Re: Voltage puzzle
If you have no heavy loads to use, disable the engine and crank the starter a bit, use the starter as a load. It should be good for a 15 second run, ten minutes of rest, another 15 second run, that way there's no danger of overheating it but you'll certainly knock off any surface charge on the batteries. And of course, make Real Damn Sure the raw water inlet is closed when you do that, so you can't hydrolock the engine.
If for some reason that's impractical, get any high-wattage auto headlamp or bulb i.e. 65-100 watts, and some jumper wires or alligators. Connect to batteries and leave it on for 5 minutes, check again at 10 minutes. If you don't mind dumpster diving, you can often find a dual-beam headlight behind any auto parts store, with the low beam burned out and the high beam (what you want) perfectly good.