Car battery failure

1 Dec 2006
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United Kingdom
Just had something new for me which I am hoping someone on here can explain in simple terms as google went way over my head.
Sister rang, car won't start ( 2006 Ford Focus ) making a clicking noise when ignition key turned to start position. A flat battery I think & take my jump start kit down to her car.
It has been fine she says just came out to this morning & no go.
Connected my jump leads & battery- no go, just the same as her battery. Connected up to my car, after manoeuvring hers into place by hand, tried to start engine -no go. Tried with my engine running- still the same
Strange I thought especially. when other faults started to appear.
Went for my meter & checked all batteries, only hers was low at 6v. but when connected to any of the others it dragged them down to 6v also even with engine running.
Fitted new battery -everything fine.
Can any one tell me why please
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The old battery was sucking the life out of the one you were using to jump start it.
Its common for batteries to fail overnight......unlike the old days when they gave some warning!
I think three out of the six cells (each 2v) have collapsed and have internally shorted, hence the alarming voltage readings - as per stivino.
What we can’t explain is the failure for the jump start to would think your donor car would bypass the original battery and kick the thing into life - but I have seen this before.
John :)
Many years ago I observed a weird thing with a battery on a Mini. I was working on it, it was running and then stalled. Tried to start it, battery was flat. Opened boot to check battery terminals and it stunk of battery acid. Battery was smoking. Pulled both terminals off, removed battery and it was still smoking. It carried on doing that for about 5 minutes until it was completely flat. And hot. So hot I couldn’t pick it up with my bare hands. A new battery cured it. That must have just been an internal short inside the battery.
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I'm sure some people will either laugh or disagree with me but from experience the battery needs to be of similar capacity as the output of the alternator. The first golf I had the battery would last around 6 months then die pretty rapidly. After the 3rd battery in 18 months and having to pay for a new one I asked the battery dealer why they kept on failing. The shop would not sell me a similar sized battery but one of twice the capacity. The battery workshop chap took in to his workshop and cut the top off the battery we had just taken off the car and showed me the plates and bottom of the battery case - what had occurred was the paste addition/filler to the plates had come out as was shorting out the lead plates in in the bottom of each cell.
He told mine wasn't the first battery he'd seen like that was due to the high charge rate from the alternator, presumably the excessive bubbling caused during charging loosening the paste filler.

Never liked small capacity batteries in car since then.
Battery and battery charging has changed over the years, I have seen batteries explode, and shorted cells, but in the main it is the battery which sets charge rate not the alternator, there are exceptions, but in the main an alternator be it 24 amp or 80 amp sets the charge voltage to around 13.8, it can be as high as 14.4 where the vehicle manufacturer uses a compromise assuming how many times engine will stop.

I have seen where an over sized battery is a problem, but only with French vehicles, Savum I think it was called, had a single phase alternator which was not continuous rated so extra large battery it would charge at max output for too long, and would over heat and fail. Not had that with British cars.

I did not start with the very early cars with a three brush dynamo and free wheel, but did work on the old dynamos, a two bobbin regulator would have an open circuit voltage of 16 volts, it would not get a battery to that level, 16 amp was normal max output, and we use to put the battery on an equalising trickle charge at least once a month and top up cells.

The alternator was around 18 amp to start with, but it could spin faster, so by fitting a smaller pulley it still charged although not much, at tick over. And in the main only voltage regulated, the CAV 208 alternator for buses had current regulations but that is the only one I know of, there were quite a few specials, found one with a three phase transformer built in giving out both 12 and 24 volt, most would not start charging without the warning light connected, however the rotor in CAT tractors did have some residual magnetism and would self excite. They could also over charge small batteries, when I started as an auto electrician some of the big CAT tractors we still 6 volt with a donkey engine starter.

But around the mid 70's there was a move to sealed for life batteries, they were not what we now call AGM or VRLA but used some cleaver venting to reduce water loss, and this meant you needed to keep battery cool, the Mini was first car I remember with battery in the boot, and an over charging alternator could easy wreck the battery. Deep cycling a battery could cause the active material to fall off the plates, so plates did not go to bottom of battery, there were pockets for it to fall into, using a car battery with a full discharge could wreak it as well, the golf trolley battery may have looked the same as car battery, but inside it was different, the material could not fall off plates as easy, but the amps it could deliver were far lower, we also got the leisure battery which was a hybrid between the two designed so could be used with engine not running so if normal battery was 60 Ah the hybrid leisure battery would need to be 90 Ah.

The VRLA (valve regulated lead acid) I first saw as very small batteries, alarms, emergency lights, stair lifts, and these unlike the flooded battery could even be used upside down, the stannah stair lift has them on their side, not sure I like the idea but seems to work, what they has is exactly the right amount of acid absorbed into a glass fibre mat also called AGM (absorbed glass mat) and it is virtually impossible for the active material to fall off. The valve is in real terms a rubber cap which seals the battery unless over charged when it will allow the gases to escape, but they hold the charge better, and they will stand cycling better, and with today's stop/start technology they are required unless we move to NiMd or the like, however the whole method of charging has also changed, the cars computer keeps track of the charge state and it holds the battery at around 80% charged most of the time but allows it fully charge every so often so sulphur does not harden, and when a battery is renewed you have to tell the cars computer it has been renewed and also what size it is.

It has also resulted in a new type of off the car charger, typical battery charger was between 8 and 12 amp rated, today more like 3.8 amp rated, and today most are stage chargers, unlike the car the home charger does not have a computer record of the battery condition, so it can only use battery voltage, so typical starts at 3.8A until 12.8 volt then 3A until 14.1 volt then 0.8 amp until 14.4 volt then 0.1 amp until either 14.4 volt hit again when it switches off, or 12.8 volt when it returns to 0.8 amp charge actual values are for the Lidi charger, the Ctek is most well know of the smart chargers, but there are many.

So the old 12 amp charger goes in the bin, odd even today Halfords sell chargers clearly marked not suitable for AGM batteries, seems odd they are still sold. But my wife's Jaguar XE would normally look after the battery well, even sends a message to the phone app to say if battery is getting low, however with lock down so few miles covered the car can't cope. I have just recharged the AGM on the Jag a 90 Ah and with a smart charger that took 8 hours, it was not flat, but taking 8 hours to drive 5 miles which is the limit it seems is really not practical, as to what will happen with adblu I don't know that has a limited life.

However comments like use the biggest battery that will fit does not any longer work.
As a P.S. you were lucky the battery did not explode, both donor and faulty one. To get a battery to drop to 6 volt your looking at 100 amp plus, a spark when removing the leads could have easy caused it to go bang, I saw it happen with a Landrover and quickly closed the bonnet, the force was enough to open the bonnet again.
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