Temperature is very important for Li-ion batteries. Low temperature will lead to lower electrical performance (capacity, multiplier performance), but can improve the storage life of Li-ion batteries, high temperature can improve electrical performance (capacity, multiplier performance), but will reduce the stability of the electrode/electrolyte interface, causing rapid decay of cycle life. For a battery pack consisting of many cells, the uneven temperature distribution within the pack can lead to large differences in the performance of the individual cells, resulting in uneven decay between the individual cells and eventually leading to the failure of the pack. For example, Quan Xia et al. of Peking University used the LFP cell of A123 for battery pack simulation and simulation tests and found that by changing the For example, Quan Xia et al. from Peking University found that by changing the structure of the battery pack and reducing the maximum temperature difference in the battery pack from 4.62K to 2.5K could improve the reliability of the battery pack from 0.0635 to 0.9328 after a cumulative charge of 600Ah.
The operating conditions of Li-ion batteries have a great influence on the heat production of ion batteries, for example, high rate charging and discharging will accumulate more heat in the battery for a short time, while small rate can almost achieve thermal equilibrium and reduce the temperature rise of the battery. Recently, Xu (first author, corresponding author) and others from Jiangsu University conducted a study to analyze the heat production power and temperature distribution of 55Ah single cell and battery pack, which showed that the heat production power of single cell battery decreases with the increase of ambient temperature and the decrease of battery SoC and charge/discharge multiplier, and the thermal analysis of the battery pack found that the highest temperature region is concentrated in the central region of the battery pack, and found that Airflow is more likely to pass over the top of the battery pack when air cooling is used, thus resulting in poor cooling.
The authors used a 55Ah square lithium-ion battery with five temperature measurement points, two of which are located at the lower part of the battery and three at the side of the lithium-ion battery, as shown in Figure a below. The heat production of the battery can be calculated by the temperature rise and the specific heat capacity of the battery (as shown in the following equation), where Q is the heat production of the battery, Cp is the specific heat capacity of the battery, m is the mass of the battery, DT is the temperature rise of the battery, and if we further divide the following equation by the time t, we are able to obtain the heat production power of the battery.
In order to ensure a consistent ambient temperature, the authors used a thermostat for precise temperature control, and the Digatron BTS-600 device was used for the battery charging and discharging equipment, and Agilent's 34970A device was used to collect the temperature information of the battery.
The temperature change curve of the above-mentioned battery at 27℃ ambient temperature during charging and discharging at 1C rate is shown in the figure below, from which it can be seen that the temperature curves of several different temperature measurement points of the battery are almost the same during charging and discharging, which also indicates that the internal temperature distribution of the lithium-ion battery is relatively uniform at this rate, so the average temperature rise of several temperature measurement points can be used in calculating heat generation. Therefore, the average temperature rise of several temperature measurement points can be used to calculate the heat generation.
1. Influence of ambient temperature
The following figure shows the temperature rise curve of 55Ah battery in the process of 1C charging and discharging at the ambient temperature of 20℃, 27℃ and 40℃ (the average temperature of 5 sampling points), from the figure we can see that the battery charging time is 74min and discharging time is 59min at 20℃, the battery charging time is 76min and discharging time is 60min at 27℃, the battery charging time is 79min and discharging time is 79min at 40℃. The following table summarizes the temperature and temperature rise of the battery during charging and discharging at different ambient temperatures, and Table 2 calculates the heating power data of the battery during charging and discharging at different ambient temperatures based on the temperature rise data in Table 1. For example, the average heating power of the battery at 20℃ is 6.51W, while the heating power of the battery at 27℃ drops to 5.36W, and the average heating power of the battery is reduced to 4.66W when the ambient temperature is further increased to 40℃.
