Re: Battery question


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Posted by D. Sherman on Wednesday, January 30, 2008 at 17:54:44 :

In Reply to: Battery question posted by Tim Holloway on Wednesday, January 30, 2008 at 17:09:38 :

It can't hurt to try, but I wouldn't hold out much hope. I've gotten a couple years' extra life out of what appear to be completely dead sulphated batteries by running 5 amps through it for a week or so. This is where an adjustable battery charger really comes in handy. My chemist friend tells me that sulphuric acid doesn't evaporate, unlike hydrochloric acid, which means that as the water evaporates, the acid gets stronger. He claims you can even turn battery acid into concentrated sulphuric acid by simply boiling it on the stove. On the other hand, concentrated sulphuric acid is incredibly strong and sucking moisture out of anything, including the air, so it will only naturally evaporate until it gets down to the point where it's strong enough to hold onto all the water it has left.

In any case, I would top it off (cover the plates) with distilled water and give it a nice gentle charge of 5 amps, checking the current and the specific gravity (get a good hydrometer, not the kind with the floating colored balls) every day or so. Most likely, cells won't charge evenly. That's okay. Over-charging doesn't hurt a wet battery like that so long as you keep the electrolyte topped off. This is a case where an "automatic" charger will not work, but a manually-adjustable one will. When you first hook it up, you might have to turn the voltage up to 20 or 30 volts in order to get it to start drawing current. Once it starts drawing, it'll rapidly draw more, so you need to turn the voltage down as needed to keep the current below 10 amps or so. Once you've got it down to 15 volts or less, and it's drawing 5 amps, you can safely go away and do something else.

If, after a week of charging, you can't get the specific gravity up to what it should be, that battery is probably junk, but there's one more thing you can try. Suck out some of the water from the weak cells and add electrolyte. I know they tell you not to, but at this point, there's nothing to lose. What's going on is that there's a certain amount of lead sulphate that you're not succeeding at converting back into lead. However, the plates of a battery, especially a high-end one, are made with more lead than all the acid in the battery can ever eat. If you add more acid, you can us up more of the remaining lead. The risk is that if you eat all the way through the plates, they'll fall apart and probably short out the cell, resulting in low voltage, but even so, you've gotten some extra "free" life out of the battery before that happened. Even if you do succeed at bringing this battery back from the dead, it will never have the same capacity it had when new. Any lead acid battery that's been completely discharged will never be the same again. But, at that point every extra year of life is free, so it'll start the engine and will hold a charge between starts, you might as well keep using it.

All of this is why I prefer wet lead-acid batteries over the modern sealed ones. There's no way to "doctor" the sealed ones, and once the water's gone it's gone, so overcharging will kill them. I have a pair of sealed 6TLs that I got cheap, and I'm keeping them on a trickle charger until I can use them, but I would much rather have the traditional kind. The only advantages I can see of the sealed ones is that the won't spill on a steep incline, and water won't get in while fording. The caps on normal batteries do a decent job of letting gases through, but keeping electrolyte in and water out, so unless it was extremely severe service, I can't see this being very important.



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