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K. Li et al. (Eds.): LSMS/ICSEE 2014, Part III, CCIS 463, pp. 476–485, 2014.
© Springer-Verlag Berlin Heidelberg 2014
LiFePO4 Optimal Operation Temperature Range
Analysis for EV/HEV
Jinlei Sun, Peng Yang , Rengui Lu, Guo Wei, and Chunbo Zhu
Harbin Institute of Technology Harbin 150001 China
{sunjinlei,lurengui}@hit.edu.cn,
{yangforyang,wg_weiguo}@sina.com,
zhuchunbo@gmail.com
Abstract. The LiFePO4 batteries are widely used in Electric Vehicle(EV)/Hybrid
Electric Vehicle(HEV) because of the high energy and power density. However,
high environment temperature could accelerate the aging of batteries, while low
temperature could reduce output power capability. Therefore, optimal working
temperature for batteries should be determined to maintain good performance in all
kinds of tough conditions. In this paper, the optimal working temperature range
for batteries is analyzed. The capacity loss model is applied to determine the upper
limit. The lower limit is calculated taking available capacity and output power loss
into consideration. Simulation and experimental results show that the working
temperature range between 10℃ and 40℃ could ensure the performance and
available capacity.
Keywords: Electric Vehicle, Hybrid Electric Vehicle, LiFePO4, optimal
working temperature.
1 Introduction
With the problems of energy crisis and environment becoming increasingly prominent,
Electric Vehicles(EVs)/Hybrid Electric Vehicles(HEVs) have attracted more and more
attention[1]. Lithium-ion batteries are becoming the best choice for solving these
problems owing to the characteristics of high energy and power density[2]. But the
drawbacks such as cost, safety and lifetime are the bottlenecks for EVs/HEVs taking the
place of traditional vehicles. The performance of power LiFePO4 tends to be greatly
affected by temperature, high temperature may accelerated aging and lead to thermal run
away[3]. It is reported that he slow charge transfer at the electrode/electrolyte interface
leads to the poor performance at low temperature[2]. At extreme low temperature the cell
capacity fades greatly comparing to the nominal capacity under room temperature [4].
Wide range working temperature has great influence on the performance and safety for
EVs/HEVs. The traditional fuel vehicles have been developed over 200 years and have
been able to withstand the harsh environment, while the EVs/HEVs must solve the
problem of battery pack thermal management to get satisfied performance at an extreme
cold or hot temperature. The optimal operation temperature range is available to provide
references for TMS and to prevent undesirable performance fade caused by environment.
LiFePO4 Optimal Operation Temperature Range Analysis for EV/HEV 477
The goal of battery thermal management is to maintain the battery within optimal
temperature range. For example, the aging and resistance rise caused by high
temperature and the available capacity and power fade caused by low temperature[5].
The battery thermal management methods mentioned in the literatures include: the
forced air cooling[6], liquid-based thermal management system [7,8],PCM based
thermal management system [9,10] and Thermo Electric Cooler(TEC) based
heating/cooling. [11,12] The forced air cooling is the traditional method for cooling,
the air flows across the surface of battery pack to take the heat away, this method has
been used in the Toyota Prius HEV application[13]. The liquid-based thermal
management takes the heat away directly or indirectly by liquid such as water, glycol,
oil, acetone or even refrigerants. Thanh-Ha Tran designed a flat heat pipe cooling
system, which could reduce the thermal resistance by 30% comparing with the natural
air cooling[7]. Zhonghao Rao[14] developed a thermal management system whose
maximum temperature could be controlled below 50℃ when the heat generation rate
was lower than 50 W and the maximum temperature difference is below 5℃. The
phase change materials(PCM) are developed rapidly recent years, PCM absorb heat
released by battery and make the temperature decrease rapidly, the heat is stored in
the form of PCM. The heat releases to the battery when in extreme cold environment.
The blower and pump are no longer needed in the PCM system. Selman and Al-Hallaj
did some research on the PCM and take the PCM to battery thermal management
system for the first time. In [15],they established 2D model for comparing four
thermal methods: (1) natural convection cooling; (2) presence of aluminum foam heat
transfer matrix;(3) use of phase change material (PCM); and (4) combination of
aluminum foam and PCM. They came to the conclusion that the use of aluminum
foam with PCM causes a significant temperature drop of about 50% compared to the
first case of no thermal management. In [16] the PCM and air-based methods are
compared and the advantages of heat pipe under extreme cold temperature were
highlighted. Chakib Alaoui worked on the TEC heater/cooler based on Peltier effect
for several years. The TEC based heater/cooler controls the temperature of cabin and
battery pack and took the place of vehicle air conditioning [12]. In [11], the TEC was
placed on the surface of each cell for the 24 series connected battery pack. The
Coefficient of Performance (COP) under the condition of US06 was as high as 1.2
and the energy consumption is only 4% of the fully charged pack.
