Grant, you may have a bad battery. I just checked a group 27 Deka AGM we have on our shelf. The battery has a date code of March 2009. It has never been on a charger and shows 12.53v. They should be closer to 12.6v when fully charged and checked after a night of resting. It is a pain but you should disconnect all the batteries. Then charge and check them individually. Really it is best to have some sort of tester that can put a load on them. If you have access to a carbon pile you can use on of those. If you you are just going to use a voltmeter to test do the charge and then check them the next day after they have time to get rid of the surface charge.
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