Marginal efficiency assessment of regenerative braking of an electric vehicle drive

Abstract

It is known that regenerative braking in small electric vehicles is not always efficient, a fact attributed to both the relatively low total weight and travel speed and the losses within the drive system. A review of existing literature indicates that research often focuses on enhancing the efficiency of this process through improvements the software, hardware, or technical solutions; however, the theoretical upper limit of this efficiency has not been clearly defined. This study develops a mathematical model of the motion of a small electric vehicle – an electric bicycle –to derive a set of braking characteristics representing the theoretically maximum regenerative braking power as a function of braking deceleration and travel speed. To evaluate actual regenerative braking power and energy returned to the battery, a computer simulation of the electric bicycle’s drive system (featuring a brushless DC motor) was conducted using the MATLAB/Simulink environment. The research methodology involved performing test runs at three distinct deceleration rates while recording mechanical and electrical power values and calculating the drive system's regenerative energy and efficiency were conducted. The obtained results lead to the conclusion that the maximum efficiency of regenerative braking increases as braking acceleration decreases; however, at the end of the braking process – at a specific speed—the energy efficiency drops dramatically to zero, a phenomenon attributed to both drive system losses and the chosen braking method.