Car Battery or Alternator

Gangster
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My car wouldn't start yesterday so I had to jump start the car. Drove the car for 20 minutes before the car turned off on me my while I was driving. The car had no power and I couldn't start up the car again.

Does this sound like a bad battery or alternator? Do cars use the car battery once started?
 
Soldato
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Oxford
Yes they do use the battery.

If you can get the car started and have a Voltmeter, you can check to see if the alternator is working.

When the engine is running it should read somewhere between 13-14volts.

What car is it ? How old ?
 
Soldato
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How old is the battery? If it was the alternator would you not get the red battery symbol lighting up in the dash?

On my car as I now hardly ever use it I have to put the battery on charge for a few days once a month to charge it back up.
 
Associate
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28 Jan 2021
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UK
I had the same thing last year. Something like below is useful to have in the car, it gives an indication if the alternator is working.
I discovered it was my battery, replaced it with a Yuasa (620CCA!), it's been great.

OIP.cNHyLbM2blgFTDKOO2rGgQHaHa
 
Soldato
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Aberwristwatch
My car wouldn't start yesterday so I had to jump start the car. Drove the car for 20 minutes before the car turned off on me my while I was driving. The car had no power and I couldn't start up the car again.

Does this sound like a bad battery or alternator? Do cars use the car battery once started?

If the engine turned off whilst you were driving I doubt it's the battery that caused it.
 
Soldato
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The alternator when running should provide the power to run all the cars electrics and put a charge into the battery (although they aren't a great battery charger). If you car stopped running it sounds like it ran on what remained in your battery after the jump start. As a guess if your alternator is dead your battery might likely be toast too. Was there no red charging light on the dashboard?
 
Soldato
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Sounds like alternator.

When mine was on its way out on a Polo, I found it wouldn't start charging until I'd hit about 2,500 revs. Something to do with the excitation current (?)

Once I'd realised the revving thing, I stopped occasionally getting flat batteries (must have been driving too gently, keeping such low revs on my commute), and the alternator lasted another 3 years or so before failing entirely.

Anyway, your issue might be different, but I'd make sure you try getting the revs up to see if it kick-starts the alternator.
 
Soldato
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After a 20 minute drive I'd expect the alternator, when I've had a dead battery in the past after about 20 minutes running the car will fire straight back up rather than cut out & be dead.
 
Soldato
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St Breward Cornwall
I think this is why what car question is important ,i.e theres a common fault on the mk1 fabia (ive had it and solved it) where the charging wires break at a weak point ,when one goes theres no charging light present ,that doesn't help diagnosing alternator ,2nd one goes intermittent so battery never reaches full, then fails totally (my car ground to a halt),but as said a voltmeter will tell you instantly the more likely fault
 
Associate
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You need to get it tested properly at a garage.

"car turned off while driving" = death trap
 
Soldato
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13 Sep 2003
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Glocestershire
There is no way we can diagnose your issue via the internet. Especially since we don't even know what the vehicle is.

If the car litterally shut off it could be many things. Did the vehicle definitely even have a flat battery, some modern vehicles can have intermittent faults that will clear after they've been off for a certain amount of time.

Edit: That said, have you checked that the terminals are tight.
 
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