and energy-savings over traditional methods are also evaluated while considering the development state and associated technical risks. It is found that only a hybrid system can meet technical requirements imposed by modular design. Based on the current state, phase change materials and thermoelectric devices are more likely to be part of this next generation thermal management system.
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Title: Selection of Thermal Management System for
Modular Battery Packs of Electric Vehicles: A Review
of Existing and Emerging Technologies
Shashank Arora*
Faculty of Science, Engineering and Technology
Swinburne University of Technology
Hawthorn, Victoria, 3122, Australia
* Corresponding author:
Tel.: +358504351511;
E-mail: shashankarora@outlook.com.au
Abstract
Li-ion battery cells are temperature sensitive devices. Their performance and cycle life are
compromised under extreme ambient environment. Efficient regulation of cell temperature is,
therefore, a pre-requisite for safe and reliable battery operation. In addition, modularity-in-
design of battery packs is required to offset high manufacturing costs of electric vehicles (EVs).
However, modularity of battery packs is restricted by flexibility of traditionally used battery
thermal management systems. For example, scalability of liquid cooled battery packs is limited
by plumbing or piping and the auxiliary equipment used in the system. An alternative thermal
management system is, therefore, required for modular EV battery packs.
In this paper, state-of-the-art developed to control battery temperature near a pre-specified state
is qualitatively reviewed with the intent to identify potential candidate for implementation in a
modular architecture. Some of the novel techniques that provide high-scalability in addition to
appreciable cost and energy-savings over traditional methods are also evaluated while
considering the development state and associated technical risks. It is found that only a hybrid
system can meet technical requirements imposed by modular design. Based on the current state,
phase change materials and thermoelectric devices are more likely to be part of this next
generation thermal management system.
Keywords:
Phase change materials; magnetic refrigeration; thermoelectric and thermo-acoustic battery
thermal management systems; heat pipes; cold plates; solid electrolyte interphase film
1. Introduction
Various chemical reactions and electrochemical transport phenomena characterise the normal
charging and discharging processes in a battery cell. Many of these reactions are exothermic in
nature [1], meaning that temperature affects the performance of a battery pack. General Motors
estimated that if an electric vehicle (EV) is operating in sub-zero temperatures, its driving range
can be reduced by several percent due to the sluggish charge kinetics in the battery cells [2].
On the other hand, if heat transfer from the battery pack to the external environment is not
sufficient, excess heat may accumulate in the battery pack, particularly when it is being
operated in a hot climate or under an insulating environment [3]. Hot spots can also develop,
leading to an uneven temperature distribution across the battery pack, which can alter charging
and discharging characteristics of the battery cells [4, 5]. More importantly, the battery cell
temperature may rise beyond the safety limits of 60 °C for Li-ion battery cells using 𝐿𝑖𝐵𝐹
4as
electrolyte, risking battery pack failure [6].
Previous studies indicate that the battery cell temperature must be regulated within a predefined
operating range to sustain a rate of reaction considered healthy for the efficient operation of
battery cells. The recommended operational range for Li-ion battery cells is generally between
25 °C and 40 °C [7, 8]. Managing large temperature spikes and non-uniform thermal gradients
across the battery pack is, therefore, a major concern in the design of large battery packs
essential for supporting an EV driveline. For these reasons, a battery thermal management
system (TMS) needs to be integrated with the EV battery pack, although the original equipment
manufacturers (OEMs) have followed different approaches [9]. Table 1 separates the OEMs
that are in favour of using a TMS from those who do not use it.
OEMs not using TMS
OEMs using TMS
Nissan
Tesla
BYD
General Motors
Volkswagen
Ford
Mitsubishi
Mercedes
Renault
Fiat
Table 1: List of OEMs distinguishing those which prefer to use a thermal management system for their EV
battery packs from those which do not (*Mitsubishi iMiEV offers a variant with forced air-cooled battery pack,
i.e. there is an optional fan that can be attached to its battery back for limiting the temperature rise during fast
charging operations)
Several incidents involving battery cells overheating and catching fire have been reported to
date. In many cases, the problem of battery overheating has caused OEMs to question the
reliability of battery packs and loss of markets for battery-powered products. A list of major
product recalls and incidents of battery-powered products catching fire in recent history is
available from [10].
It is evident that thermal stability is a major issue for Li-ion battery packs. In addition, the high
manufacturing costs of battery packs hinder the marketability of EVs. Mass market appeal of
EVs can be improved by using economies of scale generated by the implementation of modular
battery pack architecture. However, the concepts of mechanical and thermal modularity are
inter-connected. The thermal independence of each battery cell must be ensured to preserve
their interchangeability [11]. In this paper, different thermal management techniques that can
be applied for the regulation of the thermal behaviour of Li-ion battery packs are qualitatively
reviewed to ascertain their suitability for potential implementation in a modular system. In
order to provide more context for this work, the paper first presents a simplified overview of
the main issues affecting behaviour of the Li-ion battery cells in low/elevated ambient
temperatures.
2. Effect of temperature on Li-ion battery cells
Li-ion battery cells belong to a category of temperature-sensitive devices, since both their
performance and their safety are influenced by their operating temperature [12]. Research has
shown that commercial Li-ion battery cells achieve optimum performance near room
temperature [13]. This section discusses various challenges associated with operating Li-ion
battery cells in temperatures that are far from this ideal condition.
2.1 Effect of low temperature
It has been reported that 18650 type Li-ion battery cells can supply only 5% and 1.25% of the
energy capacity and power capacity available at 20 °C, respectively, in low operating
temperatures such as -40 °C [14]. Similarly, the driving range of the 2012 Nissan LEAF has
been noted to drop substantially from 138 miles in ideal conditions to 63 miles at -10 °C.
Moreover, information presented by different research groups [14-16] on the energy capacity
of Li-ion batteries available during constant current discharge/charge tests conducted in low
temperatures confirms that the usable battery capacity decreases as the operating temperature
is reduced.
It was previously believed that the unsatisfactory performance of Li-ion battery cells at low
temperatures was due to their limited electrolyte conductivity, which affects the Li-ion
transportation rate between the two electrodes at these temperatures. However, further
investigations suggest that inadequate electrode activity can also cause poor low temperature
performance in Li-ion battery cells. Electrode activity refers to the combined effect of
marginalised Li-ion transfer through surface films on Li-ion battery cell electrodes called the
solid electrolyte interphase (SEI), and the high charge-transfer resistance and slow diffusivity
of Li-ions within the anode materials [17-21].
Of the control factors [22, 23], the choice of electrolyte for Li-ion cells is critical to the
improvement of their low-temperature performance, primarily because of the intrinsic loss of
ionic conductivity associated with low operating temperatures. In addition, SEI film’s chemical
composition and physical characteristics, such as its resistance and conformability to Li
intercalation, depend on the salt forming the electrolyte, and parameters such as the quality of
the anode material, and the mode and temperature of the SEI formation [24, 25]. SEI is a surface
film approximately 5 Å to 800 Å thick, consisting of both organic and inorganic compounds,
which keeps the electrolyte kinetically stable at anode potentials of less than 0.8 V. The
thickness of the film varies with the degree of anode graphitisation [26, 27]. Interestingly, this
anodic film is highly resistive and interferes with the Li-ion transport kinetics at the
electrolyte/electrode interphase [28]. Most research activities to date have therefore focussed
on improving the conductivity and stability of electrolytes with effective SEI film formation.
Approaches that have been central to this improvement are:
1. Use of co-solvents with low viscosity and low freezing temperatures, such as glymes, esters
and lactones [29-31]
2. Formulation of new additives for electrolytes to further lower their freezing point [32-35]
3. Substitution of the existing lithium salt 𝐿𝑖𝑃𝐹6 with new mixtures to improve the charge
transfer resistance and other characteristics of the SEI film [36-40]
2.2 Effect of elevated temperatures
United States Advanced Battery Consortium (USABC) has defined a performance target of 15
years’ calendar life for all the battery packs to be used in HEVs, while the targeted calendar
life for EV battery packs is 10 years [41]. It is, therefore, of utmost concern that elevated
temperatures, i.e. temperatures greater than 40 °C, accelerate the battery ageing phenomena.
Battery ageing refers to the loss of the energy/power retention capacity of a battery as a function
of time and inhibits battery packs from meeting the USABC performance goals.
Most electrolytic compounds are not chemically stable at voltage potentials that exists on the
anode, i.e. the negative electrode of a Li-ion battery cell. When a new battery cell is charged
for the first time, some of the electrolyte is irreversibly reduced by reacting with free Li-ions
near the electrode/electrolyte interphase, forming a thin film of metastable lithium alkyl
carbonates, polymers and gaseous products on the surface of the carbonaceous anode. This film
is pervious to lithium cations but impervious to electrons and any other chemical species
floating in the electrolyte. Electrolytic reduction therefore continues until a steady-state is
reached where a surface film thick enough to block all the electrons from entry, covers the
entire anode surface. It is commonly known as SEI film and prevents the electrolyte from
corroding the charged anode due to chemical reduction with little effect on the Li-ion
transportation rate through it [42, 43].
