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post #5 of Old 10-15-2013
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Re: Problem with new batteries keeping the charge

Dog, are you absolutely, positively, checked-it-with-my-own-hands-and-eyes certain that when you leave the boat for a week, nothing is connected to the batteries? No bilge pump, no nothing?

Brand new batteries typically are rated at 12.6-12.7 volts, you would need to check with the maker to find out what the right number is for yours. And then check the date code on the batteries, it is usually stamped or embossed near the top, cleverly hidden under a label on the side. Batteries should not sit on the shelf too long before purchase, so if you bought them in August, a June/July manufacturing date is about as old as you should see. If they sat around, they probably also sulfated and wouldn't see 100% charge again.

So there's the voltage they were designed for, and a question of whether they were sitting, and then once you shut down and leave the boat it can easily take 24 hours for the float charge to dissipate then the electrolyte to equalize, and the "true" charge to show. On top of all that, you need to check your voltmeter, if it is not calibrated it can easily be off by one or two tenths of a volt on a 20 volt scale.

Check your numbers, check your measurements, odds are they are not discharging as much as you think. If everything double-checks and they really are discharging that much, you'd have a battery problem. Odds are, it is a "measurement" problem.
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