If anyone is still interested in this topic, Jerry's suggestion of adding a small amount of fresh acid to the battery to help start the charging cycle brought to mind some of the 'battery rejuvenators' that have been sold in the past. I think sulfuric acid , battery acid at 1.280 specific gravity, was sold as an additive for that purpose, but I don't know of a brand name. Another of those was a product called VX-6, which I think was cadmium sulfate, and came in a package of three or six tubes for 6- or 12-volt batteries. Another 'rejuvenator' was epsom salts, which is mangnesium sulfate, and I believe was also sold as a solution to be added to a 'dead' battery. The common point in all of these additives is the sulfate ion that transfers charges within the battery (sulfuric acid can be thought of as 'hydrogen sulfate' if you like). Add a few more sulfate ions, and you get a little more current from the battery, but none of these treatments was/is a substitute for properly charging the battery, or replacing it if necessary.
Also, Jerry's observation that 12-volt batteries don't seem to have the recovery potential of 6-volt batteries has been my observation as well. I think that difference might be because 6-volt batteries typically had/have thicker plates, which allowed a 'natural' recovery after running one down quickly, as with a hard-starting engine. It was possible to draw a 6-volt battery down to the point of no-crank, but if left alone for 10-15 minutes, it recover enough to crank the engine fairly fast for a few more seconds. A good 6-volt battery would do that two or three times before there would be no more recovery. And charging was necessary-- and correcting the engine problem.
Just in case anyone is still interested...
Harvester
