Battery warning light

My Jimny doesn’t get used a lot so the battery had lost charge.
I did jump start it and then after doing 90 miles, the following morning, wouldn’t start. Stuck it on an optimate and got error lights suggesting a shorted cell.
Ordered a replacement battery and fitted Saturday morning.started ok and left it running a while - about 20-30 minutes.
In the evening, started it up and fitted replacement air horn.
Went in the car and the battery warning light was on.

The car was idling, I revved it and the light went off, but returned as the revs dropped back to idle.

So.....assume it’s not battery (brand new Varta - apparently exactly the same as the Bosch that came off it).

So is this likely to be a slipping belt or an alternator?

Any ideas please.

Thanks Paul

Alternator.

Try removing the brush pack and see if there is a seized brush (wishful thinking), and make sure that all the connections are clean & tight.

Thank you - half expecting new alternator. It’s a 53 plate manual. Do all models fit?

Have just tried adjusting the alternator drive belt - ie loosening the top bolt on the adjustment arm and moving the alternator downwards to add tension to the belt.

It now doesn’t come on until 1,000 rpm rather than 1250 rpm.....is it possible the belt needs changing or further adjustment - not a huge amount more adjustment left and I don’t want to overdo it and risk damaging or snapping it....

Ensure you have the correct belt as they do differ in length. Don't quite understand your entry "it now doesn't come on until 1000rpm rather than 1250rpm". From cold when you start the engine revs will dictate that the light will be out if all is working OK. As engine reaches normal operating temp the revs will drop to the prescribed idle speed and the light should stay off. The best way to check the output is to use a meter set to volts and connect across the battery ensuring the red cable goes to the positive terminal and read the voltage. Should be 14 volts range.