Posted by MarkB_MI on April 29, 2009 at 02:55:38 from (216.234.126.146):
In Reply to: Electric math problem posted by jdemaris on April 28, 2009 at 09:54:13:
Amp-hour ratings of batteries are the "area under the curve". That is, when rating your batteries, they probably didn't discharge them at a fixed currect, but rather adjusted the load for a fixed voltage as the battery discharged. Hence they are rated at "12.5 volts", which might be 20 amps when fully charged and 100 milliamps when nearly discharged. They plot the resulting current versus time curve, and measure the area under that curve. The area equals the amp-hour rating for the battery.
Now for low-current loads you'll probably get very close the the rated amp-hours. But for a high current load, the voltage will quickly drop off causing the current demand to increase and a further drop in voltage. The result is that you'll be discharging the battery much faster than the rate at which it was rated, so you can't expect to get the full rated amp-hours. I'd suggest you assume you can get fifty percent of the rated amp-hours. You might do a little better, but you probably won't do any worse.
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