Battery or alternator? How to identify?

413x

413x

413x

Caporegime
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Last few days I've had to jump start the car.
It must be ( it think) either of these two.


Once it starts it seems fine, but even after 20 minute drive I came back a few hours later and had to jump start.


Any at home things I can do to identify?
 
As above, measure the battery with the car running and with the car off, there should be a few volts difference, that'll show the alternator working. If the battery shows less than 12 when sat at rest and you've seen the alternator is working then it'll be the battery or a heavy parasitic drain. I'd replace the battery or at least sub in a loaned battery to test.
 
brendy said:
As above, measure the battery with the car running and with the car off, there should be a few volts difference, that'll show the alternator working. If the battery shows less than 12 when sat at rest and you've seen the alternator is working then it'll be the battery or a heavy parasitic drain. I'd replace the battery or at least sub in a loaned battery to test.
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I did wonder about this drain.

The battery seemed fine. Then it fairly suddenly started doing this.
I stopped at shop for 5 mins and it was OK. But a few hours drain it.
 
sigma said:
A 20 min drive won't charge a low battery surely?
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Maybe not fully but more than enough to put enough charge in a useable battery to then start a cold engine after a few hours.

OP, you can do two quick and dirty tests without a multimeter. Idle the engine with headlamps, heated rear screen and heater blower on, (to put some stout electrical loads on the system). Rev it up to circa 2000 RPM, a working alternator usually brightens the headlamps and you may hear an increase in speed from the blower fan. A SLIGHTLY riskier test is idle the engine with no electrical stuff turned on. Disconnect the negative battery lead. A good alternator will keep the engine running. If it stops it's almost certain the alternator is goosed.

A meter is easiest. Measure battery voltage with engine and ignition off. Compare the reading to how it reads with the engine running with some electrical loads turned on. First should be 12 volts plus a bit. Second should be 13.5 volts or so minimum.
 
Yeah I have a multi meter.

So if the alternator is screwed I won't get the big voltage increase with the engine running?
 
You will get a voltage DROP if the alternator is shot.

If the brushes are worn out SOMETIMES tapping the alloy part of the casing hard with a big plastic screwdriver handle will shock them into contacting the slip rings for a tad longer, but if it's a rectifier or inbuilt regulator problem, or the less usual problem of burnt out or broken stator or rotor winding, it'll need rebuilding or replacing.
 
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Just been out.

Car only just started without jump. Got home last night after 20 min drive.

11.8 off (left over night)
12.8 on

Battery then?
 
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Sounds like the alternator needs attention then. It's doing something, but not a damn lot. Is the ignition warning light on or glowing with the engine running?

Best guess is the regulator is knackered, but it could have a faulty rectifier and its lost a phase or two. They can be rebuilt at home, but they may be a bit daunting at first :)
 
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Chris Wilson said:
Sounds like the alternator needs attention then. It's doing something, but not a damn lot. Is the ignition warning light on or glowing with the engine running?

Best guess is the regulator is knackered, but it could have a faulty rectifier and its lost a phase or two. They can be rebuilt at home, but they may be a bit daunting at first :)
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Probably best take it to the garage in that case.

Like you say looks like 12.8 idling is far too low. It's got its MOT soon. Probably address it then
 
With engine/accessories off:

Battery should be around 12v. Anything less that 11.7 can be a sign of a duff battery

With engine running:

You should see between 13.8-14.7. Outside this range could be a duff alternator

If those are ok then it could be a parasitic drain which you can test with a multimeter.

1. Set multimeter to 5A/10A, moving the red +ve lead to 5A/10A socket
2. Disconnect the +ve battery lead
3. Attach +ve meter lead to the +ve battery lead without reconnecting to battery
4. Attach -ve meter lead to +ve battery terminal


Once you've done this the meter will probably read a couple of amps for the first minute or so whilst everything resets so give it a few to settle. After a few mins the meter should read less than 0.10A as there will always be a small draw due to the ECU/Anti-theft systems. Given the issue description if there is a drain then you're probably see 2-3A after a few mins.

If you do see a few amps then it'll be an ardous task trying to find it. Last drain I traced took me over an hour of pulling fuses/relays to find the faulty circuit.
 
Could be anything electrical

Have a look on car forums of the make model.

I had a drain and it was the Bluetooth module

10 second job unplugged and no drain
 
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