How to check if alternator issue or battery

[Mr MXT]

Friend of mine has the voltage warning light appear on his MY02 Passat.

I'm assuming either the alternator is knackered or its just the battery.

I know you can check with a multimeter but cant remember the exact process.

Is it, measure the voltage on the battery with engine off (what should I be getting?) and then measure the voltage on the battery with engine on (14v + ?) - If im not getting over 14v on charge then its the alternator, otherwise its the battery.

Does that make sense?

[V6GTA]

Measure with car turned on. Should be around 14.20V if it's less then it's the alternator.

[Mr MXT]

Sounds simple, thanks

[rev-erend]

With a cold engine measure the voltage at the battery.

It should read approx 12.6 volts

Now start the car and check the voltage again - it should read between 13.8 to 14.4

If it is only showing 12.6 it's an alternator problem.

[Mr MXT]

What if its reading less than 12.6v?

[annodomini2]

Mr MXT said:

What if its reading less than 12.6v?

If it's under 11.5v get it on a battery tester.

If it's under 10v, it's fked!

You may also have to consider that it's both.

[rev-erend]

If it's under 12.6 v then it shows its either not in great condition or that
it has not been charged by the alternator.