Figure 1: Causes and effects of battery cell temperature on safety and performance
Battery cell
temperature
Cause
Leads to
High
Electrolyte decomposition
Irreversible lithium loss
Continuous side reactions
at low rate
Impedance Rise
Decrease of accessible
anode surface for Li-ion
intercalation
Decomposition of binder
Loss of mechanical
stability
25 °C 40 °C
Maximum cycle life
15 °C 24 °C
Superior energy Storage capacity
Low
Lithium plating
Irreversible loss of
lithium
Electrolyte decomposition
Electrolyte loss
It has been reported that at elevated temperatures, impervious SEI film starts to break down
and dissolve, leaving the anode surface exposed to electrolytic corrosion accompanied with the
irreversible loss of lithium. SEI film dissolution also disturbs the physical equilibrium of the
metastable organic components of SEI and initiates their transformation into a more stable
inorganic form like lithium carbonate. The ionic conductivity or permeability of the SEI film
gradually decreases as the percentage of inorganic carbonates in it starts to increase, marking
a significant reduction in the energy capacity and power output of the battery cell [6, 44]. Fig.
1 identifies the resulting effects of operating a battery cell at different temperatures and the
causes leading to each failure mode.
The electro-chemical breakdown of the SEI film on the anode begins to happen around 85 °C
and is the first step of a three-step process that leads to cell meltdown [45, 46]. If insufficient
heat is removed from the battery cell at this stage, a point is reached where this process and the
governing chemical reactions become self-sustaining. The battery cell then starts to self-heat
at a rate greater than 0.2 °C/min and this is classified as thermal runaway [10, 47]. The second
phase of the cell meltdown process is initiated when the battery cell temperature becomes
greater than 140 °C. This marks the start of exothermic activity at the cathode, i.e. the positive
electrode. Oxygen is rapidly released at the cathode and the battery cell now starts to self-heat
at approximately 5 °C/min. The process finishes with oxidisation of the electrolyte as the
cathode decomposes when the temperature reaches more than 180 °C. Self-heating rates around
11 °C/min have been cited for this phase, but they can increase up to 100 °C/min [48, 49]. The
process is schematically described in Fig. 2.
The onset temperature for the exothermic reactions driving thermal runaway varies with the
chemistry of the battery cells and their state of charge. In general, the higher the cell voltage or
the state of charge, the lower the onset temperature for thermal runaway. For battery cells with
the same chemistry, it varies with the load history of the specific cell and the abuse event [12].
Figure 2: Illustration of thermal runaway process in Li-ion battery cells [50]
Research into electrolyte morphology indicates that electrolyte solutions are
characterised by increased decomposition at elevated temperatures [51]. It has also been
reported that replacing cathode materials like and with or
[52], or coating the surface of cathodes with by a
synchronised lithiation method [53], results in a thermally-stable cell. These positive
discoveries have kept the research community motivated in their quest for stable electrolytes
and safer cathode materials to improve performance at elevated temperatures. However, an
appropriate thermal management strategy may provide additional safety by limiting the rate of
heat accumulation inside the pack and pushing it to the state of thermal runaway.
3. Thermal Management Techniques
In this section, techniques developed to regulate the battery cell temperature near the pre-
specified operating temperature are discussed. The first part presents methods that are
exclusively applied for heating the battery cells. These are mainly used in extremely low
ambient temperatures and are based on different strategies: internal heating, convective heating
and pulse heating. Subsequently, more conventional battery thermal management methods are
reviewed with the intent to select a TMS for modular battery packs. These methods are used
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the battery pack. Lastly, the suitability of some of the emerging TMSs is also reviewed for
modular battery pack application.
3.1 Pre-heating Strategies
Ji and Wang [54] recommend pre-heating battery cells to room temperature before normal
operation in sub-zero temperature environments. They attribute the sluggish Li-ion kinetics in
battery cells to the large change in their impedance at sub-zero temperatures. It has been noticed
that at temperatures such as -20 °C the impedance levels can increase by approximately 10
times their value at room temperature and up to 20 times at -30 °C, marking a significant
reduction in available energy and power from a Li-ion battery pack [21, 55]. High impedance
accounts for large Ohmic heat generation and may induce a notable rise in battery cell
temperature and restore the original performance index. The key is to appreciate the strong
relationship between the thermal and electrochemical interactions in the Li-ion cells at sub-
zero temperatures.
It is noteworthy that the energy balance between the electrochemical heat generation inside the
battery, given by Equation 1 below, and the heat dissipated to the surroundings defines the
battery cell temperature profile, which in turn regulates the temperature-dependent
electrochemical processes inside the cell. A good understanding of the constantly-shifting
equilibrium between heat generation and heat dissipation is fundamental to the development of
heating techniques for Li-ion battery cells operating in sub-zero temperatures.
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This principle has been utilised by Toyota and a system that uses the internal resistance of the
battery to increase its temperature during vehicle operation was disclosed in US patent
6163135. In the design, battery temperature was closely monitored by temperature sensors. As
the temperature falls below a predetermined level, a central processing unit performs a
controlled charge or discharge of the battery to bring its temperature back up to the required
level. The design exploits the basic quality of electrical machines to function as a simple load
during the battery-discharge phase, and as a prime mover or generator if operated in reverse,
i.e. the charging process. However, the system works only when the vehicle is switched on
[56].
The limitation of this system was overcome by the design disclosed in US patent 7154068. The
design includes a heater carefully placed between the top and the bottom layer of cells in the
battery pack which can be regulated via the vehicle system controller. The heater is a positive
temperature coefficient element the resistance of which to current flow increases as the ambient
temperature decreases. In addition, the heat generated by it is directly proportional to the
current flowing through it. An advantage of this heating mechanism is that it is battery-driven
and therefore does not require any power cord to be plugged into an external power distribution
system. Furthermore, the vehicle controller switches off the heater if the battery SOC falls
below a pre-set value or the vehicle has/has not been in use for a long time to save energy. This
feature is a big advantage when operating an EV in remote locations where distribution boxes
might not be readily available [57].
Another way of improving the low temperature performance of a battery pack based on the
fluid heating strategy is disclosed in US patent 7264902. The intent of this design is to provide
a system that enables rapid heating of the battery pack, especially during the start-up phase,
and to permit strict control over the battery cell temperature. Therefore, one or more good heat
conductors with heating medium flowing in them are arranged adjacent to each battery cell.
This ensures that heat transfer through conduction from the heating system to the battery cells
is not affected by the presence of any other medium between them. Each cell is formed in a
thin plate shape to further enhance the heat conduction rate in the system. Moreover, the liquid
electrolyte in the battery cells is replaced with a solid electrolyte. The replacement allows the
reduction of the liquid seals used in the pack, thereby minimising its effective heat capacity.
This design allows rapid heating and cooling of the battery pack [58].
Pesaran and co-workers compared the various preheating strategies using the finite element
modelling technique. The problem was modelled as a pure heat-transfer phenomenon and the
non-linearity introduced due to the complex electrochemical-thermal coupling in a battery pack
was ignored. The evolution of battery cell temperature as a function of used energy capacity
was then studied. Both the studies found core heating to be the most energy-efficient preheating
method. Furthermore, only alternating current (AC) signals should be employed, as direct
current (DC) pulses can cause damage to the battery core [59, 60]. In addition, Stuart and Hande
experimentally analysed the possibility of using AC signals of different amplitudes for the
external preheating of battery packs. In their investigation, lead acid batteries and nickel metal
hydride batteries were exposed to AC signals of frequencies of 60 Hz and 20 kHz, respectively.
It was observed that the speed of heating increases as the amplitude of the AC signal is
increased [61].
While several pre-heating techniques for EV battery packs have been proposed, the selection
of the most suitable strategy should be done only after ascertaining the cost and effect of heating
time on the usable battery capacity and the cycle life of the battery pack.
3.2. Conventional temperature control methods
In recent years, a number of techniques for readily removing heat from a battery pack have
been tried and tested by various research groups across the globe. They can be classified in
various ways, as seen in Fig. 3. A brief explanation is provided in the following text.
Medium used: A battery thermal management system can be differentiated by the working
fluid used in the cooling loop. They can be:
A. Air-cooled: unidirectional or reciprocating
B. Liquid cooled: Battery cells can be submerged directly in a di-electric working fluid.
Alternatively, a separate piping could be used for regulating liquid flow around the
cells. The piping could be in the form of:
a. Jacketed cooling system
b. Cold plates
c. Heat pipes
C. Phase change materials
D. Any combination of the above
Power consumed: As per the standard definition, a TMS is considered an active system if it
includes any power-consuming equipment, such as evaporators, blowers or pumps in the
cooling loop; otherwise, it is classified as a passive system. In practice though, an active TMS
is defined to be the one in which the cooling rate is controlled actively - for instance, the power-
consuming equipment is turned on and off under pre-specified conditions. In addition, TMSs
use a working fluid or coolant that absorbs excess heat and transfers it out of the system. This
heat can be absorbed either as sensible heat, thus raising the temperature of the working fluid,
or as latent heat, which causes it to undergo phase transformation. In general, all active systems
remove heat as sensible heat of the fluid, while passive systems remove it as a latent heat.
Arrangement: Refers to the method of distributing working fluid within the battery pack
A. External Internal
B. Series - Parallel
C. Direct - Indirect
Figure 3: Classification of different battery thermal management techniques
3.2.1. Air-cooling
Blowing air through a fan [62] or from a wind tunnel [63] over a battery cell is probably the
simplest and most cost-effective way of regulating its temperature. It has therefore garnered
interest from several OEMs as a prospective thermal management solution for commercial EV
battery packs. Many research activities have also been initiated mainly to consider the battery
layout and optimise the airflow or the wind speed around it, and minimise the energy
consumption and cost accrued. For instance, Xu and He [64] investigated different battery
layouts and illustrated that arranging the batteries horizontally rather than longitudinally
shortens the airflow path, thereby enhancing the heat dissipation characteristics of an air-cooled
battery pack. Their experiments also proved that a double U-type duct can readily satisfy the
heat dissipation requirements under a variety of environmental conditions,
discharging/charging rates, or SOCs.
