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A multifunctional battery module design for electric vehicle
Meng Wang
1
•Liangliang Zhu
2
•Anh V. Le
1
•Daniel J. Noelle
3
•Yang Shi
3
•
Ying Zhong
3
•Feng Hao
2
•Xi Chen
2
•Yu Qiao
1,3
Received: 5 May 2017 / Revised: 11 September 2017 / Accepted: 19 September 2017
ÓThe Author(s) 2017. This article is an open access publication
Abstract Reducing the overall vehicle weight is an effi-
cient, system-level approach to increase the drive range of
electric vehicle, for which structural parts in auto-frame may
be replaced by battery modules. Such battery modules must
be structurally functional, e.g., energy absorbing, while the
battery cells are not necessarily loading–carrying. We
designed and tested a butterfly-shaped battery module of
prismatic cells, which could self-unfold when subjected to a
compressive loading. Angle guides and frictionless joints
were employed to facilitate the large deformation. Desired
resistance to external loading was offered by additional
energy absorption elements. The battery-module behavior
and the battery-cell performance were controlled separately.
Numerical simulation verified the experimental results.
Keywords Electric vehicle Battery module
Multifunctional Energy absorption
1 Introduction
While the first electric vehicle (EV) was developed a few
decades ago [1], there are still a large number of technical
hurdles that must be overcome, before EVs can be widely
commercialized. One key issue is the drive range: To
compete with fossil-fuel vehicles, once being charged, an
EV should be able to travel 200–300 miles, for which
70–80 kW h electrical energy is needed [2]. Thus, the
battery system in an EV, including battery cells and pack
components, could be heavier than 500 kg [3].
It was suggested that the specific energy of EV battery
system must be more than *150 Wh/kg at the pack level
[4]. A promising way to achieve this goal is to render the
battery modules/packs multifunctional. For instance, they
can be energy absorbing and protective. As a first-order
approximation, assume the mass of a lightweight EV is
M=1000 kg. At the speed of v= 35 MPH—the vehicle
speed in standard crashworthiness testing [5], the EV
would carry a kinetic energy of K=120 kJ. The volume
of an EV battery system is V= 300 L [6]. If upon collision
the battery system volume could shrink by *50%, the
kinetic energy can be entirely dissipated as long as the
crushing pressure (P) is above 0.8 MPa. Disintegration of
battery pack under this condition is acceptable, since the
EV structure has been destroyed.
For another order-of-magnitude assessment, assume that
the battery system is rectangle, with the cross-sectional
area of Aand the length of L. The deceleration of the
vehicle, as the battery system absorbs energy, can be
estimated as a=PA/M. Here, for the sake of simplicity, we
assume that the crushing pressure is constant over time, so
is the deceleration. Hence, for a battery system with A=
0.3 m
2
, to keep the deceleration below 60 G [7], Pshould
be lower than 3 MPa.
Clearly, the estimations above do not facilitate an
accurate analysis. Nevertheless, it validates that there exists
a range of crushing pressure, around 1–3 MPa, at which the
battery pack can be employed as a protective structural
component to absorb vehicle kinetic energy and to keep the
&Yu Qiao
yqiao@ucsd.edu
1
Department of Structural Engineering, University of
California – San Diego, La Jolla, CA 92093-0085, USA
2
Columbia Nanomechanics Research Center, Department of
Earth and Environmental Engineering, Columbia University,
New York, NY 10027, USA
3
Program of Materials Science and Engineering, University of
California – San Diego, La Jolla, CA 92093, USA
123
J. Mod. Transport.
DOI 10.1007/s40534-017-0144-8
deceleration relatively low, meeting the requirements of
vehicle crashworthiness testing. As a result, a variety of
other structural parts of auto-frame may no longer be
necessary, leading to a considerable weight reduction. The
effective ‘‘net’’ mass of battery system can be taken as the
actual battery system mass, m
1
, subtracting the saved mass
from auto-frame, m
2
. While due to the constraints of bat-
tery chemistry, m
1
must be relatively high, (m
1
-m
2
) may
meet the functional requirement of EV, by using existing or
near-future battery chemistry.
