The auxiliary battery solenoid
When any vehicle's engine is running, it is constantly charging it's battery. When a auxiliary (or secondary) battery is installed , the "auxiliary battery solenoid" connects the main battery to the newly installed secondary battery so that they can BOTH be charged. But when the vehicles engine is turned off, the batteries are disconnected from each other. This allows the secondary battery to be used for any kind of electrical accessory, and can even be drained completely, while still having a fully charged main battery with which to start the engine.
