Vehicle start-stop system

A vehicle starter assembly 18 includes a switching circuit 34 configured to operate in a charging state and a discharging state, first 38 and second 40 energy storage devices electrically connected to the switching circuit. The energy storage devices are connected in parallel in the charging state and in series in the discharging state. A processor (50, figure 2) is programmed to detect an engine request and output a switch control signal that toggles the switching circuit between the charging the discharging states. The switching circuit may electrically connect the energy storage devices to an input terminal 44 when charging and an output terminal 48 when discharging. The assembly may also comprise a current control circuit 36 which receives a control signal from the processor to maintain a constant current flow in charging and discharging states. Either storage device may be an ultracapacitor. The engine request may include an engine start or run command. The processor may detect the magnitude of current flow and / or reduce the current flow through the storage devices to a predetermined amount before outputting the switch control signal.