What you have there is a set of completely hammered batteries. It sounds
like an off-grid system with parallel strings of batteries of different
ages being chronically undercharged., which is the renewable energy
equivalent of a car full of teenagers with guns and whiskey. Something
bad inevitably happens.
I'd recommend that you get an automotive style battery load tester and
clamp that on to each one in turn. Measure the voltage with your
multimeter, because those load testers have iffy analog meters. I'll bet
one will collapse under load.
Does this customer also have a generator? Sometimes with undersized PV
arrays and a generator you get a "gorge and starve" charging pattern.
The customer lets the batteries get totally flailed and then grudgingly
goes out and starts the generator. The batteries get blasted with high
amperage for a while, shedding positive plate material, and then get
starved again. Perfect storm.
Tell her that she needs a new set of matched batteries. If she balks,
politely tell her you can't help her, and let someone with less
judgement deal with it.
Good luck,
Hilton
--
Hilton Dier III
Renewable Energy Design
Partner, Solar Gain LLC
453 East Hill Rd.
Middlesex, VT 05602
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