Although there were many methods for battery thermal management, the
temperature control target is not uniform. Ref[17] argues that the highest battery
operating temperature should below 40℃ and the maximum temperature differences
is within 5℃. The FreedomCAR Battery Test Manual [18] defines the working
temperature range to be between -30℃ and 52℃. The wide range of working
temperature could not ensure the performance of battery pack. Thus, there should be a
specific optimal working temperature range for battery pack considering the power
and capacity characterizes.
In this paper, the experiments are taken first to test the temperature characterizes of
battery. Then the results of HPPC and cold cranking tests are analyzed. The cell
capacity loss model is used to analyze the aging of battery under high temperature the
power fade is analyzed according to capacity loss and power capability. Finally, the
optimal operation temperature range is determined.
478 J. Sun et al.
2 Experiment Design
2.1 Measurement Equipment
The commercial LiFePO4 used in the temperature characteristic experiment is
5Ah/3.2V (Voltage range 2.5V-3.65V). The test platform contains Arbin BT2000
battery tester(Output current range 0-100A, Voltage range 0-18V, Accuracy 0.02%-
0.05%FSR) and Testsky temperature control box (Temperature range -40℃-200℃,
Accuracy ±0.5℃). Fig.1 shows the devices for experiment.
Fig. 1. The devices for battery test and temperature control
2.2 HPPC Tests at Different Temperatures
The cell samples are placed in different temperature environments (-20℃,-10℃, 0℃,
10℃, 20℃, 30℃, 40℃, 50℃, 60℃) for 5 hours respectively. Each cell was fully
charged by constant current and constant voltage (CCCV) under different
temperature. The charge current was 9.37A and the discharge current was 12.5A . The
HPPC test was taken every 10% SOC intervals with 1C rate discharge current. The
HPPC test profile is shown in Fig. 2. In Fig. 2 the dotted line represents the current,
the solid line represents the voltage. The experiment is stopped as soon as the call
voltage reaching the cutoff voltage.
2.3 Cold Cranking Tests
According to the FreedomCAR Battery Test Manual [18], the pack should be replaced
when capacity fades to 80% of the rated capacity. In order to further study the output
power performance, a power pulse start test is taken according to the FreedomCAR
Battery Test Manual [18]. The pulse profile is shown in Fig.3.The tests are conducted
under different SOCs and temperatures, the maximum output power is measured
every 10% SOC internals. The steps are as follows:
1) Charge the cell to fully charged (Constant current and then constant voltage)
2) Discharge to the target SOC
3) Rest for 5 hours under the target temperature
LiFePO4 Optimal Operation Temperature Range Analysis for EV/HEV 479
4) Take 3 power pulse tests at constant power, each pulse lasts 2 seconds and rest for
10 seconds. As is shown in Fig.7
5) If the discharge cutoff voltage is met, return to step1) and decrease the power value
in step 4)
6) If the steps are finish, repeat step1 and increase the power value in step 4) until the
maximum power is found.
Fig. 2. The HPPC test profile
Fig. 3. The constant power start pulse profile
2.4 Results and Discussion
The results of the HPPC tests at different temperatures are shown in Fig.4-Fig.6 and
that for cold cranking are shown in Fig.6.
Fig.4 shows that the discharge capacity of the same cell under different
temperature conditions. It could be seen that the cell capacities are nearly the same at
the temperature between 40℃ and 60℃, while the cell capacity decreases obviously
480 J. Sun et al.
with the decrease of temperature, especially below 0℃. The cell capacity is 80% at
0℃ and could hardly discharge at -20℃.
Fig. 4. The cell capacity profile at different temperatures
Fig. 5. The cell ohmic resistance and polarization resistance at different temperatures and SOCs
The discharge capacity fades with the decrease of temperature. The ohmic
resistance and polarization resistance (Ro and Rp) under different temperatures are
indentified according to the method mentioned in FreedomCAR Battery Test Manual
[18]. Just as shown in Fig.4. The ohmic resistance changes a little at different SOCs at
the same temperature. The ohmic resistance increases with the drop of temperature.
The polarization resistance decreases with the drop of temperature, but it changes
greatly at different SOC under the same temperature. Due to the discharge capacity is
almost zero, the data at -20℃ is not universal.
