System consists of a controller that detects when a vehicle�s alternator is charging the main battery and then closes a solenoid to tie-in charge to an auxiliary battery. When the engine is off or the charge voltage is no longer present, the solenoid opens to isolate the batteries. Upon �cranking� the engine with a charged auxiliary battery, the solenoid is energized to parallel the batteries and provide a starting boost. If the auxiliary battery is low, the controller senses that condition and the batteries are not paralleled. Uses a solenoid rather than an isolation diode which typically causes a 0.7V drop, thus auxiliary battery receives full charge voltage. The starting boost provided by the paralleled batteries is especially useful in cold weather or when the battery is low. System consists of a controller and two separate solenoids, for switching the batteries. Allows a three battery system to be isolated from each other. The system receives its power from the Battery one input or it can be switched ON/OFF. When any of the three batteries go above 13.2V all three batteries are paralleled by the solenoids being energized, charging all three batteries and will disconect the batteries when the voltage drops below 12.6V