The power battery system is a key subsystem of electric vehicles, and its performance directly affects the power, economy, and safety of electric vehicles. Continuous charging and discharging during the use of power batteries can have an impact on the battery temperature, and high or low battery temperatures can affect the performance, lifespan, and safety of the battery system. Therefore, the adaptability of lithium-ion batteries to temperature has become one of the key factors restricting their application in electric vehicles, and battery thermal management has become a key technology to ensure battery performance, service life, and safety.
The optimal operating temperature for power batteries is 15 ℃ C-45 ℃, and the actual operating temperature range is -30 ℃ -60 ℃. During winter operation, electric vehicles often experience a decrease in range. The reason for this situation is that the discharge capacity of the power battery system itself in winter is lower than that at room temperature. If the car air conditioning is turned on at the same time, the range of the electric vehicle is severely insufficient during winter operation. In low temperatures, the activity of the lithium ion itself is relatively low, and the electrolyte is mostly in a solid or semi solidified state. When the lithium ion migrates, the resistance is high, Poor activity leads to poor discharge performance.
This article mainly studies the effects of environmental temperature and discharge rate on the electrical and thermal performance of lithium batteries. It analyzes the voltage platform, temperature, capacity changes of lithium batteries under different environmental temperatures and discharge rates, as well as the changes in battery performance under the discharge conditions of battery modules provided by a certain supplier.
The optimal operating temperature for power batteries is 15 ℃ -45 ℃, and the actual operating temperature range is -30 ℃ -60 ℃. During winter operation, electric vehicles often experience a decrease in range. The reason for this situation is that the discharge capacity of the power battery system itself in winter is lower than that at room temperature. If the car air conditioning is turned on at the same time, the range of the electric vehicle in winter operation is seriously insufficient.
At low temperatures, the activity of lithium ions in the power battery system is relatively low, and the electrolyte is mostly in a solid or semi solidified state. When lithium ions migrate, the resistance is high, and the poor activity leads to poor discharge performance. This article mainly studies the impact of environmental temperature and discharge rate on the electrical and thermal performance of lithium batteries, and analyzes the voltage platform, temperature, and Changes in capacity and changes in battery performance under discharge conditions of battery modules provided by a certain supplier.
1 Low temperature test
Test object: Battery module, 43.8V, 37.0Ah
Testing equipment: DGBELL constant temperature and humidity chamber, charging and discharging equipment
The testing method is to select five temperature points with ambient temperatures of 25 ℃, 10 ℃, 0 ℃, 10 ℃, and -20 ℃, and conduct discharge tests with discharge rates of 1C, 0.3C, and 0.5C, respectively. Based on the actual working condition road spectrum obtained from the vehicle test, it is converted into working condition current and subjected to discharge tests at ambient temperatures of 25 ℃, 0 ℃, and -20 ℃. Record the temperature, capacity, energy, voltage, current, and other data of the above tests.
2 Test results
2.1 Discharge Platform
At different environmental temperatures, the discharge platform of the battery decreases with the decrease of environmental temperature. At -20 ℃, the discharge platform will rapidly decrease during the initial stage of discharge, reaching a “trough” stage. This is because at low temperatures, the electrolyte is in a solidified or semi solidified state, and the conductivity of the electrolyte decreases, resulting in a rapid decrease in the discharge platform.
As the discharge process progresses, the discharge platform slowly rises to the plateau stage. During this period, as the discharge process progresses, heat is generated inside the battery, which melts the electrolyte, increases its conductivity, reduces electron flow resistance, and raises the discharge platform. After reaching a normal discharge platform, the discharge trend is the same as that at room temperature. Although the trend of battery discharge platforms is the same, as the ambient temperature decreases, the voltage platform also decreases.
The lower the ambient temperature, the lower the “trough” of the voltage platform. Comparing the three discharge rates, it can be seen that there is little difference in the discharge plateau above 0 ℃, while the discharge plateau below 0 ℃ decreases more. And compared to different ambient temperatures, the lower the ambient temperature, the shorter the discharge time.
2.2 Battery surface temperature
The temperature difference changes under different discharge rates and environmental temperatures were compared. At the same discharge rate, the lower the ambient temperature, the greater the temperature rise. For example, at an ambient temperature of -20 ℃, the average temperature rise on the surface of a 1C discharge battery is 35 ℃; At an ambient temperature of 25 ℃, the average temperature rise on the surface of a 1C discharge battery is 12 ℃. This also indicates that at low temperatures, more energy is used for battery heating, resulting in a smaller output of energy.
At the same ambient temperature, the lower the discharge rate, the smaller the temperature rise on the surface of the battery. For example, at an ambient temperature of -20 ℃, the average temperature rise on the surface of a 0.5C discharge battery is 26 ℃; At an ambient temperature of 20 ℃, the average temperature rise on the surface of a 0.3C discharge battery is 21 ℃. This indicates that when the ambient temperature is low, a smaller current should be used for discharge. The first effect is to ensure the output efficiency of battery energy; The second is to ensure the service life of the battery.
2.3 Discharge capacity
From the test results, it can be seen that under the same discharge rate, the discharge capacity gradually decreases as the ambient temperature decreases. At an ambient temperature of -20 ℃, the energy released during discharge at 0.3C is the lowest, which is 86% of that at 25 ℃. The smaller the discharge rate, the lower the ambient temperature, and the smaller the amount of electricity discharged. At an ambient temperature of 25 ℃, the discharge capacity at 0.3C is slightly higher, while at other temperature points, the discharge capacity at 1C is higher.
4 Conclusion
In low-temperature environments, the activity of lithium ions in batteries is relatively low, the electrolyte flow force is large, the conductivity is reduced, and the discharge capacity is reduced, which affects the operation of pure electric vehicles. Based on experimental testing, the actual discharge platform and surface temperature changes of the battery under different environmental temperatures, discharge rates, and operating conditions were obtained. The following conclusions were drawn:
(1) The discharge platform will decrease with the decrease of environmental temperature, and there will be a “trough” when the environmental temperature is below 0 ℃. After the “trough”, the discharge platform will slowly rise, and when it rises to a certain extent, the trend of the discharge platform is consistent with that of the discharge platform at room temperature.
(2) As the ambient temperature decreases, the lower the discharge time, the smaller the amount of electricity released. At the same temperature, 1C capacity > 0.5C capacity > 0.3C discharge capacity.
(3) As the ambient temperature decreases, the temperature rise increases. At an ambient temperature of -20 ℃, during the 1C discharge stage, the battery temperature rises by 35 ℃, with more energy used for battery heating. Reduced discharge capacity.
(4) Among the temperature points tested, the discharge capacity is the lowest at an ambient temperature of -20 ℃ and 0.3C. The discharge capacity is 86% at 25 ℃.
(5) During discharge under operating conditions, when the ambient temperature is -20 ℃, the battery is used below 0 ℃. This operating condition has a significant impact on the battery’s lifespan. It is recommended to treat the battery system with insulation, heating, and other measures at low temperatures.