In cloudy climates, like Oregon, the solar panel may not have enough power to fully charge our battery. As a backup, we hooked up the battery to the van’s alternator. When the van is running it will charge the solar battery (as well as the van’s battery).
Of course it’s not as easy as hooking up the positive terminal of the solar battery to the alternator. While this is all you need to charge, when the van is off, the van battery and the solar battery would discharge each other. The result would be a drained van battery and solar battery. To prevent this from happening, we added something called a battery isolator. In it’s most basic form the battery isolator is just a big relay that can withstand a lot of amps. Here is the battery isolator relay wiring diagram, a portion of the larger diagram from the electrical post.
There is a lot of information on the internet for battery isolation. They are common in vandwelling, RVs, trailers, and car audio applications. We chose a PAC-200 relay, which can handle 200 amps. This is well over what we expect to use, but nothing wrong with a little extra.
A relay is an electronically operated switch. Instead of turning off a circuit with a manual lever, like a light switch, a relay turns on and off a circuit via an electrical signal. The circuit that we’ll use to operate the switch is the +12V signal that is only active when the alternator is running. That is, the circuit that is +12V when the key is in the RUN position. Many other circuits in the van run only on this signal. I looked at many different places to tap into this signal, but the easier way turned out to be a Fuse Tap. What this device does is plug into a fuse socket in the van’s fuse panel, and adds a tap to the fused circuit. (It also adds a fuse to this tap, so the original circuit has its fuse, and the tap has its own fuse). I used Autozone’s vehicle repair guides to find a wiring diagram, where I found Fuse 6 was hooked up to the Hot in Run circuit.
So I hooked up the fuse tap to that fuse number, plugged in a 2 amp fuse (the coil of the relay claims it uses less than 1 amp, so 2A fuse should be fine).
While hooking up this fuse tap directly to the relay would be just fine, I decided to hook this up to a manual switch as we don’t expect to need to use this extra charging source very often, so we don’t need to put all those cycles on the relay coil if they are not needed. I chose a lighted switch to make it easier to see when the relay is on even though the relay makes a noticeable sound when it turns on and off. Here’s a picture of the back and front side of the switch. I added a wiring diagram to the back of the trim panel sbo it would be easy to hook up the switch correctly.
This LOAD side of that switch is connected to the relay coil. I mounted the relay coil to the back of the passenger side knee guard of the van. The other side of the relay coil is connected to ground. So when the relay switch is on and the van is in RUN, a complete circuit is made and the relay switches the solar battery circuit on. The solar battery circuit is simply a wire from the van’s + battery terminal to the relay, and then onto the solar battery (through a circuit breaker).