Installing dual RV batteries is fairly simple, but the results could be disastrous if done wrong.
Step 1 - Decide Which Batteries to Buy
You will first need to decide what kind of batteries to buy. If your energy use is limited to a 12-volt lighting system, water pump, refrigerator, CO alarm, 12-volt stereo system, heater, then two group 27 or group 29 deep cycle batteries will probably be enough. If you use a microwave or home theater system, you will probably need a battery bank of much larger capacity.
The batteries your RV came with are probably “RV Marine” batteries. These are a combination of deep-cycle and starting batteries. You will probably want to replace them with true deep cycle batteries, as they can be drawn down and recharged over and over again. Try to get batteries with high amp hour ratings.
Do not use regular car batteries, as they are used for smaller engines and wont last long in an RV. Never install a new battery with an old battery. It is best to buy two new batteries and install them both at the same time. Otherwise, the old battery will form a “current loop” with the new battery, drawing its charge for itself. If this happens, you would have just wasted a perfectly good new battery. The new batteries should also be of the same capacity, brand, and age.
Step 2 - Disconnect the Old Batteries
First disconnect the old battery by unscrewing the negative and positive battery clamps. It should be noted that the positive leads from the trailer are normally black wires and negative leads are white. This is different from other automobiles, where usually red is positive and black is negative. Remember that you must get this straight before proceeding!
The negative clamp should be removed first, and when you replace the battery, they should be the last two clamps connected. This is essential so that short circuits are avoided.
Step 3 - Attach The New Batteries
Place the new batteries in their proper places. You will now connect these batteries in parallel with each other, that is positive with positive and negative with negative.
The trailer positive lead should be connected with the first battery’s positive terminal. Then connect the two positive posts together with charger wire, secure the clamps, and then do the same with the negative posts. The negative lead of the trailer is connected to the the negative lead of the first battery. Ensure that the new charger wire is the same size or larger than the old charger wires.
Step 4 - Make Sure There Is Enough Air Around the Battery
It is absolutely essential that there be enough air around the battery to avoid the buildup of hydrogen. Build up of hydrogen can be very dangerous. If there is too much buildup, it can ignite, explode, and put people in danger. If you can not ensure that there is enough ventilation around the battery by yourself, be sure to consult a professional.