Design and experiment of a novel stepwise preheating system for battery packs coupled with non-dissipative balancing function based on supercapacitors

Abstract

Lithium-ion batteries have advantages such as low self-discharge rates, high energy density, and environmental benefits. They are widely used in electric vehicles. However, their further application is limited by the inconsistency between batteries after assembly and the reduced battery performance in low-temperature environments. This article proposes a novel stepwise preheating system for battery packs that integrates a non-dissipative balancing function based on supercapacitors. The supercapacitor acts as an energy transfer and storage medium for balancing the power battery pack and heating the preheating battery pack. The preheating battery pack serves as an energy supplier for heating the power battery pack and as an energy storage unit during the balancing process of the power battery pack. This article studies the influence of low-temperature environments on the discharge capacity of power batteries and supercapacitors, explores the impact of environmental temperature on the relationship between state of charge (SOC) and open-circuit voltage (OCV), and the SOC0-OCV relationship of batteries, and clarifies the relationship between temperature and SOC0 required to meet the maximum pulse discharge test. It develops a parameter matching method for a supercapacitor and a preheating battery pack that meets the preheating requirements of power battery pack. It designs the topology structure of a balancing system based on supercapacitors with the flying capacitor method. It uses the battery terminal voltage as a balancing variable and formulates a non-dissipative balancing control strategy under static conditions. Results show that the low-temperature preheating system can warm up the preheating battery pack from −25 °C to 0 °C within 7 min, raise the temperature of the power battery pack from −25 °C to 0 °C within 27 min, and further increase the temperature of the power battery pack to 8 °C, achieving its maximum pulse discharge test in low SOC0 (33.5 %) state. During the non-dissipative balancing process, the energy transfer in the battery prioritizes charging the preheating battery pack, the average balancing current is about 8 A, and the energy utilization rate is about 80 %. This study provides a new approach for coupling the preheating technology and the power battery pack balancing technology in low-temperature environments.