I think I know the answer to this, but I'd be interested to hear a proper diagnosis. My old Audi has been given a holiday while I dodge the summer showers on my motorbike, and I was attempting to keep it from neglect by starting it up and running for a while every couple of weeks. This was not quite enough to keep the battery (not the original!) afloat, and having left it for slightly longer, the next time I turned the key, everything was dead. I put it on charge (in situ) and was surprised when the charger didn't immediately go to maximum (8A).
Instead, it started at 4A and sat there for 3-4 hours, before creeping up to 8A, where I left it for a while before switching it back to 'slow charge', which then registered 2A. At this point, I thought that perhaps the charger had an overcurrent limiter that would select the lower current if a battery was very depleted, but I think this is unlikely, as it is only a cheap device.
I checked the levels, which were OK, having been topped up in the spring. I left it on the low current overnight, with a view to turning it back up again this morning, but despite the lower output setting, the current had crept up to 6A, and the battery was fizzing away nicely. I disconnected everything and checked the voltage (should have done that at the beginning, I know) and it is struggling to make 11V. That suggests a dead/shorted cell to me, but I don't know why that would manifest itself in this fashion.
I've resigned myself to replacing the battery, but any explanatory thoughts would be gratefully received.
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Have a look at the thread
www.honestjohn.co.uk/forum/post/index.htm?t=22238&...9
before jumping to any conclusions. If it has been gassing for some time during charging you may find that the internal resistance has gone very high. This may may drop when (if?) the bubbles disperse.
This however would not account for a no load terminal voltage of only 11v. (But if it was very high resistance AND there was some current drain when you measured it??? this could account for 11v).
Based on my experience of the Fiesta battery outlined in that thread, the battery recovered and is still working perfectly ok. (I still carry jump leads tho'!) It has now fully recovered from inexplicable total 'flatness' twice in 2 years, both times in identical circumstances of a sucessful high starting current after an extended period of rest. Everybody predicted terminal failure as being imminent.
PeterD gave some comprehensive answers but no one has fully explained the physics/chemistry of failure.
pmh (was peter)
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Thanks, PMH - I have now read and digested most of it!
I'll try some of the more basic suggestions (e.g. getting rid of the bubbles) and maybe even the ethylenediaminetetra-acetic acid (I shall enjoy asking for that) as the car is off the road for a few more weeks. I was surprised how suddenly it went, though!
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Bubbles from only one cell, or all of them?
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Only one cell, initially (before I left it on 'slow' charge overnight), which was what led me to believe that one cell was the problem. The following morning, with the charge current up to 6A, everything was bubbling nicely!
That was when I disconnected the charger and measured the 11V, but when I measured it again an hour or two later, it was down to 6V! Up to then, it had been connected in the car, so I removed a terminal in case something was sucking the power away, but the background consumption turned out to be only a few mA. The voltage has now fallen to below 5V, so I think I may have to pronounce it dead!
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