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Here you go.. Get your thinking caps on this.
Last Saturday I put my L320 range rover sport supercharged 2005 battery on charge for about four hrs as the car does not get used much and only short journeys anyway.
The wife used it to do two runs on about 2 miles in total and the red battery light came on during the second run on mon eve.
I put a voltmeter across the battery which was at 12.6 V before starting, 13.45 v for the first two minutes of running then 12.1v after that and the alternator started to whine.
Whats happened? Has charging the battery done something?
It's in the garage getting sorted today but I thought I would put it out there for possible answers.
It's a strange coincidence that the alternator would fail then..

themoog wrote: It's in the garage getting sorted today but I thought I would put it out there for possible answers.

The battery warning light usually signifies an alternator fault, either a broken alternator or loose/broken alternator drive belt.

How old is the battery?

No idea it was on the car when I bought it. Why do you ask?

Range rovers are lot more complex than they used to be and the charging system is also more advanced.
Ideally, you need someone who knows these cars inside out to diagnose properly. (OBD2 scan)

Good chance it will need an alternator though, providing the battery has been fitted correctly
Have you double checked the battery terminals are on good and tight after you refitted the battery

themoog wrote:No idea it was on the car when I bought it. Why do you ask?

Ok, how long have you owned the car then?

A bad battery is a super common cause of all sorts of electrical and starting issues in a car, people rarely change their batteries until they go really bad and the car doesnt start but a battery on it's way out can go for months or even years causing trouble that could be easily avoided. Changing a battery can be one of the easiest and cheapest fixes for all sorts of issues.

We had electrical problems a while ago, which could have cost a fortune in new parts if we followed online advice. Turned out it was simply the fact that my fairly new battery was faulty, once changed all the problems went away.

Done similar with my mothers car and my sisters car, both started to have trouble, even taking them to the garage to be inspected would have cost more than changing the battery and in both instances the new battery solved the problem. (both batteries were way past their service life as we later found out)

It was the alternator. Purely coincidental. Allegedly. I reckon charging the battery on the car did something to it. I've got the old one in the garage so I may see if it's repairable.