Battery indicator lit - when restarting warm engine

[Pepsimax]

Hi,

The car died when my wife was driving home today. Battery lamp lit up, then after a while car died. I went over to pick it up, got a jump and drove home with no battery lamp lit. A diagnostick cigarette device showed battery and alternator was okay.

When I was going out shortly after the battery lamp was lit and voltage showed 11,7.

Several hours later I started the car again and no problem. Used test 09 and took the car for a quick spin. The voltage reading was between 13.1 - 13.6. I put on all the electrical stuff like heated seats, window wipers etc and the voltage dropped to 12.1

Stopped the car and restarted... Battery lit again and 11.7v

Alternator failing?