Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 104 (2016) 62 – 67
CUE2016-Applied Energy Symposium and Forum 2016: Low carbon cities & urban energy systems
Researches on modeling and experiment of Li-ion battery PTC self-heating in electric vehicles Xin Jina, Jun-qiu Lia,*, Chen-ning Zhanga, Pu-en Wua aCollaborative
Innovation Center of Electric Vehicles in Beijing, Beijing Institute of Technology, Beijing, 100081,China.
Abstract
Positive Temperature Coefficient(PTC) self-heating method of battery and experiment researches are conducted in this paper; the model of discharge internal heating and the model of PTC heating for battery are established; analyses of the internal and external heat characteristics of the battery and self-heating temperature field distribution of the battery are carried out. The accuracy of those models are verified by implementing the self-heating experiments. The discharge tests are carried out under the extreme low temperature and the results show that charge and discharge rate, capacity recovery are much better than that of external-supplied power heating. © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of CUE Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, CUE2016: Low carbon cities and urban energy systems.
Keywords: Li-ion battery; PTC self-heating; thermal modeling; thermal simulation; self-heating experiment
1.
Introduction
The poor performance of the power battery in low temperature may lead to many problems of electric vehicles, such as short driving range, worse power performance and difficulty of charging, etc. A relatively feasible way is to heat the power battery. Two methods for battery heating were usually used: PTC heating and metal film heating [1-3], with all the heat being produced by resistance. Both methods will get a large area of cell heated by conduction, and short heating time and uniform temperature. However, the methods may increase the size and weight of the whole system. Another method in [4-5] proposed that the battery can be heated by convection with heated air, but there still exist some problems, such as long heating time, low heating exchange efficiency
* Corresponding author. Tel.:086-010-68940589; fax: 086-010-68914842. E-mail address:
[email protected].
1876-6102 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, CUE2016: Low carbon cities and urban energy systems. doi:10.1016/j.egypro.2016.12.012
Xin Jin et al. / Energy Procedia 104 (2016) 62 – 67
and poor consistency of battery temperature in heating process. And also the method of battery internal heating has been studied and proposed by some scholars [6-9]. With the method, battery heating is achieved by implementing an alternating current to the battery to make itself heat, but the disadvantages are that the circuits are too complex, and the influence on the battery life need to be further studied. The power of most above heating methods are mostly derived from external supplied sources, which may cause a lot of inconveniences to electric vehicles when being used. As for pure electric vehicles, the restored energy of the power battery after being heated in low temperature is much greater than the consumed energy in the self-heating process. Therefore, the self-heating method is proved to be feasible. 2.
Experimental method of PTC self-heating for Li-ion battery
Low temperature has great influence on the performance of Li-ion battery. A Li-ion cell such as the square aluminum laminated 35Ah LiMn2O4 cell, when the battery cell charged and discharged under different temperatures with different rates, its discharge, charge capacity and rate are faded greatly. Therefore, a method of PTC self-heating is proposed in this paper, as shown in Fig.1. In the design, the battery pack will be heated with its self-supplied power. When the PTC resistance band is embedded in the slotted aluminum plates which are put between the sides of cells. The heat produced by PTC will be quickly transferred to the battery. Extra slots on the aluminum plates will form air ducts to facilitate heat dissipation at high temperature. Then, the integration of low temperature heating and high temperature heat dissipating is achieved. Cell P/+ PTC heat vent
N/-
37& 0DWHULDO
:LUH ,QVXODWLQJ OD\HU
Al-plates with PTC
Fig. 1. Schematic diagram of PTC self-heating
Fig.2 Product of battery pack with PTC self-heating
The product of battery pack with PTC self-heating can be seen in Fig.2. There are 24 cells in the two columns around, and 13 aluminum plates between the 24 cells of each column, which are arranged between the sides of every two cells, to ensure that each battery cell has at least one side contact with the aluminum plates. 3.
Modeling for PTC Self-heating
3.1 Method for PTC self-heating model The heat from battery PTC self-heating is derived from two sources: internal heat and external heat. (1) Internal heat refers to the heat generated by the internal resistance of the battery when the battery is supplying power to the PTC material. (2) While external heat refers to the heat produced by the PTC material. The model for thermal analysis includes: (a) battery thermal conduction equation, (b) the internal and external heat generation rate, (c) battery thermal physical property parameters. The specific parameters are shown in Tab.1[10]. For the Li-ion battery discussed in this paper, the battery density: U 2182.7kg / m3 , the average specific heat capacity: Cp 1100 J / ( K kg) , the average heat conductivity coefficient: Ox 0.895W / (m K ) .
