A bidirectional flyback cell equalizer for series-connected lithium iron phosphate batteries

Cell equalization is a critical technology to enlarge battery pack capacity and prolong battery cycle life. A cell equalizer based on an isolated bidirectional flyback converter is introduced in this paper. Energy can be transferred from one cell to the pack or from the pack to some cell through a double winding transformer. An n-MOSFET and a milliohm resistor for current sampling are connected to each winding in series. According to the sampling current, the n-MOSFETs in both sides are switched on alternately. One is for inverting, and the other is for rectifying. Therefore, the converter achieves synchronous rectification, which contributes to higher energy efficiency. Meanwhile, the average balancing current can be set based on the sampling resistors. By the application of a controller IC for multi-cell battery stacks balancing, the implementation of the cell equalizer is simplified, and the reliability of the circuit is improved. A prototype is designed for a twelve-cell series connected lithium iron phosphate battery pack. Every cell can be charged or discharged individually at the same time, which accelerates the balancing process. Experiments have been conducted to validate the balancing circuit.