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Wednesday, April 16, 2014

The ABCs of deep cycle battery labels

My father used to muse, "Love doesn't make the world go round, numbers make the world go round." Well, on a long RV trip, a little lovin' ain't bad, but numbers, sad to say, make much of the RV world go round. And especially is this so when it comes to RV batteries. How long you can keep your electrons flowing is a critical factor in RV trips away from utility hookups.

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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But then come those "narsty" little ratings: CCA, CA, and MCA. Ah, the "A" family! CCA is the fractious one. CCA represents 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.

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.

5 comments:

  1. Good description! Deep cycle AGM batteries are in my opinion the best choice for your RV. They can be discharged much further before any damage is done than traditional lead-acid batteries.

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  2. Thank you very much for the simplified overview. I've been toying with the idea of increasing my storage capacity and this will be very helpful information.

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  3. One must remember that a 100 amp-hour lead sulphide battery has only about 50 amp-hours of usable energy while a lithium iron phosphate (LFP) has at least 80 amp-hours for several thousand cycles. We have about 10,000 watt-hours of LFP batteries on our fifth-wheel and are autonomous with 1400 W of solar and a 4.0 PSW Magnum inverter. There is an excellent discussion of LFP at Technomadia.
    http://www.technomadia.com/2011/11/lithium-update-3-lithium-battery-cost/

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  4. 6 volt golf cart batteries will last twice as long as AGM batteries, and are cheaper.. But need air circulation, and maintenance. LFP batteries cost "Way" more than other batteries, but have many more benefits. Less Weight, long life, and fewer batteries needed. Also requires special controllers that can understand lithium batteries, so you can't just change batteries alone.

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  5. And folks must take care to FULLY recharge their battery at least once a week while ion use. We read about folks boon-docking who run the battery completely down, then run the generator an hour just so the lights work again, or worse, start the rig engine for a few minutes and call that a charge. I find amp-hour tallying meters like Tri-Metric essential to get long battery life and know what is going on in the battery at any time.

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