Flat battery

I'd say it did come up to the correct voltage as the charger switched to float mode. They're off on holiday now - so I'll keep my fingers crossed for them. ;-)

If he'd left the lights on then you can quite often get one to recover enouigh voltage to start the car by just leaving it standing for an hour.

Absolutely not so. I was using sharp probes direct to the battery posts. And a recently calibrated Fluke 179.

The battery terminals were tight anyway.

I don't think that really tells you anything other than the battery wouldn't take any more charge. The post charging "surface charge", even on bad plates, will bring the battery voltage up to a level that makes the charger switch to float mode but when you remove the charger and discharge the surface charge the true voltage and battery condition will be revealed.

If you put a voltmeter on a battery, even a bad one, immediately after removing the charger you'll see very high open circuit voltages for a short while. Possibly well over 13V depending on what the final float voltage of the charger was. Those high voltages are created by an uneven mixture of sulphuric acid and water on the plates which aren't representative of the average specific gravity of the electrolyte or the true output voltage of the battery. That's why you have to remove this surface charge before you can test properly.

I've tried to soldier on with knackered batteries in old cars many times which would show a decent surface charge voltage making you think the battery was fully charged, start the car fine soon afterwards but still be dead again in a day or two. I've even trying reviving batteries by replacing the electrolyte by saving the acid from other old batteries, concentrating it by letting some of the water evaporate with it in a glass bowl on the central heating boiler until it reached an SG of 1.26, charging the battery as high as it will go and then removing and washing out all the old acid and sludge and putting the new brew in but it never achieves anything.

Some of these new fangled pulse chargers are meant to be able to remove hard sulphate to some extent but I haven't tried one. They cost more than a new battery.

I think your neighbour's experience isn't really that unusual. He started the car just to move it a few feet which took some load out of the knackered battery but didn't replace it which a longer trip would have done and it's showed its true colours. Your charger gave it enough for one more start and the overnight charge might have given it enough for a few more starts but next time he leaves it for a day or two it'll be dead again.

I think Dave knows more than enough about electrickery to measure a battery voltage properly so the above really isn't a possibility.

Yup. He didn't buy the car new - but the battery is a BMW one so in all probability original. So at 7 years old has done well enough. Eurocarparts charge 60 quid inc. for one so not the end of the world.

It's always worth ringing bmw for a price at that age, they where cheaper last time I bought one.

I nipped out and had another check on my Focus's Motorcraft battery today in the light of all this battery talk. The car is 8 years old in June so about

7.5 years now and the battery is no doubt older than that dependant on how long the car sat before it was first sold and how long before the car was assembled the battery was actually made. There's probably a code on it I could tell the actual date from. Call it close to eight years though. As I've said previously the battery has a hard life because the car gets very little mileage and usually only 1/2 mile trips to the shops so it sits partially discharged for most of the time which must sulphate it up like crazy. It was well flat last week and struggled to start the car so I charged it somewhat for an afternoon on my shagged little 30 year old Halfords charger which has a broken ammeter now anyway but never went above 3 amps at the best of times. I doubt if that brought it anywhere near full charge though.

I was quite surprised to see 12.37 volts a week later and after a couple of days of non use and in such cold weather. It ain't good but it's not as bad as I was expecting. I thought it would be closer to 12 volts. I might treat it to more frequent charges to try and keep it from sulphating further but I reckon it still has a year or two left in it. It certainly won't owe me anything when it croaks. It still starts the car fine even when it's below freezing and in the summer isn't an issue at all.

I have a general aversion to buying anything from a main dealer but I'll be well tempted to get another one from Ford when it dies if they don't charge an extortionate amount. I've had cheap batteries in the past and they are definitely not a good idea.

Must admit my motorcraft battery on my focus lasted over eight years and the car used to sit idle for weeks at a time , in the end sitting idle for two months over january seen it off

Do you know which charging mode he had it on? I have the same Lidl charger and when charging a very old battery that's on its last legs, it will charge and go into float mode if left on the slow or medium charge setting - but if on the high setting it will never go into float mode as it never reaches the required 15 or so volts.

No. A half hour of charging will allow to start the car - a decent SLI battery will enable you to start when it is nearly flat as I said above. Just because it starts and runs ok and maintans voltage doesn't mean that it cannot be nearly flat.

9.5v is flat of course but if you stuck 5 amps into such a battery for an hour that should be enough to start it. If it wasnt a diesel, hence no glow plugs clobbering it then I'd expect a petrol to get a few seconds cranking time and if the alternator can energise and start charing then it should be fine.

Both my Focus diesels have had Motorcraft batteries that did well over

120k and 8 years of service from new. I replaced my current one this winter not because I thought it would fail but I didnt want the hassle of it dying on me in the Alps. Costco sell Bosch batteries at reasonable prices and they have always given equally excellent service whenever I've used them

FWIW, I'd just change it. Mine cost =A335.

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Dave Baker used his keyboard to write :

OK, just thinking of possible reasons for a wrong reading. I would think it reasonable to also discount the idea that 3.5amps from a tiny charger would put enough in the battery to start it (3.5amp hour), especially with such a seriously discharged battery to at the beginning.

[...]

It's likely with a Focus that the first sign you will get of battery old age will be dashboard reboots at start. (The dash behaves as if you have put it in test mode.)

Chris

Agree. Always look for the simplest explanation consistent with all the known facts. Use that as a working assumption until you can disprove it.

Something left on is the simplest explanation.

John

Redwood has brought this to us :

A nice feature which the makers could include in their smart charger design, would be an override button for those occasions when you know you have the polarity correct and know the battery is simply discharged. I built a smart charger of my own design some years ago and the 'dumb button' was a modification I soon found the need to make to it.