Temperature characteristics of lithium iron phosphate batteries

The temperature of the battery is affected by many factors, such as the ambient temperature, the thermodynamic parameters of the battery itself, and the assembly and thermal management methods of the battery pack.
The battery’s capacity characteristics, internal resistance value and open circuit voltage curve are important indicators that reflect the basic performance of the battery, and are also important parameters involved in the design of the battery management system.
The heat production of the battery is related to the current and the internal resistance of the battery. For example, Qg=I²R, Qg is the heat production rate of the battery, 1 is the current flowing through the battery, and R is the total internal resistance of the battery.

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SOC-OCV curve of a certain lithium iron phosphate battery

Discharge fully charged batteries in different ambient temperatures and discuss the relationship between the discharged capacity and ambient temperature. The charging method is to charge the battery at a constant current of 1/3°C until the voltage reaches 3.65 V: then change to constant voltage charging until the current drops to 1A, then stop charging. The discharge method is to let it stand for 1 hour at ambient temperature, then discharge at a constant current of 13°C until the voltage drops to 2V, and calculate the discharged capacity. Six lithium iron phosphate batteries of the same model were placed at -40°C, -20°C, 0°C, 30°C, 50°C, and 60°C for the discharge process. The capacity released by the battery is shown in the figure below.

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LiFePO4 lithium-ion battery capacity changes with ambient temperature

It can be seen that at low temperatures, the battery capacity decays very quickly, while at around normal temperature, the capacity increases as the temperature increases, and the rate is slower than that at low temperatures. At -40℃, the battery capacity is only 1/3 of the nominal value, while at 0℃ to 60℃, the battery capacity increases from 80% to 110% of the nominal capacity.

(1) Ambient temperature has a great influence on the capacity of lithium iron phosphate batteries. The capacity decays rapidly at low temperatures and increases rapidly at high temperatures, but the rate of change is smaller than at low temperatures.

(2) The ambient temperature has a significant impact on the ohmic internal resistance and total internal resistance of the battery. Generally, the lower the temperature, the greater the internal resistance. The ohmic internal resistance is more sensitive to temperature than the polarized internal resistance. The change of ohmic internal resistance is more sensitive to low temperature. More sensitive. In addition, the lower the temperature, the earlier the polarization internal resistance rises in the small SOC value range.

(3) The difference in the SOC-OCV curve of the battery at different temperatures is small. The lower the temperature, the lower the SOC-OCV curve, and the deviation speed of the curve is greater at low temperatures.

When estimating the capacity of a battery, the impact of ambient temperature must be considered; the internal resistance of the battery is very large under low temperature and small SOC conditions, and large current charging and discharging can easily overheat and damage the battery; the lithium iron phosphate battery has a high internal resistance under low temperature conditions. Poor working performance: The SOC-OCV curve has high consistency at different temperatures.
These conclusions clarify the temperature characteristics of lithium iron phosphate batteries and are of great significance for designing battery thermal management systems.

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