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Lead/acid batterys like we all used to run required constant maintenance (which most never got) and running them above 13.8 would cook out the water pretty quick.
The gauges on these things aren't the most accurate, but 15 is good. It's likely running around 14.8 or so. The AGM batterys like being kept "hot" which allows them a longer life. They don't loose water since they're sealed, so boiling one dry doesn't happen.
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