This paper proposed a switch-based charger system instead of LDO-based charger for Li-Ion batteries. The automatic power monitor (APM) controls three main power delivery paths to generate five different charging cases. Moreover, with the proposed smooth transition loop selector (STLS), the charging process can transfer smoothly and never oscillate between two consecutive stages. Furthermore, the STLS can bear different batteries with wide built-in resistance variation by adjusting charging factors. The stability analysis proves that the stability of APM loop is guaranteed by only one Type-III compensator. The simulation results ensure the performance of the system controlled by APM even if the power source is unable to provide sufficient power.