I have been going through my late mothers and father-in-laws stuff, and there are a number of small VRLA batteries which have been flat for some time. I started with a pair originally used for a stair lift, 7 Ah each, the 3.8A Lidi charger would not recharge them as it went over voltage, so I put them in parallel with a 75 Ah leisure battery which also needed recharging, because it is a so called "Smart" charger I could just leave it on 24/7 however I powered it through a wifi connected energy meter one of the MiHome Energenie range so I could monitor progress with the PC. I was surprised, I expected a gradual recovery, this was not the case, after around 10 days within 2 hours the charge rate went from 0.1A to 0.8A and then held at 0.8A for 6 hours, then slowly back to 0.1A over another 2 hours. The charger has 4 charge rates, 0.1, 0.8, 3, and 3.8A and once charged it will jump to 0.8A when volts drop to 12.8 volt and back to 0.1A once voltage reaches 14.4 volt, so the lead in and out was a mark/space switching between the two rates, not an analogue rise and fall. Both the 7 Ah batteries did the same, and once that charge had completed they seemed to be fully functional. So second battery was in a power bank, 20 Ah this had not been left flat for so long and took just two days, so now second 20 Ah battery this one out of a wheel chair, clearly looking at the 7 Ah batteries I must give it two weeks, and since this one has been left the longest it will likely take the longest to recover. However at what point do I say scrap it? As a radio ham I do use 12 volt batteries so these fully sealed batteries are great, but I have been dumping old batteries mainly US because at some point over charged and dried out, but until this year I did not have a charger where I could look back and see charge rates change, so I was unaware of this point where the battery suddenly recovers. I thought it was a gradual thing. So asking if others have tried reviving sulphated batteries, and what they found?