I think that the capacity loss in batteries with a broken sealed top, is NOT due to drying out. Concentrated sulfuric acid will absorb moisture from the air and become more dilute! ( up to a point)
My guess is that the damage is caused by oxygen from the air reacting with the negative plate sponge lead, forming lead oxide. This locks up the active material, and massively unbalances the state of charge on the two plates. One time when I was visiting my local scrap metal yard, I saw guys cutting up large sealed batteries. They piled up the negative and positive plates separately. The brown positive plates did nothing, but the pile of negative plates began steaming in the cold winter air within 5 minutes. So fast was the rate of oxidation!!! Short term exposure to a bit of air while you add some water should not be a problem. Just be sure to reseal it! If the battery has had air constantly available, for some extended period, then nothing short of a formation charge can possibly revive it. This has lots of hydrogen produced right inside the plate by splitting water, the hydrogen "should" react with the lead oxide and turn it back to lead. Just what shape (proper porosity etc) this will leave the plate in, is anybody's guess! Regards Steve
