We're often faced with RVers (and others who should know better) spouting off about CCA numbers related to battery capacity. CCA numbers are for "Cold Cranking Amps," and are a measure of the muscle the battery has available for turning over an engine. Repeat after us: "There is no solid relationship between CCA and battery capacity."
We know we'll hear of chorus of, "Ah, you're full of it!" Full of beans, maybe, after the hot dog roast last night. But let's hold the horse for a minute and talk about the REAL figure that explains a battery's ability to do the work most RVer's need – providing juice for lights, water pumps, cell phones, etc. That number is expressed as of amp-hours capacity. 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.
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Here's where we aren't full of it. 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.