Red battery light on

[themoog]

This is not for my X5 but for our Range Rover sport supercharged.
The car does not get used regularly for long runs--just school run etc so the other day I put a battery charger on it for about four hours just to give it a boost.
The wife used the car twice yesterday for two runs of 600 metres each (i know don't get me started on that).. After the second run she said the battery light came on.
The voltage across battery this morning was 12.1v before start. First two minutes it was 13.45v then after that there was a whining noise and the voltage was 12.05 and dropping.
What do you reckon - battery or alternator? Funny coincidence that alternator would fail but maybe charging the battery killed it. I have no idea how old the battery is.
Thoughts..

[Alan Gunn]

I think it's the alternator.
You should see around 13.8/14.2v when running.
Try it again but when it drops to the 12.05v turn the rear defrost and headlights on and see what voltage you get but i think you need a new alternator or the voltage regulator changing at least.

[Greydog]

My money goes on the Alternator/regulator. With luck it's the voltage regulator either way you can bet it will be a real Beech of a job to change on a RRS.

Fingers crossed for you

[themoog]

It's going in on Thurs. I reckon it's the battery for some reason but I don't know why. Strange that an alternator would fail within 12hrs of the battery being charged..
At least I hope its the battery..

[X5Sport]

Red light is brought on if the discharge rate is higher than the charge rate.  That is usually the alternator, and given it is making an odd noise would tend to point at it being unhappy.

If the battery was dying then I would expect it to be well below 12V at rest.

Could be bad brushes in the alternator?

[A Blair]

Can I ask if you fitted a larger battery.

[themoog]

No the battery is the original. I just charged it up.