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dodgy lead acid battery

Started by Unknown April 30, 2018
Have one of those 12v jump starter light compressor things, don't know where it came from. The allegedly 17Ah battery in it showed no output, so hooked it up to an old dumb unregulated charger & left it, walking by it now & then. Charger put out about 15v.

It began to take a tiny trickle of charge, hardly any. Then to my surprise I saw it sucking up 3A of charge! What's more it appeared to work, at least on a 1W load. But the charger switches every so often between 4A charge and near zero, with the built in battery voltmeter showing about 11v when 4A, 15v when near zero amps. With charger disconnected the battery sits at about 8v - I've not yet measured its terminal volage when it's behaving better.

Obviously the battery is in a sorry state, but what exactly is wrong with it? I don't expect to replace it, not worthwile.


NT
On Monday, April 30, 2018 at 12:25:04 PM UTC-4, tabb...@gmail.com wrote:
> Have one of those 12v jump starter light compressor things, don't know where it came from. The allegedly 17Ah battery in it showed no output, so hooked it up to an old dumb unregulated charger & left it, walking by it now & then. Charger put out about 15v. > > It began to take a tiny trickle of charge, hardly any. Then to my surprise I saw it sucking up 3A of charge! What's more it appeared to work, at least on a 1W load. But the charger switches every so often between 4A charge and near zero, with the built in battery voltmeter showing about 11v when 4A, 15v when near zero amps. With charger disconnected the battery sits at about 8v - I've not yet measured its terminal volage when it's behaving better. > > Obviously the battery is in a sorry state, but what exactly is wrong with it? I don't expect to replace it, not worthwile.
It is hard to say without measuring individual cell voltages, but I bet you have one cell shorted and the others may not be in very good shape. Taking current doesn't mean it is storing the charge. The question is how much current can your draw? But with a no load voltage of 8 volts it obviously isn't going to do much. Rick C.
On Tuesday, 1 May 2018 03:05:01 UTC+1, gnuarm.del...@gmail.com  wrote:
> On Monday, April 30, 2018 at 12:25:04 PM UTC-4, tabby wrote:
> > Have one of those 12v jump starter light compressor things, don't know where it came from. The allegedly 17Ah battery in it showed no output, so hooked it up to an old dumb unregulated charger & left it, walking by it now & then. Charger put out about 15v. > > > > It began to take a tiny trickle of charge, hardly any. Then to my surprise I saw it sucking up 3A of charge! What's more it appeared to work, at least on a 1W load. But the charger switches every so often between 4A charge and near zero, with the built in battery voltmeter showing about 11v when 4A, 15v when near zero amps. With charger disconnected the battery sits at about 8v - I've not yet measured its terminal volage when it's behaving better. > > > > Obviously the battery is in a sorry state, but what exactly is wrong with it? I don't expect to replace it, not worthwile. > > It is hard to say without measuring individual cell voltages, but I bet you have one cell shorted and the others may not be in very good shape. Taking current doesn't mean it is storing the charge. The question is how much current can your draw? But with a no load voltage of 8 volts it obviously isn't going to do much. > > Rick C.
I didn't have any half decent tools or instruments with me, will get more info soon. 8v might be enough to run the compressor, but I suspect ESR will say no. Sometimes it gives around 12v, sometimes 8. NT