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Old 05-31-2012
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Re: Voltage puzzle

David-
What d0n said. You don't need a sticker on the back of your meter is you just remember that 12.6 is the normal full charge, and one volt equals 100% of the useful charge. So every tenth of a volt that you are down, is a 10% loss. A simple 1:1 relationship.
For batteries that have rested overnight, or had the surface charge pulled off by a few minutes of a decent load, this is perfectly good enough. Even the battery makers are getting away from hydrometers and nasty acid whenever they can. A hydrometer will spot a bad cell in a wet acid battery, but a voltmeter is a good enough first step.
You can use one of the electronic testers that measure internal resistance (instead of the old carbon pile load testers) for another "point of view".

I question whether those are true deep cycle batteries since no amp-hour capacity is given for them. If they are typical, i.e. 80AH each, you'd have 160AH when new. At 60%, you'd be down to 96AH. After an hour and a half of motoring, maybe with a 50A alternator putting out 35A average, you might be back close to 140-150AH capacity in a perfect world.

So by the numbers, it should be possible that they recharged. But even then, a reading of 12.9 on a battery that has been standing overnight? Should not be possible unless the meter is off. I gave up on meters, unless you've got a Fluke or similar sold pre-calibrated and still IN calibration, you need a calibrated voltage source to see if the meter is lying.

If the battery is at 12.6 and the meter is off by 1/2%, the disaply could be off by 1/10th of a volt. If the meter is off by 1%, the reading could be off by 2/10ths of a volt. Then it gets worse, since the analog-to-digital converter circuit in most meters will also be off, causing the rightmost digit to float by 2-3 places. So the voltage could well be 12.6 on the battery, showing as 12.9 (or 12.3) on a meter.

You can't simply trust your meter, just because it shows those nice clear digits. They're good for relative readings (today versus yesterday) but not absolute ones, unless you can check the calibration.
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