An electrical power system has a dual battery configuration that enables sufficient power supply for a spacecraft bus and a payload module being carried by the spacecraft. During a sunlight power mode, power is drawn from a solar array of the bus to power a low-discharge payload of the spacecraft and a high-discharge payload of a payload module. During the sunlight power mode, a low rate discharge battery and a high rate discharge battery are charged by a battery charge management unit of the spacecraft bus. During an eclipse power mode, the low rate discharge battery powers the low-discharge payload of the spacecraft and the high rate discharge battery powers the high-discharge payload of the payload module. The high-rate discharge battery may also be used to power the high-rate discharge payload in the sunlight power mode to meet its high current demands to meet a flexible mission operations.