Fan et al. numerically studied the transient effect of airflow rate and gap spacing on the
performance of an air-cooled battery pack made of eight evenly-spaced 15 Ah lithium
manganese oxide pouch cells. The Li-ion battery pack was subjected to an aggressive driving
test profile represented by the US06 profile scaled by a factor of 1.3. They reported that for a
fixed flow rate of 20.4𝑚3
, an improvement of 0.41 °C in the temperature uniformity within
the battery pack was observed as the gap spacing increased from 1 mm to 5 mm. In addition,
for a fixed spacing of 3 mm, the maximum temperature rise measured in the pack decreased by
1.8 °C when the flow-rate in an air-cooled battery was doubled from 20.4 𝑚3
to 40.8 𝑚3
[65].
In another study, Park numerically modelled the effect of an air flow manifold design for a
particular battery layout (37 coolant passages 3 mm in diameter formed between 72 battery
cells divided equally into 2 rows). Five configurations were studied: rectangular-shaped,
tapered manifolds (vertically contracting/expanding from 10 mm 20 mm), control design,
and lastly, rectangular ventilation hole placed at the outlet of a tapered manifold. He observed
that it was possible to achieve the desired cooling effect in an air-cooled pack by including
pressure-relief ventilation and a tapered manifold in the wind/cooling tunnel. Further, a
decrease from 47 W to 27 W in power consumption of fan was reported due to the presence of
a tapered manifold with a ventilation hole [66]. Giuliano et al. studied metal-foam based air-
cooled heat exchangers (HXs) for large-capacity lithium titanate battery packs. Their study
confirmed that air-cooled HXs consume less power than liquid-cooled HXs and can be used
effectively to regulate battery temperature in automotive applications [67].
While simple and sturdy in construction, air-cooled battery thermal management systems
generally employ a unidirectional coolant flow in which ambient or conditioned air is admitted
through one side of the battery pack and discharged from the opposite side. This is primarily
the reason why air-cooled systems struggle to maintain a uniform thermal distribution and a
gradient of less than 5 °C across a battery pack. This was confirmed by Lou, who designed a
cinquefoil battery pack containing 5 modules in order to understand heat dissipation through
an air-cooled Ni-MH battery pack. He noticed that although the maximum temperature was
kept in check, the battery cells near the fan were operating at a lower cell temperature than
those further away. In addition, the existence of a thermal gradient higher than 5 °C was
confirmed for the battery pack [68].
US patent 7172831 illustrates that it is possible to overcome this basic limitation of air-cooled
battery thermal management systems by using bi-directional or reciprocating airflow. The
system makes use of a controller to actively operate the fan/pump, monitor that battery cell
temperature and regulate the position of airflow valves. Active monitoring and control of the
pump and flow passages cause the direction of the coolant flow to reverse after a pre-set
(optimised) time frame. As a result, the temperatures of battery cells on opposite sides of the
pack averaged over time can be found to be approximately equal. It was further pointed out
that the temperature difference between any two adjacent battery cells can be minimised by
optimising the duration between the two successive flow reversals [69].
The authenticity of this claim has been verified by Mahamud and Park [62]. They numerically
studied the effect of reciprocating airflow on the performance of an air-cooled battery pack
made of cylindrical cells, using a lumped thermal capacitance model and a two-
dimensional computational fluid dynamics model. The computational results were validated
using an in-line tube bank set-up. It was found that a reciprocation period of 120 seconds can
reduce the maximum cell temperature and temperature non-uniformity in the battery pack by
1.5 °C and 4 °C (equivalent to a 72% reduction), respectively, in comparison with the
temperatures recorded for a battery pack with unidirectional air flow. Subsequently, He and
Ma [70] developed an observer-based control strategy to regulate the amount of cooling flow
required for reciprocating air flow systems. It was demonstrated that the cooling flow
requirement for the reduction of the non-uniformity of thermal gradients existing in the battery
back from 4.2 °C to 1 °C ( i.e. by more than 76%), is decreased by 38% if the observer-based
control strategy is used to regulate the air flow in reciprocating systems. In addition, He and
co-workers [71] also developed a hysteresis controller that can reduce the parasitic power
consumption of reciprocating air flow systems by 84%. However, the maximum cell
temperature in this case was noted to be 17% greater than the instance when no control strategy
was implemented.
3.2.2. Liquid Cooling
Studies have indicated that even extremely high air flow rates may not meet the heat dissipation
requirements for an air-cooled EV battery pack that is being discharged or charged at an
COLiMn 42
aggressive rate in a hot ambient environment [72, 73]. Furthermore, the very low thermal
conductivity of air may make it hard to cool a battery pack in a hot environment, thus
compromising its safety. An alternative way of cooling a battery pack is by circulating a liquid
coolant via jackets or through distinct tubes around it or placing it directly on a liquid-cooled
plate. In a slightly different arrangement, battery modules can be submersed in a di-electric
liquid such as de-ionised water or a silicon-based oil to increase the surface area available for
heat dissipation. Such a thermal management system has been demonstrated by Pendergast et
al., who placed a battery module with Panasonic 18650 cells arranged in an aluminium casing
under water for cooling [74]. The higher heat capacity and thermal conductivity of traditional
liquid coolants like water, acetone, glycol or oil make a liquid-cooled system more effective
than an air-cooled system, despite the added mass, complexity and higher operating costs [75,
76]. It is estimated that using liquid cooled systems can be up to 3500 times more efficient than
using air-cooled TMSs and lead to a reduction of up to 40% in parasitic load [77].
Nonetheless, not every liquid coolant may be as effective as others may for certain applications.
For example, Kim and Pesaran found while experimenting with different coolants on
cylindrical cells that waterethylene glycol achieved superior cooling performance to mineral
oil in an indirect liquid-cooled system. However, due to higher heat transfer coefficients in the
case of a direct-contact cooling system, the performance of mineral oil was comparable to that
of other coolants [78].
The previous section has described some examples of studies focussing on conventional liquid-
cooled TMSs, which remove heat from a battery module via direct submersion or by circulating
liquid in a jacket wrapped around the batteries. However, liquid-cooled systems can also be
designed using cold plates and heat pipes, and these are described in the following sections.
a. Cold Plates
Cold plates are mostly preferred where strict space-limitations apply, such as in EV
applications. They are basically thin-walled metal pressings with inbuilt channels for a liquid
coolant to carry heat. In a liquid-cooled TMS with cold plates, metal pressings are generally
arranged between adjacent battery cells and a liquid heat-transfer medium is pumped through
the inbuilt channels. The liquid coolant then removes all the excess heat from the battery cells
and transfers it to an external HX for dissipation to the ambient environment. The cooling
performance of a cold plate is assessed in terms of the heat transfer rate, the thermal distribution
across the pack, the power consumption depending upon the convective heat transfer
coefficient between the plate and the liquid coolant, or the coolant flow rate and ambient
temperature.
In the design of cold plates for EV battery packs, different channel designs and geometries may
be required for different applications, whereas most current research models of cold plates are
based on square-edged channel geometry. Furthermore, it can be concluded from the
concurrent activities in relation to fuel cells that channel geometry is a critical parameter for
cold plate design. In addition, the manufacture of square-edged channels is very expensive and
sometimes impracticable [79]. This prompted Fisher and Torrance to deviate from square-
edged channels and assess heat transfer through channels with rounded corners. They used the
boundary element method to evaluate the performance of rectangular, diamond-shaped and
elliptical channels. The results suggested that rectangular channels are more efficient than the
other two configurations. Moreover, it was found that although more channels with elliptical
design can be accommodated in the same packaging space, the heat transferred through a cold
plate with elliptical channels is approximately 5% less than that transferred through a cold plate
with rectangular channels [80, 81].
Studies by Yu et al. [82], Choi et al. [83] and Chen et al. [84] confirm that, in addition to the
channel geometry, another parameter that can have a significant influence on the performance
of a cold plate is the channel configuration, i.e. the route of coolant channels inside the metal
pressing. Broad categories under which they can be grouped are: serpentine channels, parallel
channels, and multi-channels. Jin et al. designed a thermal management system for EV battery
packs using oblique fin cold plates. It was found that in cases of heating loads below 1240 W,
this system could maintain the battery cell temperature to less than 50 °C with a flow rate lower
than 0.9 𝑙 𝑚𝑖𝑛
[85]. On the other hand, Jarrett and Kim employed cold plates with serpentine
channels for the same purpose. Based on a CFD analysis, they discovered that channels of the
greatest possible widths are necessary to achieve the lowest average temperature and minimum
coolant pressure drop. In contrast, channels with a narrow inlet and gradually widening towards
the outlet are required for maintaining thermal uniformity [79]. Huo et al. achieved the best
cooling performance for a 5C discharge of a rectangular Li-ion battery cell by directing water
into mini-channels on the sides of the electrode at a flow rate of 5𝑥10−4 𝑘𝑔 𝑠
. However, they
also observed an increased risk of failure at ambient temperatures higher than 25 °C and
recommended a two-phase TMS for high-temperature applications [86]. Recently, Saw et al.
proposed a two-phase TMS based on mist cooling or evaporative cooling for Li-ion battery
packs [87]. Zareer et al. too confirmed superiority of ammonia boiling based TMS over a
conventional single-phase liquid cooled system [88].
b. Heat Pipes
A thermal management system with heat pipes is a passive system driven by capillary action
of a wick material lining the internal surface of a vacuum-sealed shell. They have been in
existence since 1942 when they were first introduced by R.S. Gaugler, and remove heat through
the liquid-to-vapour phase change of a working liquid. Commonly used working fluids include
water, acetone, methanol and ammonia but the choice of liquid for a specific system depends
upon the heat pipe shell material characteristics. E.g., both water and glycol/water solutions
are compatible with copper shells; and in case of aluminium tubing, dielectric fluids and
mineral oils can be used as working fluids in addition to water and glycol/water solutions.