Currently, the study on multifunctional EV battery
system is still in its early stage. It was proposed that, as
cylindrical battery cells are placed in parallel and confined
by regular walls [8,9], a disintegrable structure can be
formed, which provides sufficient flexibility to protect
battery cells in a vehicle collision. A number of studies
were performed on battery safety and robustness at the cell
level [10–15]. Functional current collector (FCC) [10–12]
was developed to isolate the damaged areas in a mechan-
ically abused battery cell, which significantly decreased the
heat generation rate. Thermally sensitive binder (TSB)
[13,14] was developed to reduce the electronic conduc-
tivity once the battery was overheated, and thermal run-
away retardant (TRR) [15] was employed to lower the
ionic conductivity. The battery cells were typically tested
through nail penetration, blunt impact or indentation
[10–15]; in such tests, the cell case would largely deform
and the active materials were internally shorted. In addi-
tion, computer simulation at the vehicle level [16] has
demonstrated that strategic placement of battery pack could
reduce the impact forces on occupants and minimize the
damages of battery cells. However, little study has been
conducted on the level of battery module. To fully take
advantage of the multifunctional design of battery cells in
EV, the structure of battery module needs to be re-
investigated.
It is hypothesized that a high deformability of battery
module would be beneficial. In the current research, we
focus on designing and testing individual battery module.
Since battery cells are expensive and contain flammable
electrolyte, mechanical loading on battery module will be
carried by the energy absorbing elements, separate from the
battery cells.
2 Experiment and computer simulation
A deformable, butterfly-shaped battery module structure
was designed, as depicted in Fig. 1. Prismatic aluminum
cells were employed as analogs to hard battery cells, with
the height, width, and thickness being 25.4, 25.4, and
4.76 mm, respectively. One end of the cell was tapered at
an angle of 45°. The tip of the tapered edge was rounded,
and the length of the ramp section was 4.76 mm. The
opposite end of the cell was flat, and the two surfaces were
respectively connected to two McMaster-Carr 1603A23
brass surface-mount hinges, using McMaster-Carr
91500A086 bolts and McMaster-Carr 91841A003 nuts.
The bolt size was 2–56; the hinge was 25.4 mm wide and
0.5 mm thick, with the leaf height of 25.4 mm.
Each model battery module consisted of four cells and
four hinges. The upper and bottom hinges were fully folded;
the left and right hinges were fully open. Thus, the four cells
were aligned along the axial direction. They were mounted
vertically on a McMaster-Carr 8982K39 aluminum angle,
which served as the support. Another angle was inserted
upside down on the top, as the guide. The width, the arm
length, and the thickness of the angles were 25.4, 25.4, and
0.5 mm, respectively. The initial configuration of the mod-
ule was left–right and top-town symmetric.
Two McMaster-Carr 6100K149 steel tubes, with the
length, the outer diameter, and the wall thickness of 76.2,
7.14, and 0.127 mm, respectively, were employed as
energy absorption elements (EAE). They were attached to
the two lateral sides of the hinged cells by 3M DP110
adhesive.
Fig. 1 a Schematic of butterfly-shaped battery module: before (left), during (middle), and after (right) deformation. bPhotos of a testing sample:
before (left) and after (right) impact; the scale bars indicate 20 mm
M. Wang et al.
123 J. Mod. Transport.
The module was placed on a large flat 6.35-mm-thick
steel plate, which was firmly mounted on the base holder of
an Instron Ceast-9350 drop tower. A 12.7-mm-thick 101.6-
mm-diameter circular hardened-steel impact head was
launched onto the angle guide, with the impact velocity of
v= 7.7 m/s. The total drop mass was m= 3.65 kg. The
deceleration was measured by an accelerometer embedded
in the impact head; the drop-mass displacement was
simultaneously recorded by a linear variable differential
transformer (LVDT). Altogether four sets of modules were
tested. Their behaviors were quite similar.
