Quick question, battery warning light problem!?

[Chrisbmx92]

Hi there, My cars been sitting around for a while, it had a flat battery so i jumped it with my mates Polo. it ran fine for a few minutes then the battery warning light came on and it died. It did this a few times, so i took the battery off and charged it up fully. I put it back on Paul the Polo and he ran fine, then when it got to high revs going down the high road the battery light came on again! then it flashed and the light went off and it continued to run fine, now im so confused to what is happening! so if anyone could shed some light on to this problem it would be brilliant! sorry for the long winded explanation

[john.p.clegg]

Alternator u/s?

John

[Chrisbmx92]

yeah was going to say, the alternator belt is tight and fine, and the alternator is chucking out 50amps when the battery is fully loaded. so im just confused! its usually over 4000 rpm it comes on!