However, stainless-steel shells/channels are necessary if de-ionised water or any other
corrosive media such as ammonia and acetone is the preferred working fluid for an application
[89]. This applies to working fluid selection for cold plates as well.
A heat pipe can be usually divided into three parts, shown in Fig. 4:
1. Hot end or evaporating section
2. Adiabatic part or transport section
3. Cold end or condenser
Figure 4: Schematic of a traditional heat pipe with tubular structure and closed ends [90]
The liquid within the wick absorbs the excess heat from the battery cells, which are arranged
towards the hot end of the heat pipe, and evaporates. The increased vapour pressure and
reduced molecular density create a pressure gradient in the pipe that drives the hot vapours to
the condensing section where heat is rejected to the HX. The capillary forces developed in the
wick draw the condensed liquid back to the evaporator, thus completing the heat transfer cycle.
It is noteworthy that since a vacuum exists in the heat pipe, the working fluid vaporizes at a
temperature much below its normal boiling point, allowing the heat pipe to remove large
quantities of heat efficiently at much lower temperatures. Furthermore, the flexible geometry,
low maintenance requirements and thermal conductivities that are twice the order of magnitude
of solid conductors such as aluminium or copper, make them attractive as a TMS option for
EVs.
Mahefkey et al. have designed a TMS incorporating heat pipes for Ni-Cd battery cells while
Zhang et al. have done this for Ni-MH batteries. Both the groups found that heat pipes can
mitigate any thermal excursions in the battery pack [91, 92]. Swanepoel investigated the
possibility of using a pulsating heat pipe (PHP) with ammonia as the working fluid to regulate
the battery cell temperature of Optima Spirocell lead-acid batteries. Based on experimental
results and simulations, he discovered that a PHP with an internal diameter of less than 2.5 mm
can successfully manage the thermal variations in the pack [93]. On the other hand, Wu et al.
attempted to dissipate heat from a 12 Ah cylindrical Li-ion battery pack using two heat pipes
with aluminium fins attached to their cold ends. Their experiments also confirmed that heat
pipes can be used to keep battery cell temperatures in a safe range [94].
Various research groups have also tried to combine the benefits of heat pipes as a TMS with
other cooling methods, in order to enhance the overall system performance. In one such trial,
Jang and Rhi combined a loop thermo-siphon, which works on the same principle as a heat
pipe, with forced air-cooling. In their experiments, they managed to control battery cell
temperatures under 50 °C and 45 °C with pure water and acetone as coolants, respectively [95].
Rao et al. monitored the influence of cooling the condensing part of a heat pipe on its
performance. They reported that the system could maintain the battery cell temperature under
50 °C as long as the heat generated by each cell was less than 50 W [96].
Vibrations ranging between 0 and 100 Hz can be transmitted to the battery pack in an EV from
its top body. It is, therefore, imperative to analyse the influence of vibrations and shocks on the
performance of a heat pipe before defining it as fit-for-purpose in EVs. Connors and Zunner
studied the behaviour of a flat heat pipe under vibrations. The heat pipe had an internal lining
of copper powder to function as a wick material. They reported zero degradation in the cooling
performance of a flat heat pipe when exposed to vehicle shock and vibration [97]. Similar
conclusions could be drawn from the reports of Guo et al., who studied the effect of mechanical
vibrations on the heat-dissipation abilities of rectangular grooves. They observed that the
wetting area is enlarged by the vibrating motions of heat pipes. Vibrating motion also
intensifies the heat transfer through the microgrooves, which enables the heat pipe to operate
without any performance degradation in a vibrating environment [98].
So far, heat pipes have found limited usage in battery thermal management systems owing to
the high capital costs incurred due to the application of copper as a wall and wick material, and
the complicated fabrication process. However, recent advances in the manufacture of
aluminium heat pipes promise substantial reductions in total system costs [99-102]. In addition,
the weight savings realised by replacing heavier copper with lighter aluminum metal will
benefit the EV sector.
3.2.2.1. Fouling/Scaling in tubular structure
Closed loops are used for recirculation of liquid coolant in the EV battery packs. The water-
cooled systems are, thus, relatively less prone to biological fouling caused by exposure to light
[103]. However, large heat generation during high C-rate operation of battery packs can also
leave the wetted surface susceptible to scale formation if untreated water is used as a coolant.
Liu et al. studied the effect of scale thickness on the heat transfer in axi-symmetric channels
with water as coolant. Their work illustrated that heat transfer performance of channels covered
with a 0.225 mm thick fouling layer is 5 times better than that of a channel with 1.55 mm thick
fouling layer deposits [104].
Glycol/water solutions provide a greater resistance against biological fouling and scaling. It
must be noted that although a higher glycol concentration leads to reduced thermal performance
of the coolant, glycol concentrations less than 20% by volume have been found less effective
in providing control over scale formation [105]. Adding graphene nanoplatelets into the mix
is, therefore, recommended for improving the thermal performance of glycol solutions. Selvam
et al. demonstrated that incorporating 0.5 vol% of graphene nanoplatelets in the glycol solution,
increases thermal conductivity of water and ethylene glycol by 16% and 21%, respectively
[106]. However, nanoplatelets can precipitate over time in the (micro)-channels. Sarafraz et al.
demonstrated that a higher concentration of nanoparticles intensifies precipitation fouling
regardless of the fluid velocity. Also, for all mass concentrations, fouling thermal resistance is
defined by an inverse power function of Reynolds number [107].
Fouling of channels is partially responsible for high maintenance costs of liquid cooled
systems. HX’s surface vibrations are commonly used in the process industry to mitigate this
issue. However, this technique is laborious and costly. Vibrations of the mechanical structure
of the cooling system may affect integrity of adjoining battery cells as well. It is, therefore, not
reliable. Pulsating working fluid, oscillating between 1Hz and ultrasonic frequencies, on the
other hand, is a more practical approach. Research has shown that oscillating working fluid can
not only reduce fouling but also increase heat transfer between the heat exchanging systems
[108]. Tijing et al. also showed that a 16% to 60% reduction, depending on the frequency, in
mineral fouling in HXs could be achieved using oscillating electric field [109].
3.2.3. Phase Change Materials
Traditional battery thermal management systems using forced-air cooling and liquid cooling
are generally complex and bulky. They also add unavoidable electrical load in the form of fans,
pumps, blowers, and HXs to the limited energy storage capacity of an EV. These undesirable
disadvantages have increased the expectations for novel thermal management systems. A
simple passive solution comprising battery cells placed in a matrix of phase change materials
with zero maintenance requirements has therefore been proposed as an alternative by a research
group at the Illinois Institute of Technology. This solution employs the solid-liquid phase
transformation of organic/inorganic/eutectic phase change materials (PCMs) to remove the
thermal non-uniformities of the battery pack [110-112].
PCMs also have low maintenance requirements and have therefore piqued the interest of
several research groups. For example, Khateeb et al. studied a Li-ion battery pack made of
eighteen 18650 Li-ion cells and filled with a mixture of PCM and aluminium foam. They
examined the thermal behaviour of this pack with the help of a numerical model that was
validated through experiments at a later stage. It was also demonstrated through their
experiments that the temperature rise in a battery pack can be reduced to half by using a TMS
with PCM and aluminium foam, as opposed to the case where no TMS is applied [113]. Mills
et al. simulated a laptop battery pack with six 2.2 Ah Li-ion cells and realised a uniform thermal
distribution after using expanded graphite saturated with PCM as the thermal management
solution [114]. Sabbah et al. compared the performance of a TMS with PCM to that of an air-
cooled system using numerical methods and experiments. They demonstrated that the former
could keep the temperature of a Li-ion battery cell below 55 °C, even at a constant discharge
rate of 6.67C [115]. Kizilel et al. experimented with PCM-filled high-energy Li-ion battery
packs and achieved a uniform thermal distribution under both normal and abusive test
conditions [116]. Rao et al. also tested eutectic PCMs for 8 Ah prismatic 𝐿𝑖𝐹𝑒𝑃𝑂4battery cells.
The results of numerous experiments simulations indicate that PCMs may be a practical
solution to the thermal issues affecting EV battery packs [117]. Li et al. analysed the
effectiveness of a PCM-filled copper foam sandwich panel as a cooling system for prismatic
power batteries. The tests reflected a lower surface temperature and a better thermal uniformity
in the battery module after the integration of the PCM-filled panels [118]. Arora et al. also
demonstrated that the high capacity PCM (RT28HC) could maintain 20 Ah 𝐿𝑖𝐹𝑒𝑃𝑂4 pouch
cell in a near-isothermal state during a 3C discharge process [119].
