Implementation of an SOC-based four-stage constant current charger for Li-ion batteries

This study implements a possible use of the state of charge (SOC) instead of the charge voltage limit (Vlimit) to control the charging process for a four-stage constant current charging strategy. To determine the charging current in each stage, an iterative optimization procedure based on Taguchi method is employed to find near-optimal values. To control the change of the charging stage and terminate the charging process, the Coulomb counting method combined with battery's open circuit voltage estimation is adopted for SOC estimation. Tests of Sanyo 840 mA h, 3.6 V lithium-ion (Li-ion) batteries have been conducted with a Keithley 2230-30-1 triple power supply and a Prodigit 3332 F dual electronic load. The implemented charger has an input voltage of 12 V, output currents of 1.176 A, 0.840 A, 0.588 A, and 0.336 A, as well as an output voltage ranged from 0.168 V to 0.588 V. By performing the experiments, the proposed charging strategy has shorter charging time than the equivalent constant current constant voltage (CCCV) and the Vlimit-based charging methods. However, it yields a slightly lower charging efficiency than the equivalent CCCV and pulse current charging methods.