I just bought a very inexpensive battery maintainer from Harbor Freight to charge an extra wet-cell battery that I have. One of my intentions is to test the battery capacity this winter.
The battery is a group 27, combination starter/deep cycle battery. It is in the garage, where it will be between 50 and 20 degrees this winter. Before applying the charge, I checked the specific gravity of one of the cells and it was well into the red on the gauge. The battery needed to be charged.
The charger is spec'd for 1/2 amp and to maintain the battery at 13.2 volts. I've had it connected to the battery for a week now and the voltage is consistently maintained at 13.2 volts.
First question: At 13.2 volts, is there a limit to how long I can keep it on the charger?
After the week of charging, when I take the battery off the charger momentarily, it shows 12.5 volts. (Haven't re-tested the specific gravity yet.)
I'm planning to do a load test on the battery with a 12v, 25 watt element that I used for the wind generator's dump load. (Meaning about a 2 amp load.) I figure I'll take 25 amp-hours out, let it sit for a day and check the resulting battery voltage and specific gravity. In this way I'll know how many amp-hours the battery is still good for.
Second question: Anyone see any issues with my battery testing plans?
(I have a fantastic, Victron battery monitor on the boat, but I'm doing all this at home. I'll just keep an eye on the voltage across the 25 watt element and use the V=IR equation to track the current usage and get to a fairly exact 25 amp-hour draw-down.)