Flat battery

sometimes you can a really rapid plate voltage drop when it desulphates.

Still shouldn't damage an old basic charger. Anyway most have fuses to prevent internal damage.

Then the current goes through the roof, then they die. Obviously if the fuse works it'll be fine, but I've replaced several charger diode packs & I would suggest the fuse is normally inadequately matched to the diode pack.

IIRC, there was very little inside the back of the charger and no fuse apart from an auto reset circuit breaker that kept clicking once the rectifier had gone short circuit. I replaced the bridge rectifier £2 from maplin and good as new again.

In message , Redwood writes

Try starting the charge with a set of jump leads or boost pack connected, works on most of the 'smart' chargers I've seen.

If you have discharged it to the point that it cannot deliver the current then it has been deep discharged.

With the ignition in acc and the radio on I'd expect that the car will be taking an amp or so - that is sufficient over 2 days to totally discharge a petrol battery without problems.

Not necessarily. A small discharge can sulphate the plates very quickly: basically giving a very high internal resistance which stops it delivering the current although the electrolyte is still fizzing with stored energy. Knock the sulphate off with a short charge and the battery is, as if by magic, as good as new.

Been there, done that. Twice, in fact: once it was a radio left on overnight and once sidelights left on for three hours.

Maybe, but it's more likely that sulphated plates will have effectively choked off the discharge after just a few hours.

The OP says that everything seems OK after 30 minutes' charging - which strongly suggests sulphated plates and NOT deep discharge.

Ian

No, it implies it was flat. Small drains may sulphate a charged battery, but normally they make them go flat. Charging flat batterie normally makes them work again, charging sulphated batteries with float voltage doesn't.

The surface area of a battery plate will only fill up with sulphate when you have managed to discharge the cell more or less completely until then the softer sulphate will permit migration of electrolyte and the cell will conduct - it does not "choke off" the plate. A battery goes flat when the electrolyte is almost depelted

That's because a lead acid will still deliver a large current and maintain voltage close to the end of its discharge. A small charge will let it run without issues but you wont have much capacity and you will begin the har sulphation process that does cause problems.

Was the bollocks bit, and my answer " the battery will take as much of that as it can. " was correct.

Maybe you should give SIP your words of wisdom?

120 amp charge on a suitably specced high capacity battery and I'd not like to be doing it to anything smaller than an 100AH pack and then only for a few minutes.

Yes you *can* shove silly currents into a lead acid- anything is possible if you ram enough volts across it but you will buckle plates, you will boil electrolytes you will not charge it fully becuase the thing with melt/boil/explode first. I spent enough time in a lab researching these things to know exactly what you can and cannot get away with.

In article , Fred

Yeah will remember that for next time. I don't think my smart charger will recognise a battery if less than about 6v, Or if I use the old basic 4amp charger on a completely flat batt I'll make sure I keep switching it on & off before the amp meter goes off the scale until it gets enough juice for the smart to recognise it.

Yeah, yeah. But the OPs battery appears to have recoverd fine after half an hour of driving. That is characteristic of the effects of a small drain and NOT of a complete flattening.

Perhaps you have never had this happen to you - if you haven't, it can be difficult to imagine (a) just how flat the battery appears to be and (b) just how quickly it returns to full vigour.

Ian

The test will come if he leaves the vehicle standing for a week or the temperature drops below freezing

The OP, from what's he said, actually has no idea how well the battery has recovered other than it now starts the car. He has no way of assessing its state of charge without putting a voltmeter or hydrometer on it. It could have only a fraction of a full charge in it and still start the car fine. It might not do so in another week though or when it's below freezing.

Had an interesting one late yesterday afternoon - a neighbour knocked on the door and asked for some help. His '01 BMW 325T wouldn't start. Flat battery - just a relay or two clicking when you tried. He'd started it earlier - normally - and moved it a few yards to wash it.

Got the meter out and it was certainly flat - 9.5v on the battery posts. Loaned him my little Lidl charger and agreed if it wouldn't start the next day I'd take him to get a new battery. His lady wasn't pleased - she wanted to go shopping that evening.

About a couple of hours later I went out and noticed their car wasn't there. Sort of concluded she'd nagged him into getting the AA or whatever out.

This morning he returned the charger and said on a whim - or more likely to stop the earhole gbh - he'd tried to start it after an hour or so and it did. With a charger who's peak output is less than 5 amps?

He kept the engine running while she shopped then put it back on charge overnight. By the morning the charger had gone to float mode so had decided the battery was fully charged. What's the explanation?

On Tue, 23 Dec 2008 03:52:03 -0800 (PST), I waved a wand and this message magically appears in front of Ian:

I have had this happen to me. Putting it on a 20Ah recharger for ~36 hours cured it.