How to Check Battery Holding Time?

Assuming they are 6v batteries, the variation between 6.7 and 7 volts doesn't seem extreme, though maybe a bit unbalanced. How many amp-hours are they, and are they all the same size in that respect? (80kva x 10h seems like a lot.)

For routine maintenance, maybe check electrolyte levels biweekly, topping up with distilled water as needed. Check specific gravity about bimonthly, adjusting with battery fluid as needed. Wear face mask and gloves to guard against splashes.

Yes, I had a few of those. The floor needed reinforcement, as every battery element weighed in the neighborhood of 200 kg. You are right on the need to do individual testing, and there is no easy way to do it. Here is the basics: sulfuric acid is presents everywhere. the question is just the concentration. The only safe way to make metal to metal torqued down connection is lead to lead. It is the same metal as the battery itself, so it will withstand acid vapors. But, it is flowing under pressure, it has no springiness whatsoever. It needs retorquing every couple of months, cleaning every year or so, in order to carry the thousand Amps or so, without major voltage drop.

And that is the rub. Voltage drop on these contacts prevents full charging, and proper discharging too. You need to reform every single cell, out of the string, I am afraid. How to break the string, you might have to contact the maker to do it safely. You will need a proper charger to load up the individual cell properly, including removing any sulfatization. For the right instruments to help you in the job, check out Keithley Instruments.

Good luck, and be ready to labor long, for good results.