Simulation on the uniformity of Lithium battery pack Aug.,2012 The uniformity of Lithium battery...
Transcript of Simulation on the uniformity of Lithium battery pack Aug.,2012 The uniformity of Lithium battery...
PCG Aug.,2012
Simulation on the uniformity of
Lithium battery pack Languang Lu, Yuejiu Zheng, Minggao Ouyang
Department of Automotive of Engineering
Tsinghua University
PCG
24-8-2012
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PCG Aug.,2012
contents
Objectives and Approach
Battery pack modeling
Single factor affect simulating
Multi-factor affect simulating
Conclusions
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PCG Aug.,2012
The uniformity of Lithium battery pack
Objectives
(1)Finding the factors affecting the uniformity of
the battery pack ;
(2)Understanding the battery uniformity
processing
(3)Making suggestions for battery packing and
balancing.
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PCG Aug.,2012
The uniformity of Lithium battery pack
Research Approach
Modeling and simulating the uniformity base on
the characters of the battery:
(1)The change of the battery performance is
very slow;
(2) many factors have different levels affecting
the uniformity of the battery ;
(3)Coupling effect on the uniformity;
(4)The parameters of the battery comply
statistics. Page 4
PCG Aug.,2012
Modeling the battery
Rint modeling
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Cri=SOC0i•C0i-∫ζi(t)dt + ∫(ηi(t)i(t) + ieqi)dt
Where OCVi is cell I open circuit voltage, ri is the cell i resistance,
Cri is the remain capacity in cell i, Ci is the cell i capacity (depend
on life), SOC0i and C0i is the Cell i initial SOC and initial capacity,
∫ζi(t)dt is the self loss capacity, ∫(ηi(t)I(t)+Ieqi)dt is Ampere hour
integral, ηi(t) is the Coulomb efficiency (when charging ηi(t)<1,
when discharging ηi(t)=1), ieqi is the balancing current of cell
(charging is positive, here is 0)
PCG Aug.,2012
Modeling the battery
OCV (LFP) Suggest
One-to-one correspondence between SOC and OCV
OCV is little relation to the life cycle.
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0 20 40 60 80 1002.6
2.8
3
3.2
3.4
3.6
SOC/%
OC
V/V
M. Dubarry et al, Journal of Power Sources 174 (2007) 1121–1125
V. Pop et al, J. Electrochemical Soc.,2006, 153 (11)A2013-A2022
V Pop, et al, Meas. Sci. Technol. 16 (2005) R93–R110
I. Snihir et al, J. Power Sources 159 (2006) 1484–1487
PCG Aug.,2012
Modeling the battery
Resistance r
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( , ) ( , )r f SOC T g T t
-20 0 20 400
100
200
300
400
Temperature/℃
Resis
tance / m
Ω·A
h·V
-1
Discharge Resistance of Battery Type B
SOC=0.2
SOC=0.3
SOC=0.4
SOC=0.5
SOC=0.6
SOC=0.7
SOC=0.8
SOC=0.9
SOC=1
( , )g T tWhere is the resistance change caused
by cycle time and temperature. The r
changing rate is as the following figure
T. Matsushima / Journal of Power Sources
189 (2009) 847–854
Resistance is function of SOC, T
PCG Aug.,2012
Thermal modeling
Simple modeling as
Where T is the cell temperature, q is Coefficient of
heat transfer, A is the surface area, m is the cell
mass, Cm is the cell heat capacity.
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PCG Aug.,2012
Modeling the battery
Capacity C
The capacity fade of LFP is mainly affected by three
parameters: (1)Temperature, (2)Ah, and (3)Rate; less
depend on DOD.
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0.55Rateloss
31700 370.3= exp( ) ( )
- CQ B Ah
RT
J. Wang et al. / Journal of Power Sources
196 (2011) 3942–3948
PCG Aug.,2012
Modeling the battery
Self discharge
Lithium-ion battery self discharge is based on
time and temperature.
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T. Utsunomiya et al. / Journal of
Power Sources 196 (2011)
8598– 8603
238000.5exp( )
selfdchQ kt
kRT
PCG Aug.,2012
Modeling the battery
Coulomb efficiency
Coulomb efficiency is greatly influenced by the battery
SOC, temperature and current.
