| A soft pack 26 Ah lithium-ion battery was studied in this paper. Based on the research of intrinsic thermal characteristics for lithium ion battery, the thermal runaway behavior of battery was studied. Firstly, the thermal characteristics of the lithium-ion battery was studied under adiabatic environment during charging and discharging. Thus heat production and heat production rate was quantitatively calculated and further compared impacts on different temperature and charge-discharge rates of the battery thermal characteristics. In addition to conventional charge and discharge cycles, the research on thermal characteristics of the battery was conducted under overcharge conditions. Secondly, the critical state of thermal runaway was analyzed under different conditions, mainly in different charge states and different life cycles. The critical point temperature and voltage sags point of the battery was confirmed when thermal runaway occurred. Finally, based on the finite element method a three-dimensional thermal model during discharge at different rates was established using ANSYS modeling software. The model simulated and predicted the battery temperature distribution under different charge and discharge conditions. The main results are draw as follows:1. In the Extended Volume Accelerating Rate Calorimeter, specific heat capacity of 26 Ah soft pack type battery was measured. Its value is 1242 J kg-1 K-1. Therefore, the heat production during charging and discharging at different rates can be measured by electrochemical measurement for lithium-ion battery. Meanwhile, temperature distribution of the tested cell was measured in constant temperature tank, the results showed that the maximum surface temperature of the battery cell is located in the geometric center. On the other hand, when the discharge rate is small, the surface temperature gradient of the battery was not significant. With the increase in the discharge current, battery temperature gradient increased.2. In the EV ARC, research on overcharge process in different currents was to study thermal behavior of thermal runaway. The results showed that in the second stage during overcharging, temperature rate of battery began to increase, and positive and negative materials inside the battery reacted and continued to produce gas. When the battery charge reached to 5V, the temperature rate was into nearly 10 ℃/ min. Then it began to rapidly heat and the battery thermal runaway would occur in a short time.3. The combination of HPPC test and AC impedance test was used to study the different cycle power battery electrochemical behavior and thermal runaway behavior. The results showed that, discharge capacity decreased to 83% after 1000 cycles, the DC resistance increased with increasing cycles number. From thermal runaway curve, with the increase of the cycle number, the battery production for the heating temperature showed an overall decline, indicating that the continuous cycle of aging batteries make SEI film gradually deteriorated. Meanwhile, the apparent activation energy of lithium-ion battery was calculated. The higher state of charge, the activation energy is lower apparently. It reflected the state of charge of the battery had a significant impact on thermal runaway reaction.4. Finite element method was used to simulate the battery temperature distribution. Meanwhile discharge process was tested at different ambient temperatures to validate the model. The temperature characteristics of the battery further was analyzed and the following conclusions are draw. From the results obtained by the models and infrared thermal imaging results, maximum surface temperature of the battery cells concentrated in a central location, closer to the edge temperature decreases. Located in a central location at the cell surface, the temperature changes nonlinearly with time. With the discharge current increases, the greater the temperature rise. |
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