Optimal algorithms for the charge equalisation controller of series connected lithium-ion battery cells in electric vehicle applications

A charge equalisation controller (CEC) was developed for continuously monitoring individual battery cells and equalising the charge or voltage levels of all cells in a series pack. A charge equalisation control algorithm was developed to equalise undercharged, overcharged, and unprotected cells through the use of a bidirectional fly-back converter. The equalisation involves charging and discharging by employing constant current–constant voltage and discontinuous current mode proportional–integral (PI) control techniques. Particle swarm optimisation is applied to optimising the PI controller parameters that generate the regulated pulse width modulation switching signal for the converter. A CEC model was applied to 90 lithium-ion battery cells (nominally 15.5 Ah and 3.7 V each) connected in series. The results showed that the developed CEC model performed well at equalising both undercharged and overcharged cells with ∼92% efficiency and equalised every cell within the safe operation range of 3.73–3.87 V. The developed system realises excellent equalisation speed, a simple design and efficiency with low power loss. Thus, the CEC model has great potential for implementation in real-world electric vehicle energy storage systems.