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Posted by D. Sherman on Tuesday, May 20, 2008 at 22:16:10 :
In Reply to: First time charging of a new battery posted by Mark in NJ on Tuesday, May 20, 2008 at 21:53:30 :
"12 volt" means 12.6 volts when resting at full charge at room temperature. That's strictly a function of how many cells there are in it.
"30 ah (10hr) rated" tells how much electricity it can store. Batteries are rated in amp-hours (ah). 1 amp-hour means you can draw 1 amp out of it for an hour. 30 ah means, in theory, that you could draw 30 amps out of it for 1 hour, 1 amp out of it for 30 hours, or 300 amps out of it for 6 minutes before it went dead. In practice, it's not that simple. How many amp-hours you can get out of it is somewhat dependent on how fast you pull them out. Within reason, you get more amp-hours if you discharge a battery more slowly. That's where the "(10hr)" rating comes from. That means the 30 amp-hour rating is specified only if you discharge it over a period of 10 hours. In this case, that means a discharge rate of 10 amp-hours divided by 30 hours equals 3.33 amps.
Now to the charging part. With most batteries, it's safe to charge them at what's called a "C/10" rate indefinitely without damage. Charge them faster than that and you need clever charge controllers to avoid damaging them. "C/10" or "1/10th of its rated capacity" means you take the capacity in amp hours (30 in this case) and divide it by 10 to get the recommended charging current, which in this case would be 3 amps. The manufacturer would like you to push 3 amps into it for 3-5 hours.
There's no real way to convert this to volts since the conversion depends on the internal resistance of the battery charger and the nonlinearity of the rectifiers in it. Your best bet, assuming your battery charger has an ammeter, is to hook it up to the battery, and find the setting that gives you closest to 3 amps without going much over. Charging slower than recommended won't hurt a thing. It'll just take longer. If, for example, your battery chargers 2 amp range, really does put out about 2 amps, you might want to leave it on charge for 3/2 as long as they recommend, which would be 4.5 to 7.5 hours. Most car batteries can take 5 amps or so for a long time without damage. It's always good to top off the electrolyte using distilled water after a prolonged charge, what they're recommending is very light and isn't going to "boil" any significant amount of water out.
I hope that makes sense. Personally, I like to see a battery rated in actual amp-hours rather than just "cold cranking amps" since the latter is mostly about the surface area of the plates, whereas the former is about how much material is on the plates. They can get a lot of surface area, and hence a lot of "CCA"s by making highly-textured plates, but that makes them fragile and liable to wear out quickly or fall apart from vibration. Amp-hours (ah) is a more professional rating to my mind.
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