Abstract: The invention provides a vehicle battery system including a main and a reserve battery disposed within a battery housing (10) having conventional external dimensions and battery terminal locations. An illustrated embodiment includes a main battery, comprising a series of thin, flat cells (38) disposed along a first axis, and a reserve battery, comprising a series of cells (42) disposed along a second axis transverse to the first axis. An electrical circuit facilitates selective communication between the reserve battery and the vehicle starter in accordance with the position of a switch assembly (100). An electrically conductive fastener is disposed to simultaneously maintain electrical communication between a main battery termination and an external battery terminal and also between a reserve battery termination and the same external battery terminal.