Development of a two‐staged balancing scheme for charging lithium iron cells in series

Passive dissipation cell balance methods are predominantly used in the electrical vehicle industry to overcome inconsistency problem commonly existing in lithium ion cells in series. Some use active schemes with various intermediate storage elements to enhance cell balance efficiency. However, both active and passive balance methods suffer the setbacks of unavoidable cell overcharging or insufficient charges. The current study develops a two‐staged balance scheme to compensate cell inconsistency without the shortcomings of existing balance methods. The two‐staged balancing scheme uses an innovative switch matrix to select and equalise cell voltages in a lithium ion battery module. The developed balance module automatically selects the cell with the minimum voltage and raises the voltage of this cell with balance current until a preset voltage condition is reached. Then the system would continue to raise the next cell with the minimum voltage until any overcharged condition is reached in any cell. The system would turn to the second charging stage, charging each cell with a constant voltage mode until a preset voltage level is reached. The second charging stage charges each cell until each cell reaches fully charged state while no single cell would be overcharged by any chance. Tests showed that the developed two‐staged balance method was capable of overcoming cell inconsistency problem and charging each cell to full states without any chance of overcharging.