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Old 13th September 2021, 08:13   #1
macafee2
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Default jump starting a car with a flat battery

Went to start a car yesterday that had a flat battery, so flat the engine would not turn over.
Using jump leads I connected up a spare good battery that seemed fully charged. Still the car would either not turn over or only just turn over.

Did the flat battery nick "all" the power from the good battery and leave "nothing" for the car?

I had to connect my car up and leave the engine running for 5-10 minutes before the car with the flat battery would start.

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Old 13th September 2021, 09:00   #2
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Originally Posted by macafee2 View Post
Went to start a car yesterday that had a flat battery, so flat the engine would not turn over.
Using jump leads I connected up a spare good battery that seemed fully charged. Still the car would either not turn over or only just turn over.

Did the flat battery nick "all" the power from the good battery and leave "nothing" for the car?

I had to connect my car up and leave the engine running for 5-10 minutes before the car with the flat battery would start.

macafee2

It is far more likely to be the case that the jump lead connection was incapable of carrying the required cranking current. This could occur, for example, if your chosen -ve connection point on the chassis is not a good current conductor.
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Old 13th September 2021, 09:03   #3
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... I connected up a spare good battery that seemed fully charged.
Your use of the word "seemed" Ian probably answers your own question.
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Did the flat battery nick "all" the power from the good battery and leave "nothing" for the car?
Yes, quite possibly.

Simon
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Old 13th September 2021, 09:03   #4
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I've found "standard" jump leads to be under dimensioned, I invested in some HD leads of thicker cable which do the job when the standard ones failed.

Regards
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Old 13th September 2021, 09:14   #5
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I've found "standard" jump leads to be under dimensioned, I invested in some HD leads of thicker cable which do the job when the standard ones failed.

Regards

Likewise.

Halfords 35mm^2 jump leads are excellent and never fail to start any car provided the chosen -ve connection point and the donor battery are good.
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Old 13th September 2021, 09:45   #6
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I've found "standard" jump leads to be under dimensioned, I invested in some HD leads of thicker cable which do the job when the standard ones failed.

Regards

If you want an idea of the CSA of cables you need, just look at the + - battery cables on your vehicle, you need them to me at least the same or greater CSA.


Nothing at all wrong with the cheap ones, especially the pencil thin ones, if you don't mind the odd bit of melted red or black plastic on top of your wing.


Best way to start from a donor car, correctly connect leads - to engine + to battery + post or jump post.


Start donor car and let it run at fast idle for a few mins depending on how flat the battery is, start the flat car, run this at a fast idle for a few mins till the engine is getting warm, on the flat battery car switch on the headlights or heater on, shut down the donor car, remove leads one at a time, take it a run to Blackpool and back.


Then if there was no obvious reason for the battery to be flat, get it tested.
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Old 13th September 2021, 12:03   #7
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May I submit this in evidence
https://www.the75andztclub.co.uk/for...d.php?t=315151


even gone down in price now



C
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Old 14th September 2021, 08:32   #8
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Your use of the word "seemed" Ian probably answers your own question.

Yes, quite possibly.

Simon
when i put the spare battery on charge it went to showing fully charged and i am going to assume it is good enough to start one car once

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Old 14th September 2021, 08:36   #9
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Originally Posted by Ducati750cc View Post
If you want an idea of the CSA of cables you need, just look at the + - battery cables on your vehicle, you need them to me at least the same or greater CSA.


Nothing at all wrong with the cheap ones, especially the pencil thin ones, if you don't mind the odd bit of melted red or black plastic on top of your wing.


Best way to start from a donor car, correctly connect leads - to engine + to battery + post or jump post.


Start donor car and let it run at fast idle for a few mins depending on how flat the battery is, start the flat car, run this at a fast idle for a few mins till the engine is getting warm, on the flat battery car switch on the headlights or heater on, shut down the donor car, remove leads one at a time, take it a run to Blackpool and back.


Then if there was no obvious reason for the battery to be flat, get it tested.
I will fully charge the battery with it disconnected, check with a volt meter then leave for a few days and try again, I will then reconnect it and test to see how many amps are being drawn.
Whilst it could be a duff battery it does look and I was told is new.

If amp draw is the problem I may well leave the battery disconnected or even remove it and put it on a modern permanent charge to look after it

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Old 14th September 2021, 11:22   #10
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when i put the spare battery on charge it went to showing fully charged and i am going to assume it is good enough to start one car once
Hi Ian, thanks for your reply.

Unfortunately your assumption may be unreliable. Unless your "spare" battery receives a frequent maintenance charge, it is likely to become sulphated. This means that although it may illuminate a "fully charged" indicator, many of the plates may be rendered useless so, in fact, only a proportion of the battery is fully charged. In this condition it is unlikely to start a car and particularly when placed in parallel with the vehicle's discharged battery.

If you put your voltmeter across the "spare" battery and operate the starter, a voltage of below approximately 10.5v suggests that the battery is sulphated if it previously gave a "fully charged" indication.

To guarantee that a "spare" battery will be useful whenever needed, leave it permanently connected to a smart maintenance charger. I recommend C-tek as they are well made and very compact but there are other choices.

Simon
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