The paper presents the concept of supercapacitor based plug-in retrofit kit that can be easily installed on the electric vehicle powered by lead-acid or by other type of batteries without the intervention in the control system to store regenerative braking energy, improve battery lifetime and efficiency. The installation of the retrofit can be done in a simple manner by connecting of only two wires to the DC bus and by installing contactless current sensor on the battery wire. Many control strategies can be implemented to control such system. The energy storage system operation algorithm that stabilizes battery current is studied to extend the battery lifetime. The paper is focused on the efficiency optimization. This purpose is achieved through regulation of supercapacitor based energy storage current proportionally to the battery current in the discharge process. Theoretical equations, Matlab simulation results and experimental results on the test bench are presented in the paper and shows the validity of the approach.