An example information processing system includes a main device, and an operation device attachable to the main device. The operation device includes a first control circuit, a first battery, and a first power supply circuit. The main device includes a second control circuit, a second battery, and a second power supply circuit. The second power supply circuit supplies power from the second battery to the operation device in response to satisfaction of a condition about the amount of remaining charge indicated by the remaining charge amount information transmitted from the operation device. When power is supplied from the main device to the operation device, the first power supply circuit supplies power supplied from the main device to the first control circuit, instead of power from the first battery.