Installing a Second House Battery in a Camper, Van or RV Bus Conversion

If you have a camper or are converting an old van or bus into a camper you should add a second battery, known as a house battery to provide power to the camper while still allowing the vehicle to start in the morning. It is pretty straight forward to add a second battery to a vehicle. Find a good location for the battery, ground the negative post and run a properly sized wire to the positive post from the starting battery, but both batteries will run down together. We need a way to isolate the house battery while the vehicle is parked and charge it while the vehicle is running.

There are a few different options. The simplest and cheapest is to add a battery shut off switch. They are available from most auto parts stores and from Harbor Freight. It's just a heavy duty switch that can be turned by hand. It can handle the amps required to charge the battery. When you park, you turn the switch and break the connection between the two batteries. In the morning you start the vehicle, turn the switch, reconnect the batteries and the house battery will be charged from your motor. The driver must always remember to disconnect and reconnect the switch. It is not automatic. You could be left stranded if you park and forget to turn off the switch.

The second option is a battery isolator diode. They can be found at marine stores, camper supply stores and some auto parts stores. The power wire from the alternator connects to the common post on the isolator. Each battery connects to its own post on the isolator allowing each battery to be charged independently. These work automatically, but there is some voltage loss through the isolator and they can be a bit pricey.

The third option and the option I chose is to use a continuous duty solenoid to break the connection between the batteries. You must use a continuous duty solenoid. A standard automotive starter solenoid may look the same and cost less, but it will burn out in a few minutes. They can also be found at auto parts stores, camping supplies and online sources. I picked up my parts from a golf cart supply company because I also purchased a few fuses and quick connect plugs. The solenoid is mounted to the fender well. Some solenoids are grounded by the mounting bracket. Some need a small 14 gauge or so wire to be installed as a ground. I added a power cable to the positive post of my starter battery and protected it with a golf cart fuse. The cable connects to one side of the solenoid. I used a long length of heavy gauge cable to run the power from the other side of the solenoid to the rear bumper of my truck. I'm sure it was overkill, but I ran the cable inside of some 1/2" heater hose to add extra protection to the cable where it attached to the inside of the frame rail. There's no need to run a ground cable from the battery to the back of the truck. I just used a short length of cable to ground the quick plug to the rear bumper. Bad grounds can cause problems. I like to use a grinder to expose clean metal and I protect the connection with axle grease after it's bolted down.

You have a couple options to activate the solenoid. I simply located an ignition hot wire to power the solenoid. When the key is in the run position, an ignition hot will have power, but it will not if the key is in the accessory or off position. A test light or multi meter can be used to find the correct circuit to use. Auto parts stores have tap kits that make it easy to add a wire to a fuse panel or you just splice into a wire the old fashion way. My system will power the solenoid when the engine is running and charge my house battery, but it will automatically shut off the solenoid when the motor is off. A nice upgrade to this system is to add a momentary push button switch to active the solenoid. It will allow your house battery to " jump start " your truck battery if you should ever leave a light on or other wise rundown the starter battery. The momentary switch is available at Radio Shack and would have to be held in while cranking over the engine. Make sure to use a momentary switch to prevent it from being left on accidentally. I can keep one spare solenoid onboard and use it to replace the starter solenoid or the house battery solenoid. If I don't have a spare solenoid it is easy to temporarily bypass a damaged solenoid until it can be replaced.

Choosing a 2nd battery. A pair of 6 volt lead acid golf cart batteries will give you the most power for the least money, but lead acid batteries need to have the water level checked and they can off gas hydrogen while they are charged. They should be added under the vehicle or to a compartment that is sealed from the living space and vented to the outside. AGM batteries like Optima batteries are sealed and are safer if the battery must be mounted inside the vehicle, but they cost more and might not hold up as well as golf cart batteries. Wal-mart has a "marine" battery that is a relatively inexpensive option, but it's not a true deep cycle like the golf cart batteries so it can't take the same abuse. Batteries should be stored charged and should be recharged as soon as possible. The deeper they are discharged the faster they will fail so it is important to size the battery to match the load. If you are not familiar with how to care for the different types of batteries there is a lot of information available online.

You might find a couple battery trays in a junk yard and modify them to fit your needs. You can build a rack from some steel. If you cut the pieces and lay out the design yourself a welder can assemble it for you for a few bucks. A truck or van should have enough room along the frame for a couple extra batteries. A battery can be safely mounted inside a vehicle if care is taken. The batteries and cables must be secure in the event of a crash. Proper size cables must be used to carry the load or a fire can result. All of the connections should be protected with fuses. You can find fuses capable of handling several hundred amps. Sizing charts are available online if you need help selecting the correct size wire.

I have a slide in truck camper so my batteries are in the camper. I used a golf cart quick connect plug to attach my batteries to the power cable from my truck. My camper also has a couple solar panels. If the panels can't keep up I can run the truck and charge the house batteries when they are running low or when I am using an item like a microwave that draws a lot of power. I can also use the solar panels to recharge my truck battery should I leave the door cracked or a light on. Building in that option can save hundreds of dollars if you kill the truck battery while camping in the boonies. You can see my other articles for details about installing solar panels on my camper, adding a solar hot water shower, running the truck on waste vegetable oil and converting an alternator into a welder.

Buying cables and cable ends can be expensive. DIY cable ends can be make from scrap copper tubing. I won't repeat myself here, but you can see my DIY welder article for detailed instructions on making custom cables and cable ends.

This article has several photos attached. Be sure to see them all by clicking on the icon in the photo.