2. The impact of SoC
SoC is also a very important parameter, SoC is the state of charge of the battery, 100% is full charge, 0% is empty charge, different SoC characterizes different Li concentration distribution of positive and negative electrodes, so SoC will also have an impact on the healing power of Li-ion battery. Table 3 and Table 4 below summarize the final temperature and temperature rise of the Li-ion battery and the heat production power of the battery at 70%, 80%, 90%, and 100% SoC states, respectively. From Table 4, we can see that the average heat production power of the battery is 6.25W at 70% SoC, 6.87W at 80% SoC, and 7.19W at 90%, except for 100% SoC, the heat production power of the battery increases with the increase of the battery SoC state.
3. Effect of charge/discharge rate
Table 5 below summarizes the final temperature and temperature rise of the Li-ion battery at different charge/discharge multipliers at an ambient temperature of 20°C. Table 6 calculates the heating power of the Li-ion battery at different multipliers through the temperature rise data. From Table 6, we can see that the heat generation power of the Li-ion battery is greatly influenced by the charge/discharge rate. At 0.5C rate, the average heat generation power is only 2.31W, and when the charge/discharge rate is increased to 0.8C, the average heat generation power has increased to 5W, and when it is increased to 1.5C, it reaches 12.83W, and further increased to 5C, the average heat generation power reaches 58.51W.
Based on the above experimental data, Xu also used the model to simulate the temperature change of lithium-ion battery during charging and discharging, and the results are shown in the following figure, from which it can be seen that the model used by Xu reflects the heat production process of lithium-ion battery in the reaction very well, and the fitting result is only 1.17℃ higher than the experimental data during charging, and the fitting result is 1.1℃ higher than the experimental result during discharging. 1.1℃.
4. Temperature rise and temperature difference of the battery under different working conditions
Based on the above single cell battery heat production model, Xu built a model of the battery pack using SOLIDWORKS software to simulate the heat production of the battery pack under different usage conditions and the temperature distribution within the battery pack.
The following figure shows the battery pack temperature change curve under continuous acceleration condition (0.6C discharge for 10min, 0.8C discharge for 5min, 1C discharge for 2min). From the test results, we can see that the maximum temperature rise of the battery pack at the end of the test is 3.99℃, while the maximum temperature difference within the battery pack is 2.11℃, which is lower than the maximum temperature rise. In addition, the fitting found that although forced air cooling was used for heat dissipation, most of the airflow would flow through the upper part of the battery, and only a small amount of gas would pass through the inner part of the battery pack, which affected the heat dissipation effect of the battery pack.
The following figure shows the temperature change of the battery pack during the continuous deceleration of the electric vehicle, the discharge current of the battery pack will drop from 2C to 0.5C in steps during the deceleration process, and it can be seen from the figure that although the heat production rate of the lithium-ion battery decreases significantly with the continuous reduction of the current, the heat inside the battery cannot be taken away in time due to the poor cooling effect, and the temperature of the battery still shows a continuous increase At the end of deceleration, the maximum temperature rise of the battery pack reaches 5.22℃, and the maximum temperature difference within the battery pack is as large as 3.73℃, which indicates that although the discharge current is decreasing during deceleration, the heat dissipation system of the battery pack still has to work continuously until the temperature of the battery pack returns to normal temperature.
Pulse discharge is also a common situation in the use of electric vehicles. Xu also studied the temperature change of the battery pack in the case of a pulse, and from the test results the maximum temperature rise of the battery pack reached 5.27℃, and the maximum temperature difference within the battery pack was 2.88℃.
Professor Xu's test results show that the charge/discharge rate has the greatest effect on the heat production power of Li-ion battery, the higher the rate the greater the heat production power, followed by the ambient temperature, the higher the ambient temperature the smaller the heat production rate, the least effective is the SoC of the battery, in the range of 70%-90% SoC, the higher the SoC the greater the heat production power. In the battery pack temperature study, it is found that the battery pack will generate significant temperature rise in continuous acceleration, continuous deceleration, and pulse discharge mode, and the highest temperature rise is concentrated in the central position of the battery pack, and most of the airflow generated by forced air cooling flows through the top of the battery pack, resulting in poor heat dissipation.
