If you decide it's a battery or charging issue, remember that AGM (absorbed glass mat) recombinant batteries, are pretty sensitive to charge voltage level and temperature.  It's best to use a temp-controlled voltage regulator with them so you don't burn them up.  As they get hotter the internal resistance changes and requires that the battery be charged at a lower voltage level. The B&C regulator has this capability (along with crowbar overvoltage protection) and is a great choice if you're using today's hi-tech batteries. Check your charging specs carefully to make sure you're not overcharging. Just a FWIW.

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