The mechanical behaviour of the thermal management system is as important as its thermal
performance for making a reliable automotive-grade solution. It is beneficial to have a system
design with a higher heat absorption rate, but it also necessary to have the required strength
and stability to withstand normal stresses during daily vehicle operation. With this in mind,
Alrashdan et al. undertook a systematic study to characterise the effect of the thermo-
mechanical properties of eutectic PCMs (paraffin wax/expanded graphite), such as tensile and
compression strength and thermal conductivity, on the reliability of Li-ion battery packs. At
low as well as room temperatures, improvements in the thermo-mechanical properties,
including tensile strength, burst strength, compression strength, and thermal conductivity of
the pack were noticed, while they decreased at elevated temperatures [120].
However, phase-change materials have relatively low thermal conductivities. Consequently,
they have slower regeneration times and cannot be effective in applications that may include
fast charging followed by a quick discharge and then a fast charging of a battery pack in a short
time. Several heat transfer enhancement techniques have been investigated for PCMs, which
include the following:
1. The use of fixed and non-moving surfaces such as fins and honeycombs [121-125]
2. The employment of composite PCMs [126-128]
3. The impregnation of porous material [129-136]
4. The dispersion of high-conductivity particles in PCMs [137-141]
Table 2 summarises the research and development advancements made to the state-of-the-art
of TMS since 2016.
Source/Research
Method
Battery
type
TMS
Design Parameters
Test Conditions
Key Findings
Huang et al.
[142]/Experimental
5*6 - 1.1 Ah
cylindrical
cells
connected in
parallel
PCM + Liquid-
cooling
Paraffin-EG matrix with
embedded flat plate pipes;
Pipe thermal conductivity
= 10,000 W/m.K ; Cell
spacing: 34mm (X-) and
25 mm (Z-direction)
Coolant: ethyl alcohol
Ambient Temp: 35 °C
; Discharge rates of
1C, 2C and 3C;
5 cycles
Inter-cellular temperature difference can be maintained
below 3 °C by embedding flat pipes in PCM
Liquid assisted PCM took longer to reach the maximum
setting temperature of 44 °C in comparison to PCM
with air-cooled pipes even at 3C discharge
During cyclic discharge, liquid assisted PCM provided
better control over max. battery temperature
Liang et al. [143]/
Experimental
Battery
surrogate
generating
constant heat
load
HP
Sintered Copper HP
Evaporator section: 2mm
thick; Outer dia: 6 mm;
Horizontal working angle
Working fluid: Water
Ambient Temp: 15 °C
to 35 °C
Flow rate: 2L/min
Reducing coolant temperature can lead to constant HP
performance in ambient temperatures between 25 and
35 °C
Initial lag between starting times of battery and TMS is
preferred in low ambient temperatures. In contrast,
synchronized operation is beneficial in high ambient
temperature cases.
Wu et al. [144]/
Experimental
Five 3.2V/
12A
prismatic
cells
connected in
series
HP + PCM/EG
+ forced air-
cooling
Sintered Copper water
HP; Width: 8mm;
Thickness: 3mm;
PCM/EG Melting point:
41.71 °C
Galvanostatic charge
and discharge at rates
between 1C and 5C;
Air flow rate: 1 m/s to
4 m/s
HP assisted PCM systems are found more effective
than simple PCM system in case of repetitive cycling
Initial and maximum temperatures of 32.9 °C and 55.7
°C are noted for HP-assisted PCM/EG system during
cyclic use
Air flow of 3 m/s caused a reduction of 5.2 °C in max.
temperature
Ye et al.
[145]/Experimental
16 LFP 18Ah
prismatic
cells joined
in series
Micro-HP
array
3mm-thick flat micro-HP
array; 5° tilt angle;
Heat pipe: 200 x 60 mm
Fin: 20 x 24 x 1 mm
Room temperature:
27.56 °C; 1C charge
and 3 min rest phase
Intercellular gradient smaller than 5 °C noted in the
pack installed with micro-HP array with fins
Wang et al. [146]/
Experimental
Aluminum
block (115 x
90 mm) as
PCM +
Oscillating HP
OHP: Copper capillary
tube; Outer dia: 3mm and
Inner dia: 1.8mm;
OHP Orientations:
Horizontal, 45° tilt
and vertical;
For the same heating power, maximum temperature
decreases with increasing OHP inclination angle
battery
surrogate
Working fluid: acetone;
Paraffin melting point: 41
±1 °C
Start-up temperature:
25 ±0.05 °C
More uniform cell temperature distribution is achieved
by placing cell terminals further away from adiabatic
portion of OHP system
Start-up temperature must be below PCM transition
temperature
Hong et al. [147]/
Computational
modeling
12 x 2 battery
cells with
3mm spacing
Parallel air-
cooled system
Inlet width = Outlet width
= 20mm; Inlet length =
Outlet length = 100 mm
Inlet airflow rate =
0.012 𝑚3/s; Initial air
temperature = 300 K
Reducing inlet air temperature can only limit absolute
temperature within the battery pack and not the
intercellular gradient
Better cooling performance is achieved by placing a
secondary vent against the air outlet. Also, increasing
width of the secondary vent has a positive effect on
cooling performance of the system
Jiaqiang et al.
[148]/Computational
Modeling
Cold plate
having
rectangular
channels
Aluminium cold plate;
Coolant: Water
Discharge Rate = 0.5
to 3C; Coolant
velocity = 0.01 to 0.05
m/s; Channel number
= 2 5; Channel
Height = 3 6mm
In a cooling plate design, number of channels and
coolant flow rate are the primary design factors.
Effect of channel width and height is less in
comparison to the primary factors.
For a cold plate with 45mm wide and 5mm high
rectangular channels, optimum number of channels is 4
and preferable coolant velocity is 0.07 m/s.
Ling et al. [149]/
Experimental
Twenty 2.6
Ah 18650
Samsung
cells in 4P5S
combination
PCM/EG
PCM: 60 wt% RT44HC;
PCM/EG composite
melting point: 42.8 °C,
and PCM/fused silica
melting point: 41.5 °C
Ambient temperatures
of 5 °C and -10 °C;
Discharge Rates = 0.5
to 2C
PCM/EG composite resulted in a more uniform
temperature distribution within the pack at -10 °C.
Max intercellular voltage difference of 0.02V was
noted for case without PCM at -10 °C
PCM/fused silica caused temperature gradient greater
than 12 °C within the pack and is not recommended for
low temperature applications
Bahirei et al.
[150]/Computational
Modeling
Six 5 Ah
NCA battery
cells in
parallel
Liquid cooling
Cold plates
Aluminium cold plates;
Coolant: Water; Channel
height = 6mm; Initial
battery SOC = 70%
Laminar flow; FUDC
drive cycle; Initial
Temp = 293.15 K;
Cold plate thickness =
1 5mm
Double channel cooling system provides a more
uniform thermal distribution and a lower maximum
temperature for identical Reynolds number and cold
plate thicknesses
Thicker cold plate is preferred but increasing Reynolds
number leads to non-uniform temperature gradient
Chen et al.
[151]/Computational
Modeling
12 x 2 battery
cells
Parallel Air-
cooling
Length: inlet = outlet =
100mm; Width: inlet =
outlet = 20 mm; Adjacent
cell spacing = 2mm;
Battery heat generation
(HG) = 41408 W/𝑚3
Air flow rate = 0.012
𝑚3/𝑠; Initial Temp =
300 K;
Newton method combined with flow resistance method
is found suitable for optimization of air-cooling
systems
For fixed inlet flow rate and constant HG, optimum
width of divergence plenum and convergence plenum
are found to be 1.2mm and 20.0mm, respectively
For fixed power consumption of 0.5162 W, optimum
inlet flow rate is calculated as 0.011455 𝑚3/s
Hussain et al. [152]/
Experimental
Six 3.4 Ah
Panasonic
cells in series
PCM/Graphene
coated nickel
(GcN) foam
PCM: Paraffin
Graphene mass % =
0.5%; Adjacent cell
spacing = 5.74mm
Operating
temperatures: 25 °C,
30 °C and 33 °C;
Discharge currents:
1.7 A and 2.2 A
GcN improved thermal conductivity of pure paraffin by
23 times. However, latent heat capacity of the GcN
saturated paraffin decreased by 30%
17% lower temperature rise was recorded for pack
using GcN coated paraffin as TMS compared to the
pack using paraffin with nickel foam for 1.7 A
discharge
Yan et al. [153]/
Computational
modeling
20 Ah LFP
cell
PCM-based
composite
board
Sandwich structure PCM
composite board; Board
thickness = 10mm; PCM
melting temp = 303.15 K;
Latent heat = 225 kJ/kg
Ambient temp =
293.15 K; Discharge
rates: 1 10C
PCM composite board not only improves thermal
distribution within the battery pack but also limits
thermal runaway propagation rate
Using PCM with a higher latent heat capacity improves
thermal performance of composite board
PCM with latent heat of 1125 kJ/kg and melting
temperature in range of 303.15 K and 323.15 K is
recommended for use
Ren et al.
