This Battery Maintainer enables the auxiliary battery bank to remain topped up by using charge from the primary battery (only when the primary battery is connected to a battery charger, wind generator, solar cell, etc). The maintainer is activated when the primary battery reaches about 13.3 V and allows excess power to be transferred to the auxiliary battery. The auxiliary battery will then be charged at low current (1 or 3 amps, depending on model). It then maintains the charge on the auxiliary battery. If the primary battery's voltage drops below 12.9 V the device switches to standby. 5.6 x 1.8 x 1.6 inches.