Battery or alternator - how to tell?

EddScott · 1 Jun 2009 at 12:10

My golf won't start and I'm assuming its either the battery or the alternator. It starts with a jump and run for a while, turn off and it will restart but following morning its dead.

How do I work out if its the battery or the alternator?

saitrix · 1 Jun 2009 at 12:13

Multimeter on the battery when running. Should be 14+v if the alternator is charging the battery.

Bat · 1 Jun 2009 at 12:16

EddScott said:
My golf won't start and I'm assuming its either the battery or the alternator. It starts with a jump and run for a while, turn off and it will restart but following morning its dead.

How do I work out if its the battery or the alternator?

As it will restart straight away, but not after it's been left it will be the battery most likely.

Dogbreath · 1 Jun 2009 at 12:21

The fact that it will re-start shortly after it's been run suggests the alternator is charging. The important point here is whether something is draining the battery whilst the engine is off (which could be a faulty alternator, or car alarm etc), or whether the battery itself is unable to hold a charge.

You need to measure the current drain with the ignition off, by putting an ammeter in series with one of the battery cables. Current drain should typically be under 20mA, and usually quite a bit less than this. A low value on this test points to a duff battery. If you get a high value you need to start disconnecting things (start with alternator) until you find the culprit.

a1ex2001 · 1 Jun 2009 at 13:48

Just change the battery and see it's the cheap easy way to go and much more likely to be the faulty component.

Xez · 1 Jun 2009 at 13:50

Sounds like the battery isnt holding any charge. My fiesta was exactly the same. Take it to halfords as they test batteries for free and can tell if the battery is buggered or not.

The $6m Dan · 1 Jun 2009 at 13:50

If you can get it to a Halfords, they will test the battery for free and can tell you if the it is dead or not. Your local garage may do the same, but you'd have to ring first.

EddScott · 1 Jun 2009 at 16:06

Thanks chaps. Will take it down to HF for a check up.

Last battery I had replaced was on my wifes Leon - £110!

What happened to the days when a battery was £30?

Clarkey · 1 Jun 2009 at 16:08

EddScott said:
What happened to the days when a battery was £30?

They still exist, just don't go to Halfords.

Bennie-Mac · 1 Jun 2009 at 17:19

Clarkey said:
They still exist, just don't go to Halfords.

Or own a mega deisel battery

dalin80 · 1 Jun 2009 at 18:39

or expect to buy a decent one.