Hydrometer.Several things to do: After charging, let set a few hours and check electrolyte with a hygrometer. All cels sjould be equal in readings. A big differential means a bad cell. Do the cells have external links or are they inside the case? If external, you can read each individual cell for voltage. If you load test the batteries, use a mirror and look at the top of the electrolyte in each cell. it should be clear. if grey or black, the plates are bad - discard the bettary. If you look at the electrolyte and see one or 2 cells gasing off and the others are more quiet, then the gassing cells are bad. discard the battery. iff all cells are gassing evenly, battery probably OK. In old battery packs, that had individual cells, the man in charge would shift cell position to bring up lower charged cells and lessen charge to higher ones, using a hygrometer to check the electrolyte. Old school but it worked. cant do that with sealed batteries.
Valve of sorts. a rubber cap on a pillar/tube with slits in it.. During normal use very small amounts of hydrogen can be evolved, the pressure relief valve opens to allow the gas to escape and then reseals to prevent oxygen getting into the cell. I have stripped down lots of VRLA cells and the AGM separators require hard wringing to get out sufficient acid for a test.
Rain, collected and settled in a barrel..
Now you have added the water, and I assume it was de-ionised or distilled, you can only float charge the cells and see what happens. With great care very this insulated prods can be inserted into each cell to read individual voltages.
Far more than i ever wanted to know! but thanks for your time on this.Your cells are indeed VRLA AGM lead acid and the 150a/hr is at the 10 hour rate. The cells look to be very similar to many used in the telecoms industry for footprint. This looks to be a close match https://www.power-sonic.com/wp-content/uploads/2018/12/PGFT-12V160 technical specifications_US.pdf this data sheet gives lots of discharge data
http://www2.exide.com/Media/files/Downloads/IndustEuro/Marathon_M_FT_en.pdf and https://www.sbsbattery.com/media/PDFs/Battery-12AFT150.pdf
I don't see that happening on these. My hand's aren't steady, and the hole is less than 3/8" and I can't see in, I'd guess the top of the plates is more than an inch below.With care two insulated bare tip prods can be inserted into each cell and placed on a +VE and a -VE plate to get cell voltages. I have done it and it takes a steady hand. Insulated and thin, as a short between plates through the probes can be dangerous.
Ah good to know. They start at about $50 down here, but I suppose it's an investment in certainty.To measure the acid SG in VRLA cells we used an optical refractometer that only needed a small drop of acid; like these https://www.amazon.co.uk/Refractometers/b?ie=UTF8&node=3579771031 But for our flooded cells it was the traditional float type.