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Tab.1 Modeling method for thermal analysis of the square Li-ion battery (a)
The battery heat conduction differential equation Conduction between battery side and aluminum plate. Belong to the second boundary conditions. Heat convection among other sides. Belong to the second boundary conditions.
(b)
Heat generation inside the battery based on Bernardi heat generation model. Adopt the average heat generation rate. PTC External heat generation of battery.
(c)
Ucp
wt wW
qw
O
O
wt wn w
f ( x, y , z , W )
h(tw t f )
n
qave |
Cp
U
Battery density. Battery thickness direction(x direction) and other directions.
wt wn w
I ( E0 E) IT (dE0 / dT )
qB
qwai
Specific heat capacity.
w wt w wt w wt (Ox ) (Oy ) (Oz ) q wx wx wy wy wz wz
¦ I (i) i 1
n
n
( E0
n
n
¦ E (i) ¦ I (i) ¦ T (i) § dE i 1
n
)
i 1
n
i 1
n
0 · ¨ ¸ © dT ¹
qdian U u I
1 n ¦ i 1 mi Ci M M /V
¦
n i 1
( UV )i Ci / ¦ i 1 ( UV )i n
,O Lx z ( Lxp / O p ) ( Lxn / On ) ( Lxs / Os ) ( Lxw / Ow )
Ox
Oy =0
4.0
580 570 560
3.8
550
3.7
540
3.6
530
3.5
520 510
3.4
500
3.3 0
2
4
6
490 8 10 12 14 16 18 20
Numbers of measurement
CurrentͣAͤ
3.9
680
Heating current (A) Heating power (W)
660
3.8 Power(W)
CurrentͣAͤ
4.0
640
3.6
620
3.4
600
3.2
580
3.0
560
2.8
540
0
5
10
15
20
25
Numbers of measurement
PowerͣWͤ
Heating current (A) Heating power (W)
4.1
Heat generation rate/(kw/m^3)
3.2 Solution to PTC self-heating model Thermal characteristics simulation of PTC self-heating process is implemented with FLUENT software, the detailed steps are as follows: three-dimensional geometry model is established, and the model is imported into FLUENT, UDF procedure is written to set the quantity of heat production for the external of PTC and the internal of battery[11]. As for the heat generation rate, (1) The total voltage and current of battery in the real-time acquisition experiment is obtained with the calculation of the heat generation rate of battery and the external heat generation rate of PTC. (2) The written UDF procedure is imported into the model to calculate; (3) Convective heat transfer of the air around the battery and aluminum plates is calculated. All above calculation based on the equations in Tab.1. 3.3 Experiment results Two self-heating experiments will be conducted: The first experiment will have temperature of the battery pack increased from below -37ć to -20ć, and the second experiment will get the temperature raised to 0ć. In the process of each self-heating experiment, collect the heating voltage and current at intervals, and multiply the value of voltage by the value of current, then the heating power can be obtained as shown in Fig.3 and Fig.4. 80 70 60 50 40
External heat generation rate Internal heat generation rate
30 20 10 0 -40 -35 -30 -25 -20 -15 -10
-5
0
Temperature / Ȼ
Fig.3 Heating Current and Heating Power Fig.4 Heating Current and Heating Power Fig.5. Heat Rate Changes for Internal and in the 1st Self-heating Process
in the 2nd Self-heating Process
External of Battery in Self-heating Process
From Fig.3 and Fig.4, we can obtain the variation curve of internal and external heat generation rate which changes along with the variation of battery temperature in the whole self-heating process. From Fig.5, It can be found that when heating process being carried out, the internal heat-generation rate
Xin Jin et al. / Energy Procedia 104 (2016) 62 – 67
gradually decreases, because the battery internal resistance declines with temperature rise, leading to the decrease of internal heat-generation rate, and the change of the external heat rate caused by the combined effects between voltage rise and PTC resistance increase in self-heating process. 4.