[154]/Experimental
Samsung SDI
94 Ah cells
connected in
5S2P layout
PCM (liquid to
gas phase
change)
PCM: Sodium alignate
hydrogel film with 99%
water content; Film
thickness = 2mm
Constant current
discharge with 250 A
and NEDC drive
cycle in room
temperature
Presence of hydrogel film caused a reduction of 40.5%
in the rate of pack temperature rise during constant
current operation of the pack
During NEDC test, temperature of the pack with
hydrogel fil increased by only 0.77 °C against 4.75 °C
for the pack operating without this film
Qian et al. [155]/
Computational
modeling
5 rectangular
cells
Mini-channel
cold plate
Cold plates: Aluminium;
Coolant: Water; Inlet
placed near battery
electrodes
Laminar flow; Inlet
water temperature =
Ambient = 25 °C;
Discharge rate = 5C;
No apparent advantage is derived by using a cold plate
with more than 5 channels
Increasing inlet flow rate is a more effective means for
reducing temperature difference and maximum
temperature in the pack than changing flow direction or
channel width
Zhao et al. [156]/
Experimental
Aluminium
cylinders
with heating
rods
PCM + HP
PCM: Paraffin/EG; EG
wt fraction = 16%;
Sintered copper HP,
diameter 6mm;
Ambient = 20 ± 0.5
°C; Start-up temp =
30 °C;
Discharge rate = 5C
HP can prolong the melting duration for PCM under
dynamic situations
Maximum recorded temperature for battery pack is
reduced by 33.6% due to PCM addition to the module.
Further reduction of 28.9% is achieved after embedding
HP in PCM matrix
Al-Zareer et al.
[157]/
Computational
Modeling
LMO 18650
cylindrical
battery cells
PCM (liquid to
gas phase
change)
PCM: Propane; Pressure
of liquid propane =
8.5bar; Saturation
temperature = 293.15 K;
Submergence level: 5
- 30% of battery
height; Charge/
discharge rate = 7.5C;
Test Duration = 600s;
Initial SOC = 10%
Submerging only 5% of battery in liquid propane
maintains battery temperature below 39 °C for whole
test duration. Increasing the submergence level to 30%
brings down the battery temperature to 34 °C
Increasing propane pressure to 10 bar reduces
temperature difference in the pack at the cost of
maximum temperature, which is increased. This effect
is more dominant at low submergence levels
Smith et al. [158]/
Experimental
25 Ah
prismatic cell
Liquid cooling
Cold plates
Coolant: 50/50 Water-
glycol mixture; Cold plate
thickness: 5mm or
smaller; Plate material:
Aluminium
Ambient temps: 0 °C,
20 °C and 40 °C;
Discharge rates: 1
6C; Test range: SOC
100% to SOC 20%;
Coolant channel footprint has a greater influence on
thermal performance of a cold plate than flow pattern
Flow patterns, both I- and U-type, create similar
degrees of thermal gradient over individual cells
Increasing channel width reduces pressure drop in the
plate without affecting average pack temperature
Situ et al. [159]/
Experimental
12 Ah LFP
prismatic cell
Quaternary
PCM + Air-
cooling
PCM: Paraffin + EG +
low-density polyethylene
+ copper mesh; mesh
thickness = 0.5mm
Ambient temperature:
25 °C; Discharge
rates: 1C and 5C
Thermal conductivity of Paraffin and EG composite
increases by 36.0% after addition of copper mesh to it
Maximum temperature recorded in battery pack with
quaternary PCM was found 19.5 °C lower than that in
the pack cooled via natural air convection
At 5C discharge, air velocity of 6 m/s maximises heat
dissipation from quaternary PCM plate
Wu et al. [160]/
Computational
Modeling
12 Ah
prismatic
cells
PCM
PCM: Paraffin + EG +
PGS; Cell spacing = PGS
thickness = 1.5mm; PGS
thermal conductivity =
800 W/m.K
Initial temperature =
Ambient = 298.15 K;
Discharge Rate = 5C
EG mass faction between 15 20% is recommended
Performance of PCM/PGS modules with heat transfer
coefficients of 50 W/𝑚2. 𝐾 is found comparable to
normal PCM modules of HT coefficients 200 W/𝑚2. 𝐾
Intercellular spacing of 2mm is found sufficient to
mitigate thermal runaway propagation in PCM/PGS
module. In contrast, 14mm gap between adjacent cells
is required in modules filled with normal PCM
Table 2: State-of-the-art battery thermal management systems developed since 2016 (HP: Heat Pipe; EG: Expanded Graphite; PGS: Pyrolytic Graphite Sheets)
Mortazavi et al.
[161]/Computational
Modeling
6 Ah LCO
cell based on
4.4 Ah
Hitachi
battery
PCM
Paraffin reinforced with
graphene nano-
membranes; Nanofillers’
thickness = 10 nm;
Volume concentration:
1% and 4%
Discharge Rates:
0.2C, 1C and 3C;
Initial SOC = 98%
Hexagonal Boron Nitride (hBN) flakes are
recommended over graphene nanofillers for addition to
paraffin module owing to their superior heat capacity
and electrically insulating nature
Graphite network embedded in paraffin provides better
thermal performance than TMS designed using
graphene nanoparticles
Putra et al.
[162]/Experimental
Aluminium
alloy battery
simulator
Flat plate loop
HP
Evaporator: Copper;
Capillary wick: stainless
steel screen of mesh size
300; Coolant filling ratio:
60%;
Coolants: Distilled
water, 96% Alcohol,
95% Acetone;
Thermostatic bath
temperature: 28 °C
For heat flux of 1.61 W/𝑚2, both acetone and alcohol
maintained evaporator temperature below 50 °C. In
case of distilled water, temperature rises to 60 °C
Acetone results in minimum temperature difference,
between the evaporator and the condenser, among the
three working fluids throughout the experiment
Hussain et al.
[163]/Experimental
Six 3.4 Ah
Li-ion cells in
series
PCM with
nickel foam
Paraffin (Rubitherm 42);
Melting range: 38-41 °C;
Intercellular spacing =
5.74 mm
Ambient temp: 25 °C;
Discharge Rates:
0.5C, 1C and 2C
Nickel-paraffin composite reduces battery surface
temperature by 24% in comparison to pure paraffin
during 2C discharge
Decreasing metal foam porosity enhances heat transfer
rate thus reducing battery temperature
Discharge capacity increases with increasing porosity
Xie et al.
[164]/Computation
Modeling
10 Li-ion
battery cells
Forced air
cooling
Height: air inlet = air
outlet = 20mm;
Intercellular spacing:
6mm
Ambient temp: 25 °C;
Discharge current:
20A; Air flow
velocity: 3 m/s
The temperature difference and the maximum
temperature are reduced by 29.72% and 12.82%,
respectively, using multi-objective optimization
Evenly spaced channels with inlet angle = outlet angle
of 2.5° provides greater temperature uniformity and
lowest maximum temperature in the battery pack
Xu et al. [165]/
Computational
Modeling
Five 55 Ah
prismatic
cells
Mini-channel
cold plate
Extruded multiport
Aluminium channel (h x
w x t) = 3 x 3 x 1mm;
Coolant: Water
Initial temp = 27 °C;
Flow rate = 10 L/min;
Nail Diameter: 3 5
mm; Nail penetration
depth: 20 to 60 mm
Thermal runaway will be initiated much sooner if a
thicker nail penetrates a battery cell or if the nail
penetration depth is greater. Also, the maximum
temperature reached will be higher in these cases
Mini-channel cooling system with independent control
strategy at cellular level can prevent thermal runaway
from propagating to adjacent battery cells
4. Emerging Techniques
Several other cooling techniques have been developed in recent times. These techniques offer
many advantages, including significant energy and cost-saving potential along with high
scalability, over traditional forced-air or liquid cooling methods. In this section, some of these
emerging alternatives are discussed in relation to TMS applications.
4.1. Thermoelectric Coolers
Thermoelectric coolers (TECs) are practically maintenance-free solid-state heat pumps with no
moving parts. They utilise doped semiconductor elements, comprising of a series of p-type and
n-type thermo-elements, sandwiched in thermally-conductive but electrically-insulating
substrates to transfer heat across a junction of two dissimilar materials via the Peltier effect.
The p-type material has excess positive charge carriers called holes, whereas n-type material
carries more negative charge carriers or electrons. The direction of heat transfer depends on the
polarity of voltage applied to the TE modules. An illustration of a TEC module is provided in
Fig. 5.
As voltage is applied to a TEC, electrons jump from a lower energy level of the p-type thermo-
element to a higher energy state in the n-type thermo-element by absorbing thermal energy
from one side of the module, in effect cooling it. The electrons drop to a stable energy level by
rejecting this heat on the other side of the TEC module. Accordingly, the same TEC module
can be made to function both as a cooler and as a heater by reversing the direction of current
flow across the junction.
Figure 5: Illustration of a thermo-electric cooling module [166]
Other advantages of TEC are:
It is compact and lightweight
It is acoustically silent and operates without any vibrations
It facilitates precise temperature control to within ±0.1 °C
It has low manufacturing costs and a wide operating temperature range
It can cool below ambient temperature
It is location independent, and can operate in any spatial orientation, at high G-levels
or in zero gravity
More importantly, the cooling elements of a TE module are easily scalable [167-169]. For these
reasons, TECs have been previously applied for climate control in EVs [170-173]. However,
the figure of merit (ZT) for the currently available bellerium telluride based TECs is
approximately 1. Consequently, the maximum coefficient of performance obtainable with these
devices is limited to 10%. It has been suggested that a ZT close to 4 is required to achieve
cooling performance comparable to other thermal management techniques [174]. Research to
develop new thermoelectric material and achieve much higher ZT is, therefore, required to
promote TECs as a viable solution for the temperature control of EV battery packs.
Nonetheless, the thermoelectric refrigeration method has been used in the new battery thermal
management system developed by the Gentherm Incorporation, details of which are disclosed
in US patent 8974942. This patent presents the design of a highly integrated yet simplified
assembly of thermoelectric modules that can heat and cool separate battery cells
simultaneously [175]. It is, therefore, believed that owing to their flexible form and scalability,
TECs may prove pivotal to the development of modular battery packs.
