This work proposes a CMOS reconfigurable charge pump (CP) for a low-voltage energy harvesting system. It utilizes the low effective resistance from the parallel CP to enhance its power conversion efficiency (PCE). The CP exhibits an adaptive configuration with different stages depending on the input voltage, changing its voltage conversion ratio (VCR) to limit the output voltage under 1.8-V. Additionally, this work develops a novel dynamic source-fed oscillator that modulates the oscillating frequency by utilizing a dynamic source for the ring-voltage controlled oscillator (RVCO). The independent source from the RVCO and the clock-generating units from the proposed technique permit the implementation of frequency modulation without affecting the clock amplitude. Fabricated in 65-nm CMOS, the proposed prototype measures 62% peak PCE with an input voltage range of 0.26-V to 0.64-V.