Refurbishing dead SLABs?

I use quite a few 12V 7Ah Sealed Lead Acid batteries (SLABs), which I normally obtain as surplus due to routine replacement in UPS equipment. Recently, this source hasn't been available due to the pandemic, and I find myself with one usable battery and two dead ones.

The two dead ones are reading 5V and 10V. Shaking the one reading 5V, it audibly rattled! Clearly, it wasnt as wet in there as it should be!

So, I thought i'd try and refurbish it! The lid of these is lightly glued on, and it was fairly easy to prise it of using a sharp chisel. Under the lid, the cells are closed off using pushed on rubber caps. These easily lifted off, revealing the plates of the cells, all but dry!

As it happens, I had a bottle of de-ionised water on the shelf in the workshop. Using a syringe, ive refilled the cells up to the top, and then gently sucked the excess out of the filler holes. This proved to be a rather messy job, so a roll of paper towel was kept handy to mop up any spills and splashes.

With each cell refilled, the caps replaced, and the lid put loosely back on, it was time to try it on the charger. For this, I am using my 'semi-intelligent' SLAB charger, and monitoring the current into the battery using a multi-meter. This showed the current rapidly ramping up! Ive left it for now as the current was passing 60mA and climbing. A check of the battery voltage showed it to have risen to well over 7V at this point.

Whether the battery will recover enough to be useful, who knows. Its certainly worth a try.

Now, one thing to note - I have added more water to this first test battery than required. There is a reason for this - I dont know yet how easily the glass matting absorbs the water. It may be that it takes several hours to do so. Supposedly, the correct amount of water is just enough for the matting to appear wet.  I will top up the second battery more slowly and observe.

The first battery had dropped to about 6.5V over the 7h since I stopped charging. On restarting the charge, the current input was up at 400mA and rising. The charger can supply 1.5A. As it is now daytime and I can keep an eye on it, I will allow it to charge during the rest of the day and see how it does. For safety, it is in a plastic box and covered with paper towels.