In this paper, a fast li-ion battery charger microcontroller (MCU) based on configurable pulse width modulation (PWM) controller by tracking the polarization curve of battery cells is proposed in order to inject the maximum charge-current without battery degradation and its fully custom-designed MCU is implemented as a system-on-chip (SoC) using 0.18 um Embedded Flash CMOS process. The evaluation results show that a newly designed charger is about 18.6% faster than the conventional constant-current (CC) charging method with the temperature rise within a reasonable range. The implemented charger system by tracking the battery cell polarization curve results in charging 4 cells of battery to about 80% of its maximum capacity in less than 56 minutes and 13 degrees C maximum temperature rise without damaging the battery.