Model validation and analysis of heating effect
In the first process of self-heating experiment, the curve of battery total voltage and average temperature change with time is as shown in Fig.6. The value of initial total voltage is 190V, and the value of initial temperature is -36.4ć. After heating for 34.2min, the value of average temperature rises to -20.7ć. Analysis of thermal simulation with FLUENT software is conducted according to two cases: (1) Heating is implemented both externally and internally, namely, heat is produced by PTC and the battery; (2) Heat is only generated by PTC. The simulation results can be referred to Fig.7. When heating is occurring both inside and outside, the curve of the simulation are consistent with that of the experimental temperature rise, with 0.201ć average temperature difference and 0.938 ć maximum temperature difference, which proves the correctness of the thermal model. At the same time, comparison between heat generation of both externally and internally and heat generation of the external PTC. The simulation results show that the temperature in the former case is 2.44ćhigher than that in the latter case, which illustrate the importance of the internal heat generation. Fig.8 indicates the temperature distribution inside the battery pack, and the average cell temperature on the far left rises to -18.8ć, the average cell temperature on the far right to -23.47ć, with a difference of 4.67ć, and the surface temperature is controlled within the range of 5ć. Total voltage Average temperature
Self-heating only by external PTC Self-heating by internal and external
-18
-20
190 -21
-27
160
-30
150
-33
140
-36
-24 -26 -28 -30 -32 -34 -36 -38
500
0
1000 1500 2000 Time / s
Fig.6 Voltage and Temperature in 1st Self-heating Process Total voltage Average temperature
-4
184
-8
180
-12 -16
172
-20 0
800
1600
2400
1500
2000
Fig.8 Temperature distribution of the 12 cells after the 1st self-heating process
The actual average temperature Self-heating by internal and external Self-heating only by external PTC
0
188
176
1000
Fig.7 Comparison of Simulation and Experiment in the 1st Self-heating Process
Temperature/ Ȼ
192
500
Time/s
-3 Temperature / Ȼ
0
Voltage / V
Temperature / Ȼ
-24
170
The actual average temperature
-22
Temperature/ Ȼ
Voltage / V
180
-6 -9 -12 -15 -18
3200
Time / s
-21
0
800
1600
2400
3200
Time/s
Fig.9 Voltage and Temperature in 2nd Self-heating Process
Fig.10 Comparison of Simulation and Experiment in the 2nd Self-heating Process
Fig.11 Temperature distribution of the 12 cells after the 2nd self-heating process
In the second process of self-heating experiment, the variation curve of battery total voltage and average temperature change with time is shown in Fig.9. The value of initial temperature is -19.3ć. The total voltage is reduced to 172V during self-heating and the partial voltage of the internal resistance is 18V. After the power of battery being supplied to PTC, the value of total voltage drops to 172V, and the partial voltage of the internal resistance is 18V. After heating for 48min, the value of average temperature rises to -2.4ć, and the value of total voltage rises to 186V, and at the end of heating, the partial voltage of the internal resistance drops to 4V. The curves of comparison between the result of self-heating simulation and experiment data are shown in Fig.10, and the two curves are highly fitting with each other,
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Xin Jin et al. / Energy Procedia 104 (2016) 62 – 67
the average temperature difference is 0.164ć, and the maximum temperature difference is 0.783ć. Similarly, comparison between heat generation of both externally and internally and heat generation of the external PTC. After the simulation, the temperature in the latter case is 1.93ćlower than that in the former case. Fig.11 indicates the temperature distribution after the completion of the 2nd self-heating, and the average cell temperature on the far left rises to 0.2482ć, the average cell temperature on the far right is -3.825ć, with the temperature being still controlled within a reasonable range of 5ć. 5.
Comparison of battery performance with self-heating and external power heating
In order to compare with the self-heating experiments, the external power heating experiments are conducted in the same way, the external power source used is 220V AC. Comparative experiments including tests of HPPC and constant-current discharge. (1) Comparison of HPPC discharge capability: firstly, two battery packs are heated in the -40ć environment with self-heating method and external power heating method respectively. Then, two packs are discharged based on the HPPC test respectively. The experiment results of self-heating and the external power heating are shown in Fig.12 and Fig.13 respectively. From Fig.12 and Fig.13, we can see that the pack heated with self-heating can discharge with current 3C (105A) for 10s, but the discharge duration time of the pack heated with external power heating is less than 10s. The above results illustrate the self-heating method is more effective. (2) Comparison of constant discharge capability: firstly, two battery packs are heated in the -40ć environment with self-heating method and external power heating method respectively. Then, two packs are discharged with constant current 1C(35A) respectively, and the result is shown in Fig.14, which indicates that the discharge capacity and voltage platform of the pack heated with self-heating method are all obviously higher than the pack heated with external power heating method. Thus, the result further proves the superiority of self-heating method.
