A modular battery and power control system provides a range of electrical power to high-voltage and low-voltage power demand systems in a hybrid vehicle. The modular battery and power control system provides a single high-voltage modular battery pack that is selectively connected to high-voltage and low-voltage demand systems by a switch system. The switch system is controlled by a voltage controller module which monitors power demand and charge states of the electrical system and determines the switch positions to effect the indicated electrical demands of the hybrid vehicle.