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Thursday, July 23, 2009

Demystifying RV Battery Capacity

A frequent question appearing on RV forums, "What is the formula to convert CCA (cold cranking amps) to amp-hours?" It's a good question, because sadly, many battery manufacturers don't really provide information that's useful for RVers. It's important to have a handle on battery capacity--how much juice you can take out of a battery--as you'll need a sufficient amount of battery power "in the bank" when you go away from shore power hookups.

First, the most useful figure is that of amp-hours. Some battery manufacturers use a different term, the "20 hour rating." Simply stated, amp-hour capacity tells you how many amps can be taken from the battery over a 20 hour period before it is completely discharged. For example, a 110 amp-hour rating means you could pull 5.5 amps per hour continuously for 20 hours before the battery would be depleted. How'd we do that? Divide the amp-hour rating (110) by 20 (hours) and the result is 5.5 amps.

A related rating is "reserve capacity." It's a rating of how many minutes a battery can continuously produce 25 amps before complete discharge. By multiplying the reserve capacity minutes by a factor of .65, you'll get an approximation of that battery's amp-hour capacity. We say approximation because 25 amps is a pretty big load. Most RVers won't be using 25 amps, certainly not continuously until discharge, so the reality is, the actual "usable" amp-hour capacity would probably work out a bit higher.

But then come those "narsty" little ratings: CCA, CA, and MCA. Ah, the "A" family! CCA is for "cold cranking amps," that is the maximum amount of amps that can be produced by a battery in 30 seconds at 0 degrees Fahrenheit. MCA is for "marine cranking amps," similar to CCA, only at 32 degrees. And CA, or "cranking amps" is the same rating as CCA. All three of these characters are useful for comparing batteries you need to start your engine with. BUT for RVers, when considering storage of electricity for use in running lights, pumps, fans, televisions, etcetera, they are USELESS.

Some have tried to advance the idea that you simply divide the CCA number by 6, and viola! The result is an approximate amp-hour capacity figure. But Billy Joe, it just ain't so. Here's a couple of real-world examples. A Yuasa 68MF battery is rated by its manufacturer as a 525 CCA battery. By this "divide by 6" theory, the amp-hour capacity should be 87.5. But in reality, Yuasa says the amp-hour capacity here is but 68 amp-hours. Meanwhile an Optima D34 battery has a much larger CCA rating--870. Divide that by 6, the"predicted" amp-hour capacity is 145, bad sadly, the reality is the rated amp-hour capacity here is even less than the Yuasa. Optima says the D34 is rated for only 55 amp-hours. So in our photo, that huge 1465 CCA might not be worth a hoot for powering your bedside reading light--you just can't tell.

So what gives? Battery plate design and size and other factors simply make a "CCA to amp-hour" comparison an impossible, "apples to oranges" scenario. When shopping for RV "house" batteries, those that will be used as "deep cycle" storage units for operating your coach equipment, stick with amp-hour capacity wherever possible, or gamble a bit with "reserve minutes."

And remember too, discharging a deep cycle battery to the very bottom means you'll get very few discharge-recharge cycles. Bottom line there--expect to buy house batteries far more often. The old rule of thumb for boondockers really does apply: Don't discharge your deep cycle batteries to less than half their capacity before recharging them. In practical terms as we've often said, 12.2 volts (without a load on) is the recharging point for better battery life expectancy.

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