help needed asap

just moved it to front of drive for someone avin a look at it n bat light came on had it on charge other day when put add up to sell it,i tuched acelleraor n it went off,had a play with it n it seems to be when i start it up it revs to 2000 then goes down stright away near enuf when it goes under 1100 revs bat light comes on,i ajusted revs to tick over at just over 1100 and bat light dose not come on,and like i say wen it does come on if it went under 1100 revs as soon as u rev it the light goes out,electrics are not my thing i dont tuch them other than changing a plug fuse or bulb.any help much appreciated,not got a muti meter but guna try n get one now.

Think you need a reading of the charge rate first but sounds like the alternator from what you have said.