So there is a different test for house batteries. Remember a deep cycle battery has rating for capacity. This is usually referred to as the 20 hr capacity. That means for a battery like the Lifeline 4D which has a 210 Amp-hr capacity that it can take a 10.5 amp discharge for twenty hours (210/20).
You could take the batteries to some place that perform this test, I would try marine repair locations. It generally takes an electronic load, and temperature controlled water bath (battery should be at 77įF)
The fact that you said you admittedly abused the batteries would lead me to believe that you are experiencing normal capacity loss. Since it was a daily driver, I expect that it's main charging mode was through the alternator, which will take off some capacity in it's own right. You have probably also left the battery in a partial state of charge causing sulfation and accelerating capacity loss.
You should invest in a good battery monitor, and also make sure you have a switch to turn on your inverter only when you need inverted power. And then come up with a good charging scheme.
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