Tech Tips – Batteries in a Smaller RV

As we write this article, our view out the window is of endless desert, margined off only by distant mountain ranges. The nearest house of sticks or bricks is literally miles away—yep, we’re “boondocking,” or dry camping, far away from utility hookups. Our computer is presently powered by our RV’s battery system, itself being charged by solar panels. But there’s a bit of a problem—while we have plenty of solar power, at night we could easily use up most of our available battery power. We don’t have enough storage capacity.

You may face a similar problem. Perhaps your RV is small in scale, and there just isn’t a lot of room or weight-carrying capacity. Our “field exploration RV” is a truck camper, and the manufacturer allowed but one tiny compartment for a battery, and that for our lifestyle “just ain’t cuttin’ it.” What’s to be done?

Adding Batteries
First, a quick discussion of RV batteries: Batteries in “house” use (lights, water pump, 12-volt appliances, etc.) are a deep cycle variety. These are designed to give small amounts of current over a long period of time. This is opposite the design of a “starting battery,” used to crank up your engine and operate running lights. While a deep cycle battery is meant to be used and recharged many, many times, the more deeply discharged it is before recharging, the fewer cycles of charging and discharging it can provide.

For example, our truck camper has a smaller, 12-volt deep-cycle battery, a “Group 27” battery that can provide about 110 amp-hours of current before being completely discharged. Simply put, we could operate one of our fluorescent lamps (which uses about 2 amps per hour) for about 55 hours straight before completely discharging our battery. If we were to completely flatten our battery—or even take it to within 20 percent of completely discharged every time before recharging—we could expect it to give us fewer than 200 such cycles. On the other hand, if we allowed the battery to discharge down only by a quarter to a half, we could reasonably expect that battery to provide almost twice that number of cycles. RV batteries are not cheap, so one can see this could make a significant difference.

How can you “extend” the life of your batteries and still get use out of them? Where possible, add more storage capacity. For us, this will mean having to add another battery. But where and how?
As we mentioned, our battery compartment is oh-so-small. To add another battery, we’ll have to put it somewhere else. Whenever you need more batteries, there are several factors to consider:

Venting: If you intend to add “flooded acid” batteries, those “normal” deep cycle batteries give off hydrogen gas when charging. Since it is highly explosive, you don’t want hydrogen gas floating around in your rig where it could encounter a spark or flame. Hence, it’s best to install these batteries in a vented compartment. Another alternative is a specially designed, sealed battery box equipped with a vent tube that can lead outside the rig.

There is an alternative here: There are special “gel” batteries that are completely sealed and do not “gas” when recharging. These can be placed inside the living area of a coach. However, they are more expensive than flooded acid batteries, and typically don’t have as much capacity as a flooded acid battery of the same size. They have the advantage of being virtually maintenance free and they recharge faster than their flooded acid cousins.

Distance: It’s best to keep batteries close together wherever possible. RV makers typically place batteries near the electrical distribution panel, as heavy-gauge wiring is required between the house battery and that panel.

Weight: Any deep cycle battery is a heavy critter. You must see to it there is adequate support for the battery, and that it won’t throw your rig’s weight balance off.

Wiring Up Batteries
So let’s assume you’ve found a suitable location to tuck away an extra battery or two. How does this all work out in practice? There are a couple of simple rules of thumb: When you parallel batteries together (of the same voltage) the capacity of the batteries “adds up.” In our case, we will soon be adding an identical 12-volt, 110 amp-hour battery to our system. Two 110 amp-hour batteries paralleled together will give us a net storage capacity of 220 amp-hours. What’s the other rule of thumb? Where batteries are connected in series, the voltage is additive, but the storage capacity remains the same. For example, two 6-volt golf cart batteries, each providing 220 amp-hours of capacity, series wired will produce 12 volts of power, with an available storage capacity of 220 amp-hours.

If such is the case, why would anyone bother hooking up two 6-volt golf cart batteries, since they’re a bit more expensive to purchase than 12-volt batteries? Because 6-volt golf cart batteries will give you many, many more recharge cycles than the smaller 12-volt ones. Had we the choice, we’d immediately throw in for the golf cart solution, but our rig doesn’t have the space or weight-carrying capacity for golf cart batteries.

For us, we’ll have to “hang” our second battery in a battery box on the rear bumper of the rig. Happily, within a couple of feet of the new location is our rig’s generator, already wired with heavy-gauge cables for the starting motor. We won’t have to run a lot of heavy cable, just far enough to reach that existing wire run that leads back to the existing battery.

But how do you “series” or “parallel” wire a battery system? To add another 12-volt battery to the system, you’ll parallel wire the batteries together. This means connecting the positive terminals of the batteries to one another, and likewise connecting the negative terminals of the batteries together. With the batteries located close to each other, this is easily accomplished by using pre-made battery cables from the auto parts store.

But let’s say you’re going to dump your existing 12-volt battery and install two 6-volt batteries. What now? To increase the voltage to meet your 12-volt needs, you’ll series wire those batteries. One of the battery’s negative posts is connected to the other’s positive post. The two remaining free posts, a positive on one battery and the negative on the other, are then connected to the respective lead wires from the RV.

In all cases, we highly recommend you start with new batteries, identical to one another (same model, same manufacturer). If you parallel wire two 12-volt batteries together, if they are not exactly equal in construction and in amount of use, their internal resistances will be different. One will “work harder” than the other to power a load; and one will “see” the other needs to be charged and will work at equalizing the situation. The result will be an early demise of both batteries.

And as always, a little disclaimer: If you’re not sure of what you’re doing, or feel uncomfortable about carrying out this work, get experienced help. And always, when working with batteries, TAKE OFF YOUR JEWELRY AND WATCH before getting to work. Batteries have plenty of potential power stored up in them. More than one person has lost a finger when their ring “fried” into an electrical circuit.

Russ and Tiña De Maris are authors of RV Boondocking Basics—A Guide to Living Without Hookups, which covers a full range of dry camping topics.  They also provide great resources in their book, Camp Hosting USA—Your Guide to State Park Volunteering.  Visit www.icanrv.com for more information.

Russ and Tiña De Maris