In recent years, electric vehicles have achieved unprecedented development, and countries are actively carrying out related research work. In civilian vehicles, major automobile manufacturers continue to introduce technologically advanced hybrid electric vehicles and pure electric vehicles. On Military vehicle, many countries have also carried out a lot of research work. Compared with mechanical transmission vehicles, military electric vehicles have the following advantages: convenient power system layout; The vehicle is easy to start and accelerate; Can achieve silent driving with good concealment; Can provide high-power power supply for vehicle mounted weapon systems; Can absorb regenerative braking energy, etc.

The United States, Germany, Britain and other countries have successively launched military electric vehicles such as electric armored vehicles and Infantry fighting vehicle. In the rapid development of electric technology, some issues have also become prominent, among which the loud noise of power bubbles is particularly significant. The performance and service life of power batteries directly affect the performance and cost of electric vehicles. At present, the main power batteries used in electric vehicles are lead-acid batteries, Nickel–cadmium battery, Nickel–metal hydride battery, lithium ion batteries and supercapacitors.

Lithium ion power batteries have gradually replaced lead-acid batteries, Nickel–cadmium battery and Nickel–metal hydride battery as the main power batteries for electric vehicles because of their high specific power, large energy density, long life, low self discharge rate, long storage time and no pollution. Military vehicle often need to work in cold regions, and they are required to work normally at -40 ℃. However, at low temperatures, the battery charging and discharging performance has significantly declined. This article will conduct low-temperature tests on a 35A • h lithium-ion battery.

1 Lithium ion battery test

Battery charging and discharging equipment, with a maximum voltage of 5V and a testing accuracy of 0.1 mV; During the testing process, the tested battery is placed in a temperature chamber to maintain the required ambient temperature for testing. The test battery is a soft pack (30cmx16.8cmx1.5cm) 35 Ah energy and power balanced lithium manganese battery, with an aluminum plastic film as the outer shell.

2 The effect of low temperature on battery voltage

2.1 Effect of Low Temperature on Battery Discharge Voltage

To investigate the effect of low temperature on the discharge performance of batteries, the battery is first charged at a constant current to constant voltage rate of 1/3 C at room temperature, fully charged, and then left to stand in a temperature chamber for 5 hours. Then, the battery is discharged at a constant current rate with a cut-off voltage of 3V. Within the temperature range of 0-40 ℃, discharge at constant currents of 10, 35, 70, and 140A, respectively.

The experimental results show that at the same discharge rate, the discharge voltage of the battery decreases with the decrease of temperature. Taking 10A constant current discharge as an example, compared to 20 ℃ at 40 ℃, the average discharge voltage of the battery decreases by 1V, which is 27% of the nominal voltage. As the temperature decreases, the maximum current that the battery can discharge gradually decreases. At -10 ℃, the battery can discharge at a constant current of 140A , at 0 ℃, it can discharge at a constant current of 70A, at -30 ℃, it can only discharge at a current of 35A, and at -40 ℃, it can only discharge at a small current of 10 A.

When discharging at low temperature and high current, the discharge curve shows a nonlinear state with obvious valley and peak shapes, and the discharge voltage fluctuates greatly. Taking 70A constant current discharge as an example, at 20 ℃ and 0 ℃, the discharge curve is relatively normal without any valley or peak.

When the environmental temperature drops to -10 ℃, the discharge curve shows obvious valley and peak shapes. When the environmental temperature drops to -20 ℃, the discharge curve shows obvious valley and peak shapes, The voltage at both ends of the battery decreased from 4.15 before discharge to 3.07V, and the voltage drop reached 1.08V.

Subsequently, the voltage began to rise, reaching a maximum of 3.35V, and then began to decrease. This indicates that during high current discharge at low temperatures, the active substances in the battery cannot be fully utilized due to the low temperature of the battery, resulting in severe electrode polarization and high internal resistance of the battery. Therefore, in the early stage of discharge, the discharge voltage of the battery rapidly decreases.

As the discharge progresses, due to the high internal resistance of the battery, a large amount of heat is generated inside the battery, causing the temperature of the battery to rapidly rise, activating the active material part of the battery. Therefore, the discharge voltage of the battery begins to rise. As the battery temperature rises, the internal resistance of the battery begins to decrease, and the heat generated decreases. As the ambient temperature remains at -20 ℃ C, the temperature of the battery decreases, and the discharge voltage of the battery also decreases.

2.2 The effect of low temperature on battery charging voltage

To study the effect of low temperature on battery charging performance, the battery was placed at different environmental temperatures and charged at constant current and voltage at the same rate. The battery first undergoes constant current discharge at a rate of 1/3 C at room temperature, with a cut-off voltage of 3V. After discharge, it is allowed to stand in a temperature chamber for 5 hours. Then, it is charged at a certain rate at different temperatures, with a cut-off current of 1A for 10A charging and 3A for 35 and 70A charging.

From the charging curves at different temperatures, it can be seen that compared to the discharge characteristics of low-temperature batteries, the charging performance of the battery decays more significantly. Below 0 ℃, the battery is no longer able to charge normally. With the same charging current, as the temperature decreases, the charging voltage continuously increases during the constant current charging stage, especially during high current charging. Below 0 ℃, there is no constant current charging process at all. At the moment of charging current loading, the battery terminal voltage rapidly increases to the cut-off voltage of 4.2V, directly entering the constant voltage charging stage.

3 Low Temperature on the Charging and Discharging Capacitance of Batteries

3.1 Impact of Low Temperature on Battery Discharge Capacity

In order to compare the attenuation degree of battery discharge capacity at different temperatures, the available capacity ratio is used here. The available capacity ratio refers to the ratio of battery discharge capacity to Nameplate capacity.

At the same discharge rate, as the ambient temperature decreases, the available capacity ratio rapidly decreases; When the ambient temperature drops to -20 ℃, the battery cannot discharge at 4C; When the ambient temperature drops to 30 ℃, the available capacity ratio for constant current discharge at 10 A decreases to 60.33%, and the battery cannot discharge at 2C or higher; When the temperature drops to -40 ℃, the available capacity ratio for constant current discharge at 10 A is only 22.31%, and cannot be discharged at 1 C or higher.

3.2 Impact of Low Temperature on Battery Charging Capacity

At the same charging rate, as the ambient temperature decreases, the constant current charging capacity of the battery rapidly decays, and the attenuation is more severe compared to the available discharge capacity; When the temperature drops to 0 ℃, charge at the rate of 1C, and the constant current charging capacity is only 52.05% of the Nameplate capacity; The constant current charging capacity is only 42.55% of the Nameplate capacity when charged at 2C; When the temperature drops to -10 ℃, only 60.23% of Nameplate capacity can be charged at 10A constant current, and it is unable to charge at 1C and 2C times; When the temperature is below -30 ℃, the battery cannot perform constant current charging.

4 Conclusion

(1) In a low-temperature environment, under the same discharge rate, the discharge voltage and discharge capacity of lithium-ion power batteries significantly decrease. Compared with discharge, the charging performance of the battery decays more significantly, and the constant current charging voltage of the battery significantly increases, while the charging capacity significantly decreases.

(2) As the temperature decreases, the internal resistance of the battery during charging and discharging increases, especially when the temperature is below -20 ℃.

(3) By analyzing the range of discharge voltages of four batteries, it was found that at room temperature, the consistency of the batteries at the end of discharge deteriorates, while as the temperature decreases, the consistency of the batteries throughout the entire discharge process deteriorates.

(4) In practical use, in low-temperature environments, a heating system must be used to heat the power battery pack to improve its performance.