Pulse charging has shown some very appealing advantages that can maintain the battery’s optimal performance during cycling. It has been shown that pulse charging can reduce the build-up of film at the solid electrolyte interface (SEI), allowing ions to flow back into the electrolyte, provide shorter equalization cycles, and allows more of an efficient charge after each short relaxation period than the conventional constant current charge method [1]. Some pulse charging strategies use a high voltage pulse with a relaxation period, this can result in an unexpected variation of charge current which can lead to performance loss in the cell and error in state-of-charge (SOC) estimation [2,3]. In this study, we propose a current pulse charge topology that can charge and equalize a series connected battery string.

Methodology

This system uses a dc-dc converter mainly to maintain a constant current in each cell, 2N low resistance solid state switches are used to control the pulse current with short relaxation periods for each cell as shown in figure 1. Where N is the number of cells. During the charging process, the individual batteries in series are connected to the power and ground bus via a microcontroller to be charge one at a time. This configuration provides a means of isolation or bypassing capabilities for each cell in the stack without the need of a solid-state transformer (SST). The pulses are precisely controlled by varying the duty cycle for a specific switching period which is based on the average current represented by equation (1).

Equation (1)

I_avg is the average current, T is the period of the pulse and i(τ) is the pulse current induced in battery. The duration for each pulse is given by equation (2).

 Equation (2)

Using this approach, constant-current/constant-voltage can also be achieved by using pulse charging simply by varying the duty cycle. Furthermore, this approach allows each battery to receive its own independent charge and increasing the equalization rate for the pack itself. Figure (1)

[2] Yin, Meng Di, Jeonghun Cho, and Daejin Park. "Pulsed-Based Fast Battery IoT Charger Using Dynamic Frequency and Duty Control Techniques Based on Multi-Sensing of Polarization Curve." Energies (2016): 1-20. Print

[3] Li, Jun, Edward Murphy, Jack Winnick, and Paul A. Kohl. "The Effects of Pulse Charging on Cycling Characteristics of Commercial Lithium-ion Batteries." Journal of Power Source (2001): 302-09. Print.