If you have a small-ish 4WD, or it’s also your daily driver, you might not want to carry your second battery all the time. In my case, my deep-cycle battery is strapped down in the back of the wagon, and if the fridge isn’t in the car, the battery usually doesn’t need to be either.

Most 4WD 12v outlets are either connected to one battery or the other. If you’re carrying both batteries all the time, that’s fine, but I needed another solution. I wanted to know that all my 12v outlets would at least work while the car is running, regardless of whether the second battery was connected.

The diagram below shows what I came up with. With just the cranking battery connected, the 12v sockets operate exactly as normal – they come on when the key is turned to ACC, and turn off when the key is turned to OFF. With the second battery connected, the sockets automatically switch from the cranking battery to the second battery when the key is turned to OFF.

The result is no load on the cranking battery when the car is OFF, and no load on the deep cycle battery when the car is running. This has the knock-on effect of putting less load on your charging system, since the second battery is either charging or working, but never both.

In my case (as in the diagram) I have an Optima Blue dual-purpose battery as my cranking battery, but unless you’re drawing a crazy amount of current when you’re driving, that’s not necessary. That just gives me a little more latitude if I choose to leave the second battery at home.

The other thing to note is that I have a Redarc DC-DC Charger, rather than a voltage-sensing relay. Cranking batteries might love getting a flood of charge from the alternator, but other battery types really don’t. It’s not the way a smart mains charger or battery tender would do it, so it doesn’t make sense to charge a deep-cycle battery that way in a vehicle.