-80
150
-100
140 0
-120 500 1000 1500 2000 2500 3000 3500
180
-30
170
-60
160
-90
150
-120
140
-150 0
1000
2000
3000
4000
5000
Discharge voltage / V
160
Voltage/ V
-60
0
190
Current/ A
voltage / V
-40 170
Self-heating ਘԆࢽ External power heating
4.0 30
200
-20
180
Total voltage Total current
210
0
Current / A
Voltage (V) Current (A)
190
3.8 3.6 3.4 3.2 0
4
8
12
16
20
Dischaege capacity / Ah
Time/0.1s
Time/0.1s
Fig. 12. The curve of pulse discharge after the two times of self-heating
6.
Fig.13 The curve of pulse discharge after the two times of external power heating
Fig.14 Comparison of 1C Constant Current Discharge after Self Heating and External Power Heating
Conclusion
(1) PTC self-heating method is proposed and model of PTC self-heating is established. Numerical simulation and model experiment are carried out to verify the feasibility and effectiveness of the selfheating method. (2) Collecting the data of battery voltage and current in the process of self-heating experiment are collected, and calculating the internal heat generation rate of battery and external heat generation rate of PTC are calculated. The experiment data are imported into FLUENT software for simulation. Subsequently, comparison between simulation results and experimental results are performed to illustrate that heating inside the battery is effective and the temperature difference can be controlled within 5ć.
Xin Jin et al. / Energy Procedia 104 (2016) 62 – 67
(3) Further contrastive analysis on the effect of external heating and self-heating has been implemented. The results show that discharge capability of HPPC and constant current heated by selfheating method is obviously higher than that heated by external power heating method and it further illustrate the superiority of self-heating method. Copyright Authors keep full copyright over papers published in Energy Procedia. Acknowledgement The authors would like to thank the Collaborative Innovation Center of Electric Vehicles in Beijing Institute of Technology for the support of this research project. Reference [1] Liu Cunshan, Zhang Hongwei. Research on heating method at low temperature of electric vehicle battery[J]. Chinese Journal of Power Sources. 2015.8:39(8). [2] Zhang Chengning, Lei zhiguo. Dong Yugang. Method for Heating Low-Temperature Lithium Battery in Electric Vehicle [J]. Transactions of Beijing Institute of Technologyˈ2012.9:32˄9˅. [3] Liu Bin. Study of the Control Strategy for Power Battery Thermal Management on Electric Vehicles [D]. Beijing Institute of Technologyˈ2015.1:25-32. [4] Lu Chun. Research on the Control Strategy for Power Battery Thermal Management on Electric Vehicles[D]. Beijing Institute of Technology,2011.12:58-65. [5] Wang Facheng, Zhang Junzhi, Wang Lifang. Design of electric air-heated box for batteries in electric vehicles[J]. Chinese Journal of Power Sources,2013,7:37(7). [6] T.A. Stuart , A. Hande. HEV battery heating using AC currents[J].Journal of Power Sources 129(2004) 368-378. [7] Hande A,Stuart T. Effects of high frequency AC currents on cold temperature battery performance[C]. Proceedings of the 2nd IEEE India International Congress on Power Electronics(IICPE 2004), Mumbai, India. 2004. [8] Zhao, Xiao Wei,Zhang, Guo Yu,Yang, Lin,Qiang, Jia Xi,Chen, Zi Qiang. A new charging mode of Li-ion batteries with LiFePO4/C composites under low temperature[J]. Journal of thermal analysis and calorimetry, 2011. 104(2): 561-567. [9] Jianbo Zhang , Hao Ge , Zhe Li , Zhanming Ding. Internal heating of lithium-ion batteries using alternating current based on the heat generation model in frequency domain[J]. Journal of Power Sources, 273(2015)1030-1037. [10] Bernardi D, Pawiikowski E, Newman J. A General energy Balance for Battery System[J]. Journal of Electrochemical Society, 1985(5): 132. [11] Fan Guangchong. Thermal management modeling and experimental research of lithium-ion battery at low temperature[D]. Beijing Institute of Technology,2013.
Biography Jun-qiu Li, Doctor of Engineering, associate professor, engaged in the researches of energy management and control strategy for electric vehicles. Used to take charge of a national defense fund project, win a second prize of National Technology Invention Award and a second prize of Technology Invention Award of the Commission of Science.
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