As everyone knows, the best performance of lithium ion battery should be kept in the range of 20~30℃, and it’s comfortable to keep the temperature in the range of 0~45℃。It is easy to cause lithium precipitation or discharge power reduction and cycle life shortening when the temperature of the accumulator is below 0℃.And we also may face the problem of thermal safety if we don’t control the power.
Therefore, lithium ion battery have many problems in low temperature environment application, which limit the scale of application of lithium ion battery.
At present, the research on the performance of vehicle battery mainly focuses on the problem of heat dissipation under high temperature environment. With the increasing demand for battery charging and discharging in low temperature environment,more and more researches have been done on the low temperature performance of battery.
Improving the low temperature performance of lithium ion batteries is one of the key problems to be solved in broadening the application field of lithium ion batteries. In this article, the capacity, internal resistance and discharge power in ultra-low temperature of battery module and battery cell at low temperature are discussed, which provides a certain reference for subsequent battery selection and battery system design and development.
We put the battery cell/battery module in the incubator, connect to charge and discharge test equipment and battery test equipment specification:5V，400A，2kW；The test battery unit is 51ahncm battery, and the battery module is 20ah1p9s LiFePO4 battery module. The schematic diagram of the test bench is shown as following picture:
After the battery cell is connected with the charging and discharging test equipment, the battery is charged to the cut-off voltage at 1C rate under 25 ℃and converted to constant voltage charging. After charging, it is set to the target temperature. Then discharge at 1C rate to the cut-off voltage (the cut-off voltage for discharge at - 20 ℃ and below is 2.4V, and that at temperature above 20 ℃ is 2.8V). The target temperatures are - 30 ℃, 20 ℃, 55 ℃ respectively. The discharge capacity test results are shown as the following sheet:
It can be seen from the chart that the discharge capacity at 1C rate decreases with the decrease of temperature. From 54.306ah at 55 ℃ to 41.327ah at - 30 ℃, the attenuation rate is 23.9%. In order to study the reason of discharge capacity attenuation at different temperatures, 1c discharge capacity voltage curves at different temperatures are made, as shown in pic. 2 and pic. 3:
The results show that with the decrease of temperature, the battery voltage decreases continuously. When the temperature drops to - 20 ℃ or below, the discharge voltage of 1C rate will decrease first and then rise. This is mainly because the internal resistance of the battery is very large at low temperature (Chart 2), and the electrode polarization is serious. When discharging, the battery voltage is low. As the discharge continues, the heat generated in the discharge process of the battery makes the temperature of the battery rise rapidly, and the internal resistance decreases. Therefore, during the discharge process, when the discharge voltage reaches a trough, it will rebound and then decrease again. The temperature change is shown in pic. 3:
The temperature data of single battery is the temperature data at the middle position of the monomer. From the diagram, the temperature rise of single battery at - 30 ℃, - 20 ℃, 20 ℃ and 55 ℃ is 1.6 ℃, 1.2 ℃, 1.1 ℃ and 0.3 ℃ respectively when the discharge capacity reaches 10Ah. It can be seen from the data that the heat generation rate at low temperature is higher than that at normal temperature and high temperature. However, due to the large heat exchange between low temperature and environment, the temperature rise at - 20 ℃ is only 0.1 ℃ higher than that at 20 ℃. The larger heat generation rate at low temperature makes the temperature rise rapidly in the process of discharge, which leads to the decrease of internal resistance of battery monomer, which is consistent with the change trend of 1C discharge voltage at different temperatures.
The internal resistance of battery can be divided into polarization resistance and ohmic internal resistance: polarization internal resistance is mainly caused by concentration polarization and electrochemical polarization. Ohmic internal resistance refers to the ohmic internal resistance of storage battery formed by electrode material, electrolyte, diaphragm and contact resistance between various structural parts in battery monomer. In order to explore the internal resistance of battery cell in DC discharge at different temperatures and verify the voltage and temperature changes of battery cell at different temperatures, 50% SOC DC discharge internal resistance of battery cell at different temperatures was tested. In this test, mixed pulse power performance test method (HPPC) was used to measure the DC discharge internal resistance at different temperatures. The
test results are shown in the following chart:
4.Study on high rate discharge at ultra-low temperature
At present, the electrolyte of lithium battery keeps liquid in a wide temperature range, and the general temperature range is - 40 ~ 70 ℃. Therefore, this paper studies the high rate discharge performance of battery at - 40 ℃. Due to the sudden drop of battery voltage when discharging at a high rate at ultra-low temperature (- 40 ℃), the high power output is limited.
In order to study the high rate discharge performance of battery at ultra-low temperature, 20Ah · h 1p9s LFP battery module was used in this test. Therefore, in this test, the PTC with 270W mounted power at the bottom of the module is used, and the external regulated power supply is used to heat the module, and then the battery module is connected to the charging and discharging test equipment of the module. The discharge process is as follows: discharge the module at 270W for 30s, and switch on PTC at the same time; discharge at 3516w, if sudden drop occurs during the test, return to 270W discharge for 30s and switch on PTC, until the module discharges at 3516w without sudden drop, the module is continuously discharged at 3516w to the cut-off voltage. The test bench diagram is shown in Pic. 5.
Due to the heating of the module itself and the PTC heating, the temperature of the module gradually increases, and the temperature change is shown in Fig. 7. When the temperature in the middle of the top cover reaches - 31.51 ℃, there is no voltage jump when the module is discharged at 3516w, and the battery module voltage is 17.48v. In the process of 3516w discharge, the change curve of ꎬ voltage first increases and then decreases, which is mainly due to the large internal resistance at low temperature. With the discharge process going on, the battery module generates more heat, which reduces the internal resistance, so the battery module voltage will rise. When the voltage reaches the highest point (25.11v), it will continue to discharge at 3516w. At this time, the voltage change will be affected by the internal resistance of battery module and SOC, and finally the battery voltage will gradually decrease.
The capacity internal resistance and power performance of battery unit and battery module at low temperature are discussed, and the capacity internal resistance of single battery and battery module work under low temperature are analyzed. The following conclusions are obtained
1) At low temperature, the discharge capacity of battery cell is obviously reduced. In the process, the discharge voltage curve shows a trend of first decreasing, then rising and finally decreasing. This is mainly caused by the decrease of internal resistance due to the increase of temperature in the process of discharge.
2) The internal resistance of DC discharge at low temperature is more obvious than that at normal temperature. Compared with 50% SOC at 20 ℃. The DC discharge resistance of 50% SOC at - 20 ℃ increased by 275%, and that at - 30 ℃ increased by 381.25%.
3) At ultra-low temperature, the voltage of battery module will drop suddenly when discharging at high rate. Using PTC to increase the temperature of battery module can effectively solve the problem of voltage sudden drop of battery module in high rate discharge at ultra-low temperature.
To sum up, in the low temperature environment, the thermal management system of the battery pack is used for storage. Heating the battery can effectively reduce the impact of low temperature on the performance of battery pack, improve the performance of battery pack, and broaden the application scenarios of battery pack.
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