4.2. Thermo-acoustic refrigeration
It is known that thermal gradients can result in sound generation; the interaction between
thermodynamics and acoustics can therefore also be utilised to produce a refrigerating effect.
Thermo-acoustic refrigerators (TARs) are based on the Stirling cycle and use resonant high
intensity sound waves and a compressible mixture of inert gases as working fluid to pump heat.
A TAR assembly includes a gas-filled resonance tube containing a regenerative unit, called a
stack or regenerator for the respective heat pumping processes, and two heat exchangers. Fig.
6 shows the cross-sectional layout of a TAR with the different components.
The regenerative unit is a set of concentric cylinders, square rods or uniformly spaced parallel
plates strategically arranged between two heat exchangers, such that they reside between the
velocity node and antinodes within the resonance tube. Further, an acoustic driver like an
electric transducer, a moving-coil loudspeaker or a sinusoidal drive mechanism, is placed on
one side of the resonance tube while the other end is closed [176, 177]. The selection of the
appropriate driver is based on the design requirements, such as light weight, weight-to-volume
ratio, high BI-factor and low vibration losses [178].
Figure 6: Cross-section of thermoacoustic refrigerator illustrating various parts [178]
The acoustic driver produces cyclic variations of acoustic pressure in the resonance tube, which
causes expansion and compression of the inert gases contained in it. In response, the working
fluid starts to oscillate in the resonance tube. Imperfect thermal contact of the regenerative unit
with the acoustically oscillating gaseous mixture creates necessary phasing. This enables the
regenerative unit to continuously absorb heat from the resonance tube at the end near the
velocity antinode and reject it to tube walls closer to the velocity node. This natural phasing
also allows the TAR to operate without requiring moving parts other than the oscillating
working fluid [179, 180]. TARs can achieve heat transfer either via a standing pressure wave
or by means of a travelling pressure wave. These two differ in phasing between pressure and
velocity. As a result of this phasing, regenerative units with large channels are required in
standing pressure wave devices to artificially delay the heat exchange between the tube walls
and the stack. Conversely, much smaller flow channels can be utilised for travelling pressure
wave refrigerators, as heat transfer starts immediately after the working fluid is subjected to a
pressure change. They are therefore more efficient and compact than standing pressure wave
pumps.
Thermoacoustic pumps utilising standing pressure waves perform a surface heat pumping
process, whereas travelling pressure waves are used in a conventional Stirling-cycle heat
pumping process [181, 182]. TARs require no lubrication, sliding seals or expensive
components. They can be manufactured using only low-tolerance machined parts.
Furthermore, existing vibrations can be readily isolated in TARs, as they use compressors of
low moving mass (approximately 15 gm) and high oscillation frequencies (~ 400 Hz) [179].
The absence of moving parts and vibrations and low manufacturing costs make TARs excellent
candidates for battery TMS in EVs.
4.3. Magnetic Refrigeration
Certain ferromagnetic materials and paramagnetic solids are characterised by the intrinsic
coupling of their crystal lattice to the external magnetic field. They are called magnetocaloric
materials and this coupling is known as the magnetocaloric effect. Under adiabatic conditions,
it is quantified by a reversible temperature change observed in the magnetic solid due to
variation in its magnetic entropy upon exposure to a varying external magnetic field [183].
The application of a magnetic field induces spin polarisation in magnetocaloric materials,
which makes their molecules more stable, thus reducing their degrees of freedom and magnetic
entropy. As the total entropy of a magnetic solid remains constant if adiabatic conditions are
maintained, vibrations in the crystal lattice and the entropy of free electrons in the material
increase to compensate for the lost magnetic entropy. Consequently, an increase in material
temperature is noted. In contrast, a cooling effect is observed as the molecular magnetic
ordering returns to the initial alignment state by absorbing thermal energy from the crystal
lattice and free electrons when the externally-applied magnetic field is removed [184].
Typically, a field change of 1 Tesla can cause the material temperature to change between 1.5
to 2 K, depending upon the strength of the applied magnetic field and the absolute temperature,
but the effect maximises around the Curie or phase transition temperature of the magnetocaloric
material [185, 186].
Figure 7: Illustration of an active magnetic regenerator used for room-temperature applications [187].
Magnetic refrigerators for room temperature applications utilise active magnetic regenerators
(AMRs) similar to that shown in Fig. 7. An AMR combines the functionality of an indirect
heat exchanger with a heating/cooling mechanism. It has a porous structure and is made up of
magnetocaloric materials like gadolinium and perovskite manganese oxides [188], to maximise
the adiabatic temperature span associated with the refrigeration cycle. In this device, the
regenerator is maintained near the Curie temperature and an external magnetic field is
periodically applied to it, making it circle in a magnetic entropy-temperature loop.
Subsequently, a heat transfer fluid, usually helium or hydrogen gas or a water-based solution,
is circulated through the porous structure of the regenerator from the cold end to the hot end.
A thermal wave front is established as the working fluid traverses the structure absorbing heat
from it. The heat is then discharged to a hot bath at temperatures greater than the bulk
temperature. The fluid flow is terminated when the wave front reaches the hot end of the
regenerator, i.e. the exiting temperature drops to the hot bath temperature. This also acts as a
signal to start the adiabatic demagnetisation of the regenerator followed by the circulation of
the working fluid in the reverse direction, completing one refrigeration cycle [189, 190].
Magnetic refrigerators can operate at 30 to 60% efficiency of the Carnot cycle without needing
much maintenance [191]. Moreover, they do not need components with large mass rotating or
reciprocating at high speeds. They can therefore be compact and virtually noise-free. In
addition, they can also provide energy savings of up to 50% in comparison with conventional
refrigeration methods [192, 193]. Magnetic refrigeration systems with cooling powers between
200 W and 700 W and a quasi-indefinite lifespan are commercially available for applications
such as beverage dispensers, medical refrigerators and wine cellars [194]. It is likely that EV
battery packs can also benefit from TMSs based on magnetic refrigeration techniques.
4.4. Internal Cooling
Conventional TMSs minimise thermal gradients across the battery pack by maintaining the
exterior cell surface temperature in a pre-specified range. However, a variety of thermal
resistances and consequently a large thermal gradient exist in the space separating the heat-
generation sites inside the cell from the heat-transfer medium outside it. Estimates for a 26650
Li-ion battery cell suggest that the cell surface temperature and the core temperature may differ
by as much as 24 °C for a 10C discharge rate [195]. To address this issue, US patent 6653002
discloses an internal cooling strategy aimed at minimising the thermal resistance between the
internal heat-generation sites and the heat-transfer medium.
In one of the embodiments of this invention, microporous TECs are integrated both on the
internal and on the external sections of the battery cell. In addition, an array of small heat pipes
with a loop or any other open shape is sandwiched between the TECs, forming a cooling
module. The open shape of the heat pipes facilitates electrolyte movement in the battery cell.
It is also suggested that TECs could be replaced with micro-coolers to accomplish an ultrathin
module assembly. If the temperature inside the battery cell becomes greater than the pre-set
threshold values established by a temperature controller, electric current is applied to the TEC
module and as a result the Peltier effect develops across it. The hot electrolyte flows through
the microporous material of the TEC and comes into thermal contact with a cold junction plate,
which is essentially a heat sink. The cold plate removes heat from the hot electrolyte until a
lower threshold value is attained. The temperature controller immediately stops current flow to
the TEC module and a uniform temperature profile is established inside the battery cell [196].
Driven by a similar objective, Bandhauer and Garimella [197] developed a passive system that
uses the liquid-to-vapour phase change process to remove heat generated at local internal
locations during battery operation. In their concept, an internal evaporator with micro-channels
can be incorporated either directly in the thick current collector or embedded in a sheet of inert
material compressed between split current collectors. Excess thermal energy is transferred at
the appropriate saturation temperature and pressure to a working fluid flowing through
chemically inert micro-channels. The working fluid subsequently undergoes liquid-to-vapour
phase change and flows under the buoyancy effect to an external condenser, where it is
condensed. The gravitational forces transport the condensed fluid back to the evaporator inlet.
The work of these researchers shows that the saturation temperature has a small influence on
system performance.
More recently, Mohammadian et al. [198] demonstrated that the internal cooling technique
involving electrolyte as a coolant flowing in rectangular micro-channels is more effective in
decreasing the bulk temperature of a battery cell than a water-based external cooling system.
These researchers also showed that the same pumping power for the internal cooling system
could achieve up to five times the uniform temperature distribution in a battery cell. Shah et al.
[199] also investigated the effectiveness of annular air passages and heat pipes and plain copper
rods inserted along the axis of cells as an internal cooling system. They found that, depending
on the inner diameter of the heat pipes, it is possible to reduce the cell core temperature of
26650 Li-ion battery cells by approximately 18 to 20 °C through internal cooling. An
appreciable decrease in core temperature can also be achieved by embedding thin copper rods
instead of heat pipes in the core of the battery cell. It was noted that internal cooling with heat
pipes can delay the onset of thermal runaway events by facilitating rapid heat dissipation from
battery cells.
5. Selection of TMS for Modular Battery Pack
Previous research studies have established several techniques as suitable candidates for
application in battery thermal management systems. The selection of a specific technique can
therefore be made only after evaluating all the available alternatives. The factors that are
traditionally considered in the TMS trade-off analysis include energy efficiency, capital costs,
ease of operation, maintenance requirements and reliability. A comprehensive assessment of
these techniques was last presented by Rao et al [200].
