With most battery isolators you get three terminals, one input from the alternator and then one output to each battery Bank. The problem is most alternators since wire were they since the voltage it's hooked to the vehicle's primary battery and it really can't see the extra battery in the back of the RV. This one has an extra port on it for a voltage sense wire since the voltage of whichever battery is lower until they both come up to par and that's a rare hen in this hen house. It made a phenomenal difference for me always having a good hot starting battery but a mediocre house battery now while it's running I can test the voltage at both batteries and they are identical for the first time. At the price point you might think you're cheaping out but this is money well spent because I don't care how much you spend you're not going to find a better isolator it's been working flawlessly and I couldn't wish for something better.