This kit allows two batteries to be charged from your engine alternator at the same time. When your engine is on and the start battery is sufficiently charged the isolator(VSR) will begin charging the second (auxiliary) battery simultaneously. When the voltage of the start battery drops (i.e.. the engine is stopped) the VSR disengages from charging the auxiliary battery. This isolates the start battery and eliminates the possibility of draining it, ensuring that you always have enough charge to start your engine. It also has the ability to protect sensitive electronic equipment powered from the auxiliary battery from harmful engine start-up spikes, and an emergency override feature to allow you to jump start your vehicle from your auxiliary battery. Suitable for 12VDC Marine, 4WD, caravan, and solar applications. This kit includes everything you need like pre crimped marine tinned cable, battery terminals, heatshrink, cable lugs, cable ties and more