A bicycle power supply system comprises a first power supply, a second power supply, and a power supply level sensing structure. The power supply level sensing structure is operatively coupled to the first and second power supplies. The first and second power supplies are selectively electrically coupled with electrical power being supplied from the second power supply to the first power supply while a first power supply level is detected by the power supply level sensing structure to be below a first prescribed power threshold, and with electrical power being supplied from the first power supply to the second power supply while a second power supply level is detected by the power supply level sensing structure to be below a second prescribed power threshold.