RV Battery Basics: A Beginner’s Guide

With the proliferation of electronic devices and more and more accessories relying on them, storage batteries are essential for RVing. Many owners ignore their batteries until there’s a problem. Unfortunately, that may be too late to save the expensive items, so it’s worth taking a little time to understand the RV battery basics and how to care for them to get the best service.

RV batteries consist of individual cells inside the case that are connected together in series. Each cell produces about two volts. Therefore, three cells in series yields six volts, and six in series gives you a 12-volt battery.

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Types of RV Batteries

Batteries used to power the coach in an RV must be a deep-cycle design to withstand the frequent and deep discharges and recharging they typically receive. Engine-starting batteries won’t last long if used in place of deep-cycle batteries, so this practice should be avoided.

Deep-cycle RV batteries are made in three different common designs listed here in ascending order of price:

• Liquid electrolyte wet cells (conventional flooded-cell batteries)
• Gelled electrolyte (gel batteries)
• Absorbed glass mat (AGM or “dry” batteries)

Lightweight lithium-ion (li-Ion) batteries are starting to enter the market. Li-Ion batteries are maintenance free, extremely light, and provide far more power storage for a given size and weight, but they are still too costly for most RVers.

Flooded-cell batteries are often installed as original equipment and deliver good capacity at a relatively low price. These “wet” cells require periodic filling with water, which should be distilled to prevent mineral buildups. If batteries run low on water and electrolyte to the point where plates are exposed, they may be ruined. Wet batteries use more water in hot weather and when frequently discharged and recharged, so service them more frequently in these conditions. Wet batteries also need regular terminal cleaning.

Gel batteries contain a thick gelled electrolyte instead of liquid, so they don’t spill if briefly tipped on their side and don’t require refilling with water. They cost more and typically last longer than wet-cell batteries.

AGM batteries also don’t require water, as their electrolyte is soaked into fiberglass matting, which is wrapped around their lead plates. They can be mounted on their sides and are costlier than gel or wet cells, but last considerably longer and are maintenance free.

12-Volt vs. 6-Volt

Typically, smaller coaches will have one or two 12-volt batteries wired together in parallel to provide 12 volts to the electrical system. Some larger coaches have either one pair of large six-volt batteries wired in series, or even two pairs of six-volt batteries wired in series/parallel to deliver 12 volts with enough wattage when running the coach from the batteries.

Related video: RV Battery Maintenance Techniques for Extending RV Battery Life

Six-volt batteries must be installed in pairs wired in series so their combined voltage will be 12. When 12-volt batteries are used in pairs they must be wired in parallel. If 12-volt batteries are parallel-wired in pairs, there’s a problem called “cannibalism” where the weaker battery draws current from the stronger one. That’s why matching batteries should be replaced in pairs, even if only one dies. A significant advantage of six-volt batteries paired in series is they don’t cannibalize each other. Also, for heavy power demands, six-volt golf-cart type batteries offer the most storage for your money. Many RV owners upgrade to them when replacing batteries, especially those using solar panels.

Battery Ratings

Battery ratings are important to know. Ampere-hour (A-H) ratings are a way of rating how long a battery can handle a load. A-H ratings are determined by multiplying output current in amps by time (in hours) that the load is applied, until the voltage drops to 1.75 volts per cell (considered to be discharged). The higher the A-H rating, the more powerful the battery.

Reserve Capacity (RC) is how long in minutes (at 80°F) a 100% charged battery may handle a load of 25 amps, until it dips to 1.75 volts per cell (10.5 volts for a 12-volt battery, or 5.25V for a 6-volt battery). This is more for engine-starting use, where the engine may be off but lights and accessories left on.

Cold cranking amperes (CCA) is the amount of amps a 100% charged battery can provider at 0°F for 30 seconds while maintaining a minimum 7.2 volts (with a 12-volt battery). Marine Cranking Amperes (or MCA) is the same as CCA, only measured at 32°F, which makes it a less difficult test. Cranking ratings are only important for engine-starting batteries.

Charging Batteries

Discharging batteries deeply (below 50% capacity) shortens their life. Therefore, it’s important to have sufficient battery capacity, particularly if you dry camp. Charge discharged batteries as soon as possible. Use a maximum charging rate of 20% of the amp-hour rating (for example: with 100 A-H batteries, use maximum 20 amps charging rate). Don’t allow a battery to remain at a low state of charge for an extended period, even overnight. Leaving a battery discharged causes the cells to become “sulfated” and they lose their ability to receive and hold a charge.

Related video: Testing RV Battery for Charge & Condition

Batteries lose charge over time, and most RVs also have small parasitic current draws that will kill a battery in days or weeks. Non-smart basic power converters damage batteries if left on for long periods such as during storage. That’s because they produce a steady current that overcharges and causes damage once the battery is fully charged. If you don’t have a “smart”-type power converter with a sophisticated multi-stage battery maintenance program, use a separate portable maintenance-type charger during storage (not a trickle charger, which can overcharge and damage a battery if left on).

Cautions

Battery acid is extremely corrosive. Wear rubber gloves and eye protection whenever working with batteries. Water and baking soda can neutralize the acid and should be kept on hand when working around batteries.

Related video: Simple RV Battery Maintenance To Keep Them Going Longer

Batteries produce hydrogen when being charged, which can explode if there’s a spark nearby. Connect the positive (red clamp) jumper cable to the positive (+) battery terminals first. Then to keep sparks away from the battery, connect the final negative (black/ground) jumper cable to a grounded unpainted metal portion of the chassis, rather than on the battery itself.

Closing Thoughts

To get the best service and longest life from your batteries, be sure to use the right type for your particular RV’s needs and power usage. Add distilled water to replace water lost during operation in conventional flooded-type batteries. Avoid discharging batteries below 50% of their capacity, and recharge them as soon as possible. Many RVs have monitor panels which display battery states of charge. This can also be measured with a voltmeter at the battery terminals.

A fully charged battery at rest (no load or charging being done) should read 12.6 volts or higher. A 75% charge should yield a 12.4 volt reading, 50% charge would be 12.0 volts, and a 25% charge 11.7 volts. Use a maintenance type charger (or smart-type power converter) during periods of storage.

With a comprehensive understanding of your RV batteries and proper ongoing care, you can add years to a battery’s life.

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