Coulomb efficiency is linear with SOC
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1 21+ ( 298)k SOC k T
K.S. Ng et al. / Applied Energy 86
(2009) 1506–1511
Zhe Li, et al, J Tsinghua Univ (Sci &
Tech) ,2010 , Vol. 50 , No. 8
PCG Aug.,2012
Parameters variation
the parameters are Normal Distribution
(Gaussian distribution)
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PCG Aug.,2012
Simulating the battery uniformity changing process and
analyzing the Sensitivity of each parameters.
Each parameters is divided into five cases(uniformity is very
good X1, good X2, medium X3, poor X4, very poor X5)
Simulating parameters
factors Expec
tation ()
Standard Deviation ()
Very good
(X1)
Good
(X2)
Medium
(X3)
Poor
(X4)
Very poor
(X5)
Coulomb efficiency 0.9995
0.00003 0.00006 0.00009 0.00012 0.00015
Relative Self
discharge rate 1 0.05 0.1 0.15 0.20 0.25
Relative capacity
fade 1 0.05 0.1 0.15 0.20 0.25
Relative resistance
changed 1 0.05 0.1 0.15 0.20 0.25
Initial capacity (Ah) 70 0.7 1.4 2.1 2.8 3.5
Initial SOC 0.9 0.0045 0.009 0.0135 0.018 0.0225
Ambient temperature
(℃) 25 0.5 1 1.5 2 2.5
PCG Aug.,2012
Single parameter Simulating results
1. Cause by the variation of Coulomb efficiency The battery pack capacity is strongly affected by the uniformity of
Coulomb efficiency.
The balancing maintenance time interval is strongly affected by the
uniformity of Coulomb efficiency. The poor, the shorter (3months or
shorter).
The BMS designed balancing current is strongly affected by the uniform
of Coulomb efficiency. The poor, the larger (larger than 3.5mA for cell
70Ah).
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Pack capacity and
the max capacity
after balancing
PCG Aug.,2012
Single parameter Simulating results
It is not necessary to adopted complicated balancing circuit, the simple
balancing circuit (resistance dissipation) is enough.
The more sensitive to the SOC,
the lower fade of pack capacity.(as Lead
-acid battery)
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Pack capacity with simple balancing circuit
The effect
of SOC
sensitivity
to Coulomb
Pack capacity different between
balancing with complicated and
simple balancing circuit
PCG Aug.,2012
The uniform of Coulomb efficiency has strong effect on SOC uniform,
but has little effect on other parameters.
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The effect of coulomb efficiency on SOC different
Single parameter Simulating results
PCG Aug.,2012
2. Cause by the variation of self discharge The battery pack capacity is strongly affected by the uniform of self
discharge.
The balancing maintenance time interval is strongly affected by the
uniform of self discharge.
The BMS designed
balancing current is strongly
affected by the uniform of
self discharge. The poor,
the larger (larger than
3.5mA for cell 70Ah).
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Pack capacity and the max capacity after balancing
Single parameter Simulating results
PCG Aug.,2012
It is not necessary to adopted complicated balancing circuit,
the simple balancing circuit (resistance dissipation) is
enough.
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Pack capacity with simple balancing circuit Pack capacity different between
balancing with complicated and
simple balancing circuit
Single parameter Simulating results
PCG Aug.,2012
The uniform of self discharge first affect SOC uniform, then affect the
coulomb efficiency, but has little effect on other parameters.
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The effect of self discharge on SOC
different The effect of self discharge on
coulomb efficiency different
Single parameter Simulating results
PCG Aug.,2012
3. Cause by the variation of capacity fade The battery pack capacity is slightly affected by the uniform of capacity
fade.
It is not necessary to adopted complicated balancing circuit.
The uniform of capacity fade has little effect on other parameters.
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Pack capacity after simple circuit balancing Pack capacity after complicated
circuit balancing
Single parameter Simulating results
PCG Aug.,2012
4. Cause by the variation of resistance change rate The battery pack capacity is strongly affected by the uniform of
resistance change.
The balancing maintenance time interval is strongly affected by the
uniform of resistance change. The poor, the shorter .
The BMS designed balancing current is strongly affected by the uniform
of resistance change. The poor, the larger (larger than 3.5mA for cell
70Ah).
