Preserve Your Reserve Power: Helping Your RV Battery Last

By Dan Sanchez and Fred Wehmeyer

Many RV owners have a deep-cycle battery to power the living quarters of their vehicle. At a developed campsite, we mostly plug into the electrical outlets and don’t think about how much power we’re using. But when we venture to primitive campsites or take boondocking trips to areas without power, keeping these batteries in top condition is essential.

Deep-cycle RV batteries are made to provide longlasting power, but it’s important to make sure they’re not drained beyond the manufacturer’s recommended depth of discharge (DOD). The DOD is the total amount of power drain on your deep-cycle battery versus its rated capacity at that rate. In general, battery manufacturers do not recommend draining your battery beyond 50 percent of its rated capacity (50% DOD), or it will deliver shorter cycle life. Therefore, while boondocking or when parked at primitive campsites, it’s important to be aware of how much of a load you’re placing on your battery over time.

Preventing Deep Discharges

The best way to prevent draining your deep-cycle battery past 50% DOD is keeping an eye on what state of charge it’s in. A battery monitor is an easy way to do this as it provides a visual way to see the battery’s actual voltage or state of charge at any given time. These require installation, and depending on the models, some can be expensive, but you will see at a glance what condition your battery is in, which makes it easy to monitor your usage when it’s your only source of power. Other methods include checking the batteries with a hand-held voltmeter set to measure DC voltage in the 12-volt range. Another inexpensive but more accurate way is to use a battery hydrometer, which looks somewhat like a turkey baster with a float and measures the density of the battery’s electrolyte (specific gravity) to determine its state of charge. Some indicate this in green, yellow, or red for high, medium or low, and others use actual numbers for specific gravity. Deep cycle batteries should read 1.270 or higher at full charge.

Using a battery monitor or voltmeter, if a battery open circuit voltage (OCV) reads 12.7 volts or higher, it’s in a good state of charge. If the OCV drops to 12.1 volts, it’s 50% discharged, and for optimum cycle life, it needs to be fully charged at that point. If you’re using a hydrometer, remove the vent caps on each cell of the battery and measure the specific gravity of the electrolyte in each cell. This measurement is very accurate, but must be done with protective eyewear, rubber gloves, and plenty of ventilation. Specific gravity readings of 1.270–1.280 average across all the cells indicate a fully charged battery when checked at an 80-degree battery temperature. Check with the battery manufacturer’s specifications to determine full charge specific gravity for your battery.

If you’re using the hydrometer method, battery manufacturers also recommend using a simple correction factor to compensate for temperature variations. Subtract 0.004 from your hydrometer reading for every 10 degrees below 80 degrees fahrenheit, or 5.6 degrees below 27 degrees celsius. Let’s look at an example—say the temperature of the electrolyte is 50 degrees fahrenheit, and your battery specific gravity reading is 1.250. In this case, .004 for every 10 degrees equals .012, so you must subtract .012 from your reading. Subtracting this from 1.250, your corrected specific gravity reading is 1.238. For higher temperatures, add 0.004 to your hydrometer reading for every 10 degrees above 80 degrees fahrenheit, or 5.6 degrees above 27 degrees celsius.

If you’re able to keep your deep-cycle battery at or below 50% DOD, you can make it last much longer and avoid having to replace it as often. Keep in mind that flooded lead-acid deep-cycle RV batteries also require watering from time to time to ensure the battery cells are properly covered. Following these methods will help you increase your RV’s home power substantially and conserve the life of the battery. ■

About the Authors

Dan Sanchez is a journalist with more than 30 years in the publishing and marketing industry focusing on the outdoor, energy, and automotive fields.

Fred Wehmeyer is Senior VP of engineering at U.S. Battery Manufacturing and has more than 40 years of experience in design, manufacture, and quality assurance testing of various battery chemistries.