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lead acid battery not recharging


I just took apart a battery backup surge protecter and I want to keep the lead acid battery and charger except the battery is bad. I tried leaving it to charge for a while, nothing. I filled the battery compartments with water as a youtube video suggested, still nothing. I've left it to charge for hours, but it only reads around 2 volts and doesn't get any higher. Any suggestions on how I can bring this battery back to life? Or if it's just a dud.


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Most Helpful Member
Probably dead..

There are two common reasons lead-acid batteries fail.

The plates simply break up over time and loose connectivity through all the material - or the loose bits end up in the bottom of the battery and eventually short it out.

Or, the plates can form an insulating or high-resistance surface film, but still be mechanically sound.
And both can happen to some extent at the same time.

With the first, it's scrap. With the second, a high voltage pulse "desulfator" can sometimes recover a lot of the battery capacity.

I've got several UPSs, plus alarm systems (mine and family) that use lead-acid batteries, and I've experimented a lot with a desulfator I got from ebay some years ago.

I've found that roughly half the "dead" batteries which have been on float charge but otherwise little used for 3 - 5 years before failing can be recovered to a fair proportion of original capacity, though I've never put those back in to anything critical.

The desulfator design is a simple circuit and easy to build, if you want to experiment.
A couple of examples:

I've never had any "wet" cells to experiment with, but according to the theory a layer of a lead sulphate type compound can form over time on the surface of the plates, which is high resistance and is taking sulphur from the sulphuric acid electrolyte, weakening that.

The high voltage pulses are supposed to electrolyse the compound back to lead and release the sulphur, so if it works the specific gravity of the electrolyte should increase as the process happens, over a week or two.

With the sealed / gel cells I have played with, I have no way of knowing what state they are in internally, if they have cracked up, dried out or whatever. I do know that some of them go from high resistance / low voltage and not taking charge to behaving like you would expect a good battery to, while it has no effect at all on others.

I use the gadget connected to a battery for a week or two, with a small plugtop/wallwart style automatic trickle charger connected at the same time. I've added ferrite chokes with several turns to the leads from that to minimise the loading it has on the device pulses.


Alright, well if this lead acid is most likely unusable would the charger work for any 12 volt lead acid battery? Or is there a current limit for the input?


Well-Known Member
Most Helpful Member
It should be OK with any lead-acid battery of equal or greater capacity; lower capacity ones could be charged at too high a current, depending on the UPS design.

Note that normal car batteries are not deep cycle rated and will be wrecked within a very few cycles, if run down to anywhere near flat.

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