Finite element analysis (FEA) was conducted using
ABAQUS with explicit package. The FEA model is shown in
Fig. 2a, identical to the experimental setup. Linear elasto-
plastic material models were employed. For the EAE ele-
ments, the Young’s modulus E
Fe
= 200 GPa, the yield stress
r
Y-Fe
= 500 MPa, the Poisson’s ratio m
Fe
= 0.33, and the
mass density q
Fe
= 7800 kg/m
3
. For the cells, E
Al
= 70 GPa,
r
Y-Al
= 300 MPa, m
Al
= 0.33, and q
Al
= 2700 kg/m
3
. Friction
coefficient was set to l= 0.3. Mesh convergence study was
carried out to ensure appropriate mesh density.
3 Results and discussion
Currently, the energy storage of EV relies on lithium (Li)
ion batteries, which have the highest specific energy per kg
and the lowest specific cost per kWh, compared with lead
acid batteries, nickel-metal hydride (NiMH) batteries, and
double-layer and pseudo-supercapacitors [17]. Lithium-ion
battery cells contain electrolytes based on highly flam-
mable organic solutions such as dimethyl carbonate
(DMC), diethyl carbonate (DEC), and ethyl methyl car-
bonate (EMC), since Li phase is incompatible with aqueous
solutions [18]. The organic solvents are volatile, having
relatively low boiling points around 110 °C and low flash
points below room temperature; their combustion heats are
quite high, around 1 kJ/g [19], lower than yet on the same
scale as that of gasoline [20]. Thus, if the battery cells
largely deform and the electrolyte solutions spill out, tough
challenges would be imposed to system safety. To be
conservative, in the current research, the high deformabil-
ity of battery module is achieved by using connection
hinges; the battery cells behave as ‘‘rigid’’ parts.
x
z
x
z
x
z
y
z
y
z
Impact head (3.65 kg)
7.7 m/s
Cell
Hinge Steel tube
Steel plate
Support
(a)
(b)
(c)
Fig. 2 Computer simulation of battery module abefore and bafter impact; cstress distribution in the battery module at 10 ms during impact
A multifunctional battery module design for electric vehicle
123
J. Mod. Transport.
The initial configuration of the battery module is fully
folded. Upon the dynamic compressive loading from the
drop mass, the tapered cell edge and the impact guide
trigger a smooth self-unfolding, aided by the angle support
from the bottom. The cells are freely jointed by the hinges,
and the motion is quite resistance free and robust. Even
when the angle guide and support are damaged, once the
self-unfolding begins it would continue until the cell
assembly is flattened. During the deformation process, the
cells are designed to be ‘‘loading-free’’, as long as the
frictions are small. As shown in Fig. 2b, during impact, the
stress level in the cells is quite low; stress concentration
occurs around the hinges and the top of upper cells. Most of
the loadings are carried by the EAE. Therefore, the cell
assembly can reach a high deformability, while the cell
damage is trivial. According to necessity, additional pro-
tection components may be included in the bottom section
of the module; hence, when the module self-unfolding
completes, direct impact on cell surfaces is minimized.
The favorable working pressure of multifunctional bat-
tery system, as approximately analyzed in the introductory
section, is 1–3 MPa. Because the volume of battery mod-
ules is only a fraction of that of the battery pack [9] and
also because higher working pressure leads to better energy
absorption capacity, the crushing pressure at the module
level should be close to the high end, i.e., 3–5 MPa. The
resistance to external loading is mainly offered by EAE,
separately assembled next to the battery cells. The modu-
larized structure enables independent adjustment of the
performance of individual battery cells and the mechanical
responses of module. The EAE is lightweight and highly
deformable. As their wall thickness and size are controlled,
the buckling pressure can be tailored in a broad range. As
shown in Figs. 2c and 3a, shortly after the drop mass
impacts the cell assembly, in less than 1 ms, the tube
buckling takes place, resulting in a buckling plateau in the
stress–strain curve. Figure 3a shows the profiles of engi-
neering stress, r=F/A
0
, and engineering strain, e=DH/H
0
,
where F=ma is the impact force, ais the deceleration of
drop mass, A
0
is the initial cross-sectional area, DHis the
system deformation, and H
0
is the initial height. As the
EAE is compressed, elastic energy is stored by the tube
wall, and when it exceeds the limit of structural stability,
wrinkles would form and propagate along the axial direc-
tion. The buckling pressure, P, is relatively constant as the
wrinkles develop.
