*Article* **An Improved SOC Control Strategy for Electric Vehicle Hybrid Energy Storage Systems**

**Kai Wang 1,\*, Wanli Wang 1, Licheng Wang <sup>2</sup> and Liwei Li 1,\***


Received: 27 July 2020; Accepted: 2 October 2020; Published: 12 October 2020

**Abstract:** In this paper, we propose an optimized power distribution method for hybrid electric energy storage systems for electric vehicles (EVs). The hybrid energy storage system (HESS) uses two isolated soft-switching symmetrical half-bridge bidirectional converters connected to the battery and supercapacitor (SC) as a composite structure of the protection structure. The bidirectional converter can precisely control the charge and discharge of the SC and battery. Spiral wound SCs with mesoporous carbon electrodes are used as the energy storage units of EVs. Under the 1050 operating conditions of the EV driving cycle, the SC acts as a "peak load transfer" with a charge and discharge current of 2*isc*~3*ibat*. An improved energy allocation strategy under state of charge (SOC) control is proposed, that enables SC to charge and discharge with a peak current of approximately 4*ibat*. Compared with the pure battery mode, the acceleration performance of the EV is improved by approximately 50%, and the energy loss is reduced by approximately 69%. This strategy accommodates different types of load curves, and helps improve the energy utilization rate and reduce the battery aging effect.

**Keywords:** hybrid energy storage system; supercapacitor; energy allocation
