An on-board battery supervisor that monitors the state-of-charge (SOC) of a lead-acid vehicle battery and automatically disconnects loads from the battery to prevent over-discharging of the battery, is disclosed. As the battery is being discharged, the on-board battery supervisor disconnects loads at an SOC level that leaves enough energy in the battery to reliably start the vehicle engine, while providing as much energy as possible to accessory loads. The loads are automatically reconnected as the battery is charged. Some preferred embodiments of the invention are designed specifically for use in watercraft and a dual output version of the invention simplifies the electrical system. All embodiments have circuit breaker outputs, high peak current capability, a low quiescent current drain and a dual timer that allows accurate determination of the SOC of the battery.