Figure 3a indicates that as the engineering strain is
*5%, the buckling pressure reaches 2 MPa, and it varies
around 2–5 MPa until the engineering strain exceeds
*60%, close to the desired level. The maximum strain is
around 70%, mainly determined by the volume fraction of
the hollow space, measured by
e
t¼t=R, with tbeing the
wall thickness and Rthe radius. According to the Pthe-
orem [21], the buckling pressure (P) is a functional of E,
H
0
,R, and
e
t, with Ebeing the Young’s modulus; thus, P/E
=f(q,
e
t), where q=H
0
/Ris the aspect ratio and fis a certain
function. By adjusting E,q, and
e
t, the working pressure of
EAE can be varied to meet different functional require-
ments. As a new wrinkle is nucleated, the pressure would
drop; as the wrinkle is folded, the pressure would increase.
Therefore, the buckling plateau of a smooth tube is inevi-
tably jerky. If needed, the buckling plateau can be
smoothened by using tubes of crimped walls.
Although the buckling pressure fluctuates, because the
fluctuation frequency is high, it does not have any pro-
nounced influence on the drop-mass velocity, as shown in
Fig. 3b. After the initial setting period of a couple of ms,
the drop-mass velocity decreases nearly linearly, with the
deceleration of *500 m/s
2
,or*50 G. The simulation
results agree well with the experiment. The relatively
Fig. 3 Typical impact testing and computer simulation results: athe
dynamic stress–strain relation; bthe drop-mass velocity; and cthe
absorbed energy
M. Wang et al.
123 J. Mod. Transport.
constant velocity changing rate suggests that the protective
cell module works smoothly.
The absorbed energy is shown in Fig. 3c. Due to the
reduction in drop-mass velocity, the energy dissipation rate
keeps descending. After the initial setting period, most of
the kinetic energy is absorbed in the following *5 ms,
after which the deformation of the cell assembly becomes
less intensive.
4 Concluding remarks
Butterfly-shaped self-unfolding battery module is designed
and tested. The goal is to achieve a controlled resistance
and a large deformability, with the impact loading on
battery cells being minimized. The battery cells are freely
jointed by a set of hinges and initially fully folded. Upon a
compressive loading, the hinges, aided by a guide from the
top and an angle support from the bottom, help unfold the
cells. The resistance is offered by a set of energy absorp-
tion elements (EAE) in parallel to the battery cells. The
resistance and the cell motion are decoupled, so as to
independently adjust the performance of battery cells and
the mechanical responses of module. The resistance pres-
sure offered by the cell assembly is a few MPa; the
deformability is more than 60%. The relatively large
fluctuation of resistance pressure does not have much
detrimental effect on the smooth reduction of drop-mass
velocity. During impact, most of the kinetic energy is
absorbed in a few ms after the initial setting period. Such a
multifunctional battery module may be employed as
structural components in electric vehicles, so as to lower
the overall vehicle mass and increase the drive range.
Acknowledgements This research was supported by the Advanced
Research Projects Agency-Energy (ARPA-E) under Grant No. DE-
AR0000396, for which we are grateful to Dr. Ping Liu, Dr. John
Lemmon, Dr. Grigorii Soloveichik, Dr. Chris Atkinson and Dr.
Dawson Cagle.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestricted
use, distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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