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12AH is the "Capacity" (C) of the battery and it stands for "12 Ampere Hours" which means that if the battery starts out at 12 volts, fully charged, and you pull 12 amps out of it for 1 hour, its voltage should fall to a certain voltage, (the cutoff voltage) usually about 10 volts, at the end of that time, and then you'll have to recharge it. If you don't, and continue to use the battery it'll get damaged. More or less, depending on whether it's lead-acid or NiCd or whatever.BUT, there's a catch with C. To get the full capacity out of the battery, you're only allowed to pull current out of it at some rate smaller than C, usually C/10 or C/20. That means that if you have a
12AH battery and its rate of discharge is specified as C/10, you'll only get to use its full capacity if you pull 1.2A out of it for 10 hours, then you'll have to recharge it.In your case, since you have 12AH batteries and you want to pull 1.2A out of them they should last for 10 hours (1.2 amps X 10 hours = 12AH) if they're rated for C/10 until they have to be recharged but, not knowing what rate they're allowed to discharge at to realize C, it's hard to say. If you can find out who made the batteries, though, you can go to their website and they'll probably have the spec's there. If you can't, but you know the chemistry of the batteries, then you can got to anybody's website who manufactures batteries with the same chemistry and get their specs for a similar capacity vanilla battery. Chances are the spec's for yours will be pretty close to theirs.
-- John Fields