Folks new to RVing can get a nasty shock, if you’ll pardon the pun, when they first learn about RV batteries. We got an early education on our first trip out with a truck camper. A day away from home, we got up in the morning after spending the night in the middle of nowhere and found to our chagrin that our truck wouldn’t start. We had no idea that our camper had no battery of its own, and our little furnace fan had sucked the truck battery dry. After a jump, we were on our way with new lessons learned.
Happily, most RVs have their own batteries to handle lights, water pumps, and of course, furnace fans. And these are quite different from the battery or batteries under the hood of your towing vehicle, or that start the engine in your motorhome. These two types of batteries should be called by different names, and they are. Those used for engine operation—starting up and running lights—are called “SLI” batteries (starting, lighting, ignition). The ones used to operate interior lights, water pump, furnace, and other in-the-RV jobs are called either “house” batteries, or by some, “coach” batteries.
SLI batteries are built to put off a lot of power in a hurry—and then immediately be recharged. Starting your engine takes a lot of power. House batteries, called “deep-cycle” batteries, are designed to put out less power at once, but over a longer period of time. If you try to use a typical SLI or car battery to power your RV’s internal needs, you’ll soon kill off that battery—it’s just not designed for the job.
When shopping for a deep-cycle battery for your RV, you’ll find some choices. There are three basic types of deep-cycle batteries, each with its advantages and disadvantages.
The most typical RV house battery is a lead-acid or “flooded acid” battery. Filled with a liquid electrolyte, they are the least expensive of all the deep-cycle batteries, and have the longest track record, having been around for decades. There are vented varieties where the cell lids come off easily to refill the cells as needed, and non-vented or so-called “maintenance free” versions.
These batteries must be kept upright to prevent spilling of the electrolyte. When charging, they give off hydrogen gas and must be kept in an area vented to the air outside the RV. In terms of longevity, flooded acid batteries don’t last as long as other types, but if cared for properly—meaning keeping the electrolyte level filled, and generally not discharging them to less than half their rated capacity—they are a good choice for most RVers.
Absorbed Glass Mat (AGM) batteries are probably the second most popular type of deep-cycle battery among RVers. Here the electrolyte is not a free-flowing solution of acid sloshing around in the cells, instead the acid is saturated into a fiberglass mat. They can be mounted at odd angles since they don’t have to be kept upright. Properly cared for, they don’t “off-gas” like flooded acid batteries, and hence don’t require venting to the outside.
AGM batteries are said to have greater longevity than flooded acid batteries, meaning that they can be discharged and recharged more cycles. Still, they are more costly than an equivalent flooded acid battery. They are also more finicky about charging: Overcharging an AGM battery can lead to swelling, even bursting, of the battery case. While charging to 2.4 volts per cell (14.4 volts on a 12-volt battery) is OK, a “float charge” (a charge left on an unused battery) of only 13.5 volts is a safe level.
Finally come gel cell batteries. Like AGM batteries, these characters don’t have a liquid electrolyte. They can be mounted in any direction you wish, and they don’t off-gas, so don’t require outside venting. They are the most difficult to care for in terms of maintaining a proper charge in that they are the MOST sensitive about over-charging. A conventional off-the-shelf battery charger will destroy them.
How do you decide which deep-cycle battery is best for your “house” needs? Unless you have an unusual need for a battery that can’t vent to the outside, or can’t be mounted vertically, many RVers will tell you, stick with a conventional flooded acid battery until you’ve been in the game for a while. With experience and after hobnobbing with other RVers, you’ll get a feel as to whether you’ll want to switch to a different breed of battery.
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. Visit icanrv.com for more information.