Many vehicles and other types of equipment have multiple batteries, one to start the engine and others to power accessories. To eliminate the multi-battery drain problem, Sure Power Isolators act as a check valve between the batteries, preventing current from flowing from one battery to another. Each battery is isolated, and acts as an independent power source. So, no matter how drained your accessory batteries become, they will never drain power from the battery you are depending on to start your engine.One main and one auxiliary battery (bank) for use with alternators up to 160 amp output. 9"L x 4-1/2"W x 3-1/4"H.