Flat battery and booster battery problem

I'm in the UK. Car is 1.3 litre.
QUESTION (1) I know this may not be the best way of doing it but, in theory, if a car battery got flattened by leaving the headlights on
then why would a fully charged second battery attached in parallel to the flat battery give almost no help at all?
It's was almost as if the second battery lost nearly all its charge to the flat battery.
Can someone explain what is happening with the battery charge please.
QUESTION (2) If that's so then, to get the car started, is it absolutely necessary to put a charger on the flat battery which is in the car?
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beccy wrote:

the booster battery may have been flat? Was it one of those portable emergency start ones? The battery in those is tiny and rarely last more than a year. Additionally the cables and connectors are often quite insubstantial.
Assuming that the battery booster pack you are trying to use is in good condition then it is necessary to connect the booster pack correctly and then leave it for about five minutes to transfer some charge into the flat battery. Then try and start it.
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On 08:26 14 Oct 2009, Mrcheerful wrote:

The booster battery was a proper charged up battery and not part of a special emergency "unit".
I'll try it with the five minute arangement.
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beccy wrote:

then probably the jumper cables were a bit on the weak side, so a connect up for a few minutes should do the job. It is quite possible in emergency to jump start a car using thin bits of household flex, just leave them connected for half an hour or so (then remove the cables) and enough charge will have transfered to start it up. They even make a dedicated jump start device that just plugs into the cigarette lighteres of the dead car and a good one, you leave those connected for about twenty minutes till the light goes green and away it should go. Cigarette lighter circuits can only carry a few amps in any case, so it is all done by transfer over some time.
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That's true if using a car to jump start from and you leave the engine running. Just coupling two batteries together won't really charge the flat one by much.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

it works with a battery pack and is the official way to use them, so some charge does transfer even without one being on charge.
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It really depends. If, as the OP says, a battery has been run totally flat under load it's likely sulphated and will need a much higher voltage at low current trickle for a long time to help reverse the process. Maybe even days.
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beccy says...

1) You're using those stupid cheapass thin jumpleads
2) The flat battery presents a high load to the second battery.

Far better to to use decent jump leads and possibly remove the earth terminal on the flat battery until its running then replace without turning off the car, assuming that's an option.
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If the second battery is good it means the connection between them isn't. Most jump leads are incapable of carrying the full starter current. Ones which can cost upwards of 50 quid.

If the battery isn't permanently knackered through being run totally flat, another car connected to it and the engine run for a while might then charge it enough. But an overnight charge with a charger is a better bet. Lidl have some decent ones in at the moment for only 14 quid. Small enough to keep in the car.
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On 10:05 14 Oct 2009, Dave Plowman (News) wrote:

The jump leads have worked well in the past but they're not 50+ leads! Maybe this time the spare battery is having to provide even more power than the times in the past.

"Pernanently knackered through being run totally flat"? Can it be permanent? Is there an simple way to test this without going to a garage?
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beccy wrote:

Go to a motor factors that sells batteries. Most will do a free test. Halfords, if you must, but they'll rob you blind for the new one...
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You're missing the point - the battery is flat. Deal with that problem first and then the car can be driven to places you suggest.
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Roger wrote:

Indeed, but the question was 'how can the battery be tested'. If the OP gets the car started, she can then go and have the battery tested. Others have offered plenty of advice on getting it started.
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If the battery isn't totally flat it will provide some of the current needed. It's totally flat ones that sort out the jump leads. ;-)

Try charging it. If it takes a charge you might be ok.
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A lead-acid battery that has been run completely flat will never be quite as good as it was before, but if it was in good shape before and wasn't flat for too long, it should mostly recover.
Most auto parts stores will test batteries for free, otherwise if the car cranks fine on a cold morning then the battery is probably fine.
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wrote:

Qu is la Quid
-- Tom Horne
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Tom Horne wrote:

google is your friend
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beccy wrote:

Jump leads not making electrical contact. Some cheap ones have insufficient metal in the plastic jaws and I've come across ones where the rest of the plastic prevents metal to metal contact. Get a torch and take a good look when you have them 'attached'. It's probably not connecting ...
--
Adrian C

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On 14:18 14 Oct 2009, Adrian C wrote:

I wondered about the area of contact because the clips had a sort of lipped shape on each surface which meant the contact area was a metal edge. IYSWIM. It didn't seem much to me. But if the area is too small then wouldn't the jump lead clip get hot where it did make contact?
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beccy wrote:

cheap leads often corrode where the cables meet the clips and no contact results. A good healthy crackle as you connect the final clip shows something is moving, then just leave them connected for a while before trying to start the dead one.
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