Fig.6 shows the maximum charging and discharging power at different
temperatures and SOCs. The maximum charging and discharging power at target
SOC is defined as the product of maximum charging/discharging voltage during pulse
and the current. The charging power increases with the drop of temperature, while the
LiFePO4 Optimal Operation Temperature Range Analysis for EV/HEV 481
discharging power fades with the decrease of temperature. At the same temperature
the charging and discharging power in the full SOC range are nearly the same.
Fig. 6. The maximum charge and discharge power at different temperatures and SOCs
3 The Optimal Operation Temperature Range
As is analyzed in Section 2.3, the target cell capacities at a temperatures higher than
40℃ are nearly the same, while the discharge capacities begin to fade below 0℃.
Many researches claim that high temperature accelerates the aging[19] and the
performance fades during low temperature [20].In this section, the operation
temperature range is determined considering the current output power capability and
long term lifetime.
3.1 The Determination of Operation Temperature Range Upper Limit
John Wang established the capacity loss model taking DOD, temperature, discharge
rate into consideration in his research[19].
0.55
31700 370.3
exp( )( )
rate
loss h
C
QB A
R
T
−+×
=⋅ (1)
Where is the percentage of capacity loss, B represents the pre-exponential
factor, the Ah-throughput, which is expressed as Ah = (cycle
number)×(DOD)×(full cell capacity), and z is the power law factor, R is the gas
constant. T is the absolute temperature.
Yuejiu Zheng[21] further developed the model and have confirmed the parameter B.
1/3
15
10000( )
rate
BC
=
(2)
loss
Q
h
A
482 J. Sun et al.
The 1C rate discharge capacity loss is calculated according to equals (1) and (2)
0.55
31329.7
24662 exp( )( )
loss h
QA
R
T
−
=⋅ (3)
The aging experiment takes considerable time and work. To explain the
temperature influence on aging, we take the 1C discharge rate with 80% DOD
capacity loss model to simulate and analyze. The simulations under the conditions of
10℃ to 60℃ (10℃ internals) are taken. The results are plotted every 50 points, as is
shown in Fig.7.
Fig. 7. The capacity loss simulation at different temperatures
When the cycle number comes up to 2000, the capacity losses below 40 are
lower than 20%. The 2000 times cycle is enough for the lifetime of both the battery
and vehicle. Additionally, the maximum average temperature in summer is 40, the
maximum capability for thermal management system is to make the temperatures in
and out of the EV/HEV nearly the same. To sum up, 40 is determined to be the
upper limit of operation temperature range to maintain the performance and prevent
accurate aging caused by high temperature.
3.2 The Determination of Operation Temperature Range Lower Limit
The low temperature affects the charge transfer at the electrode/electrolyte interface,
which leads to the significant plating on the negative electrode during charging. It
irreversibly causes the capacity loss. Low temperature affects the driving distance and
output power performance for EVs/HEVs.
The maximum output power test results at different SOCs and temperatures are
shown in Fig.8. It shows that at room temperature, the maximum output power is
38W with almost no change within the whole SOC range. With the decrease of
temperature, the maximum output power fades gradually at the same temperature and
different SOCs. For example, at -10℃ and 100% SOC the maximum output power is
LiFePO4 Optimal Operation Temperature Range Analysis for EV/HEV 483
the same as that at room temperature. However, the power differences between 10%
and 100% are 10W. When the temperature comes to 10℃, the output power is similar
to that of room temperature and the power differences are little with different SOC.
Thus, the lower limit of operation temperature range is determined to be 10℃.
Fig. 8. The maximum output power at different temperature and SOC
4 Conclusion
In this paper, we proposed the optimal temperature operation range for batteries in
EV/HEV. We first take the HPPC and cold cranking tests under different
temperatures to obtain the temperature characteristics of LiFePO4. And then the
upper limit is determined according to the aging model. The lower limit is
determined considering discharge capacity loss and output power fades. Finally,
the optimal operation temperature range is proved to be 10℃ to 40℃ according to
the experimental results. The range provides a temperature control target for pack
thermal management. Working in the proposed temperature range is good for
maintaining vehicle in good performance and reducing energy loss during heating
or cooling.
Acknowledgments. This research was supported by the National High Technology
Research and Development Program of China (2012AA111003) in part and the
NSFC-EPSRC Collaborative Research Initiative in Smart Grids and the Integration of
Electric Vehicles (51361130153) and Science and Technology Project of State Grid
Corporation of China and the Fundamental Research Funds for the Central
Universities (Grant No.HIT.IBRSEM.201306).
484 J. Sun et al.
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