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Posted by D Sherman on Tuesday, September 30, 2008 at 20:14:22 :
In Reply to: Battery Acid posted by Joe Cimoch on Monday, September 29, 2008 at 06:56:06 :
The only correct way to add acid is to fully charge the battery (e.g. 5 amps for a couple days), check the electrolyte with a hydrometer, and add acid only if the specific gravity is still too low. Add distilled water up to the mark if the specific gravity is high. I have resurrected many "won't hold a charge" batteries by this method. What happens, especially on heavy duty batteries, is that even though a lot of the acid is permanently locked up as lead sulphate, and full charging therefore won't get the gravity up to what it should be, there's still plenty of lead left on the plates. Thus, by adding concentrated acid until the gravity is what it should be, you get some more life out of the remaining lead. By doing this, I've typically gotten 1 to 2 years out of 6TLs that I would otherwise have scrapped. Lighter duty batteries with lots of "cold cranking amps", and not much weight, can't be rejuvenated like this. They get their "CCA"s by having lots of intricate surface area on the plates, so more lead is in contact with the acid, but the fragile plates tend to fall apart and get sulphated into oblivion sooner.
As for shelf life, electrolyte doesn't go bad. Theoretically, it might absorb water from the air (concentrated acid definitely does), and dilute itself over time if stored in an open container. If you had to, you could theoretically make it more concentrated by boiling it; the water will boil off, but the acid won't. I haven't tried that yet. I used to get straight sulphuric acid as drain cleaner, and use that to rejuvenate the "lost cause" batteries, but now all the drain cleaner acid I've seen has other junk added to it.