According to their assessment, cold plate and thermoelectric devices are not recommended for
use in commercial systems, owing to their high thermal resistance and low coefficient of
performance, respectively.
However, Gentherm Incorporation employs thermoelectric devices in their commercial design,
whereas cold plates are used in the GM Volt and Tesla Model S battery packs, signifying that
both technologies have improved significantly in recent times. A trade-off analysis, updated
based on new information, is presented in Table 3. In addition to the traditional factors, cooling
techniques are also evaluated for scalability, development state and associated technical risks
in the present study. More importantly, emerging alternatives, such as magnetic refrigeration
and thermoacoustic refrigeration, are included in the comparison.
It is clear that liquid cooling systems are more effective than air-cooled TMS for large battery
packs operating at high discharge rates in ambient temperatures higher than room temperature.
Research has also shown that required pumping power for a microchannel cold plate system
with a 10.0 L/min water flow rate is just 1.3 W. However, pumping power varies with pressure
drop, which increases significantly as viscosity of the coolant in increases. For example, TMS
using ethylene glycol as coolant sustains nearly six times more pressure drop than the water
cooled TMS with same coolant flow rate [165]. The cold plate generally has a flat shape, which
makes its application easy in case of battery packs made of pouch cells and prismatic cells.
However, the same shape marginalises contact area in case of cylindrical cells, making heat
transfer using a cold plate a less effective process. Although the spot welding requirements and
potential leak points are much higher in a coolant-jacket design, it ensures optimal thermal
contact for cylindrical cells. A coolant jacket system is designed by integrating fluid inlet and
outlet with segregation walls or liquid guideways joining a plurality of cell apertures. Cell
apertures are hollow cylinders that are made-to-size for a specific battery cell type.
Consequentially, they limit scalability of the jacketed system.
Criteria
Forced Air
Liquid
PCM
Thermoelectric
Thermoacoustic
Magnetic
Jacket
Cold Plate
Heat Pipe
Ease of use
High
Low
Moderate
Moderate
High
Moderate
Moderate
Moderate
Integration
Simple
Difficult
Intermediate
Intermediate
Simple
Intermediate
Intermediate
Difficult
Energy efficiency
Low
High
Medium
High
High
Medium
Medium
High
Thermal gradient
High
Low
Moderate
Moderate
Low
Moderate
Moderate
Low
Cooling level
Small
Large
Medium
Large
Large
Medium
Medium
High
Regeneration rate
High
Medium
High
Medium
Low
High
Medium
High
COP @ room
temperature
0.4 0.7
1.8 2.1
1.5 1.9
N/A
N/A
0.7 - 1.2
Up to 1.0
1.8
Maintenance
Low
High
Medium
Medium
Low
Medium
Low
Low
First cost
Low
High
High
High
Moderate
High
Low
Medium
Scalability
High
Low
Low
Low
High
Medium
Medium
High
Technical risks
Low
High
Medium
Medium
Low
Medium
High
Medium
Development state
Commercial
Prototype
Commercial
Prototype
Prototype
Commercial
Experimental
Experimental
Table 3: Qualitative analysis of various battery thermal management methods
Battery packs are designed by connecting multiple battery cells in a series-parallel
combination. As a result, they can be rescaled simply by modifying the number of battery cells
involved in this combination. However, as soon as a liquid thermal management system is
integrated within the electrical-mechanical-structural framework of the batteries, the battery
pack loses its configurability. For instance, if the number of modules in the current battery pack
architecture needs to be increased to meet energy requirements of a certain application, or if
the current pack is too big for the envisioned application and it is decided to do away with the
pack’s excess energy storage capacity, then the pre-existing piping/plumbing and the auxiliary
devices would either be over-engineered or under-designed for the new application and will
need to be redesigned. It is, therefore, imperative for battery packs to have a modular thermal
management system for them to retain their scalability. In other words, mechanical modularity
is dependent on thermal modularity of the system, as heat exchange between neighbouring
battery modules cannot be fully eliminated. Hence, thermal independence of adjoining battery
modules must be ensured so that structural and mechanical design requirements can be satisfied
for the battery pack. Inefficiency in heat transfer due to incompatible geometries and
marginalised contact area can be removed via PCMs. PCMs will not only enable a greater
control over battery cell temperature and but also provide tight packaging by functioning as a
suitable cell spacer and filling up the voids between neighbouring cells. However, low thermal
conductivity of PCMs needs to be considered while selecting a suitable thermal management
system for EV battery packs.
It is evident from the analysis that none of the thermal management techniques alone can meet
the operational requirements of a modular TMS. It can therefore be inferred that a modular
TMS would involve a combination of these techniques. In other words, a modular TMS needs
to be a hybrid system. After careful consideration of all the likely scenarios, it is estimated that
this hybrid system will be developed around PCM. An example of one such hybrid system is
demonstrated in Ref. [201].
Lastly, there are several approaches to designing a TMS. The most appropriate approach
depends on the desired level of sophistication, the availability of information, and the
timeline/budget for a particular project. A systematic approach, proposed by Pesaran [75, 202],
can be adopted for the design of a TMS in general.
6. Conclusions
Different thermal issues affecting performance, cycle life and safety of Li-ion battery packs are
briefly discussed in this paper, and various thermal management techniques to address these
issues are qualitatively reviewed. The purpose of the review is to assess their suitability for a
modular battery pack.
Conventional TMSs like fans and cold plates either have low scalability or deliver marginal
performance (estimated from combined sets of cooling levels and regeneration rates) under
challenging conditions. Other interesting alternatives are available but they have not yet
attained full technological maturity. It is, therefore, concluded that a robust modular TMS
would be a hybrid system, designed through the union of at least two different TMSs.
Considering factors such as technical risks, ease of integration, cost and energy efficiency, it
can be inferred that PCMs would form an integral part of the modular TMS assembly.
Thermoelectric devices also appear to be a promising candidate for this application, due to their
superior state of development and acceptable COP at room temperatures.
Acknowledgement
This study was supported by a research grant from the Cooperative Research Centre for
Advanced Automotive Technology (AutoCRC), Australia.
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... The thermal runaway occurs when the reaction rate is increased by high temperature with high power output and later then it will increase the heat dissipation that will generate even higher temperatures. Fig. 1 shows the stages of the thermal runway in the battery mechanism [14]. Generally, it happens when heating starts to generate at the anode. ...
... Thermal runaway in lithium-ion mechanism[14]. ...
... The thermoelectric device is successful for the battery's preheating and cooling with a little input electrical energy, which improved the battery's reliability and operating efficiency. Although there have been various attempts to employ TEC for BTM, the Seebeck method's poor efficiency continues to limit its usage in BTMS [257]. However, when thermoelectric materials with a higher Seebeck coefficient are produced in the future, it provides a possible answer for BTMS. ...
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... The conventional techniques used for thermal management of electronic devices are not suitable. The traditional techniques include fan cooling with large equipment, which increases the cost and power consumption [31,32]. Fan cooling was widely used in the past for the cooling of computer electronics and several numerical studies were carried out using natural and forced convection in electronic components [33À35]. ...
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Full-text available
Rechargeable Li-ion batteries are widely used in renewable energy storage and automotive powertrain systems, and therefore an efficient thermal management system is imperative for maximum battery life and safety. Battery heat generation and dissipation rates primarily depend on the battery surface temperatures, which are effected by the coolant system design and coolant inlet conditions. In this paper, a two-way coupled electrochemical-thermal simulations with selected experimental validation has been done and analyzed the effect of water coolant inlet conditions on the effectiveness of a commercial minichannel cold-plates for 20 Ah LiFePO4 prismatic batteries. Three coolant inlet temperatures (25 – 45 ℃) and four flow rates (150 – 600 mL/min) are being tested at three different discharge rates (2 – 4 C) and the performance of coolant system design has been analyzed in terms of battery peak (maximum) temperature and temperature difference (i.e., non-uniformity) across the battery. The coupled model predicted results indicate that the coolant flow rate has profound effect on the battery temperature non-uniformity, while the coolant inlet temperature has significant effect on the battery peak temperature. At high coolant flow rates, battery surface temperature difference is within the acceptable range (∆T < 5 ℃), but maximum temperatures are high at all discharge rates. Further, at low coolant inlet temperature of 25 ℃ and high coolant flow rate of 600 mL/min, the battery temperature rise at top and bottom locations during constant current discharge process are high, indicating the battery heat generation rate is high at low coolant inlet temperature. Keywords: Li-ion battery, minichannel cold-plates, coolant flow rate, peak temperature, temperature non-uniformity, electrochemical-thermal model, COMSOL software
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The thermal performance of a battery thermal management system (BTMS) can be enhanced by cooling strategies, which are seldom taken into account in the study of heat pipe-based BTMS (HP-BTMS). The effects of coolant flow rate, ambient temperature, coolant temperature and start-up time on the thermal performance of HP-BTMS are crucial for the development of cooling strategies and are experimentally investigated in the present study. Results show that the thermal performance of HP-BTMS increases slightly with the decrease of ambient temperature as it is under 25 °C. When the ambient temperature is under 35 °C, the thermal performance of HP-BTMS can be kept nearly unchanged by reducing coolant temperature. The enhancement is little when ambient temperature is under 25 °C. Additionally, a drastic rise in the non-uniformity of battery temperature is observed at the moment of HP-BTMS initiation if HP-BTMS starts operating after battery temperature exceeds equilibrium value. Finally, intermittent cooling and constant cooling can achieve similar battery cooling performance, which indicates that the power consumption can be reduced by decreasing running time of HP-BTMS.