Research on control strategy of rapid preheating for power battery in electric vehicle at low temperatures

Abstract

Due to cold weather conditions, it is not advisable to use the remaining capacity of the power battery to heat it. Instead, the battery can only be preheated during charging when the battery power is almost consumed up. The traditional positive temperature coefficient (PTC) heating system combines the cockpit air conditioning and heating system with the low-temperature preheating system for the power battery cells. The PTC controller is integrated into the air conditioning control system. However, this integration requires the air conditioning to be activated in order to preheat the power battery. As a result, there is energy waste during the charging process. To address the aforementioned issues, this article proposes a rapid preheating system that utilizes an independent liquid circuit equipped with a high-voltage heater. The system achieves a preheating time of 2000 s for a ternary lithium battery with a capacity of 73kWh and a rated voltage of 770 V. Another important finding is that fluctuations in the current of the battery management system (BMS) occur during the rapid preheating process, leading to uncontrolled battery charging and discharging. The battery rapid preheating control strategy has been redesigned to rapidly heat the battery system by disconnecting the rapid charging relay of the high-voltage circuit, thereby prevents over-discharge and overcharge of the power battery. Experiments have shown that the BMS current increases or decreases in a stepwise manner, as expected by the design. The average temperature of the battery pack during onboard charge module slow charging and direct current fast charging is below 35 °C, and the maximum temperature difference is less than 6 °C. The proposed rapid preheating system and improved battery charging architecture can shorten the charging time and reduce energy consumption. This advancement will open up new possibilities for power battery protection and contribute to the development of lithium-ion batteries for electric vehicles at low temperatures.