an apparatus which connects / disconnects battery from a load such as a computer system includes a first circuit arrangement which sets a first reference voltage level at which the energy source is connected through an fet device to the computer system , a second circuit arrangement that sets a second voltage level at which the energy source is disconnected by the fet device from the computer system and a third circuit arrangement which controls the fet device so that said fet device is forced into either its on or off state once one of the reference voltages is reached . a second apparatus which reduces power dissipation in a battery charger power device by forcing the input voltage to track the battery voltage as the battery charges / discharges .