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Pack capacity and
the max capacity
after balancing
Single parameter Simulating results
PCG Aug.,2012
It is not necessary to adopted complicated balancing circuit,
the simple balancing circuit (resistance dissipation) is
enough.
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Pack capacity with simple balancing circuit Pack capacity different between
balancing with complicated and
simple balancing circuit
Single parameter Simulating results
PCG Aug.,2012
The uniform of resistance has positive feed-back on other parameters. It
causes the non-uniform of the temperature, then the non-uniform of self-
discharge, causes the non-uniform of SOC, causes non-uniform of
coulomb efficiency, causes the resistance much different.
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The effect on SOC different The effect on temperature different
The effect on self-discharge
different
Single parameter Simulating results
PCG Aug.,2012
5. Cause by the variation of initial capacity The battery pack capacity is affected by the uniform of initial capacity .
If the uniform of initial capacity is good (generally <5%), it is not
necessary to adopted complicated balancing circuit.
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Pack capacity Pack capacity different between
balancing with complicated and
simple balancing circuit
Single parameter Simulating results
PCG Aug.,2012
The uniform of resistance has effect on other parameters. It causes the
non-uniform of the SOC, the non-uniform of coulomb efficiency and the non-uniform of self-discharge.
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The effect on SOC different
The effect on self-discharge
different
The effect on coulomb efficiency different
Single parameter Simulating results
PCG Aug.,2012
6. Cause by the variation of initial SOC The battery pack capacity is slightly affected by the uniform of initial
SOC .
It is not necessary to adopted complicated balancing circuit.
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Pack capacity different between
balancing with complicated and
simple balancing circuit
Pack capacity and the max
capacity after balancing
Single parameter Simulating results
PCG Aug.,2012
7. Cause by the variation of ambient temperature The battery pack capacity is strongly affected by the uniform of ambient
temperature .
The balancing maintenance time interval is strongly affected by the
uniform of ambient temperature . The poor, the shorter (3months or
shorter).
The BMS designed balancing current is strongly affected by the uniform
of ambient temperature . The poor, the larger (larger than 3.5mA for
cell 70Ah).
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Pack capacity fade
Single parameter Simulating results
PCG Aug.,2012
It is not necessary to adopted complicated balancing circuit,
the simple balancing circuit (resistance dissipation) is
enough.
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Pack capacity different between
balancing with complicated and
simple balancing circuit
Single parameter Simulating results
PCG Aug.,2012
The variation of ambient temperature has effect on other parameters. It
influent the uniform of self-discharge, SOC ,coulomb efficiency, and the
resistance.
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The effect on SOC different The effect on self-discharge
different
Single parameter Simulating results
PCG Aug.,2012
Consider all the 7 parameters 1/3C CCCV charge and 1/3 C CC discharge
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factors Expectation
()
Standard
Deviation ()
Coulomb efficiency 0.9993 0.0003
Self discharge rate 1.5 0.3
capacity fade 1.2 0.2
resistance changed 1 0.1
Initial capacity (Ah) 70 0.7
Initial SOC 0.9 0.01
Ambient temperature (℃) 25 0.25
PCG Aug.,2012
Simulation result is similar to other
experimental curve.
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Pack capacity and the minimum cell capacity
PCG Aug.,2012
The main factors impact the pack performance is the uniform
of coulomb efficiency , self-discharge, internal resistance
and ambient temperature.
After carefully cell selected, pack with simple balancing
circuit is enough.
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Pack capacity recovered under different
Non-uniformity
Pack capacity recovered by balancing
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Conclusions After simulating the following conclusions are achieved: The uniformity of coulomb efficiency, self-discharge rate, resistance
and ambient temperature are the main reason to impact the
performance of the battery pack .
The correlation of the coulomb efficiency to SOC is negative , and the
uniformity will maintain a constant finally.
The cell temperature difference of the battery pack temperature field
design should be less 5C, otherwise the balancing current or time will
increase.
Generally, it is good enough to adopt simple balancing circuit for BMS.
The balancing interval time and the balancing current are depended
on the cell uniformity. (generally, 2-3 moths,>3.5mA).
Cell selected should consider the uniformity of coulomb efficiency,
self-discharge, capacity (<5%) . When packaging the initial SOC is
also be uniform.
PCG Aug.,2012 Page 34
Thank you for your attention!