as above, if outputting more than 15v, then you've got regulation issues. not 100% sure on the golf, but is it not a 3 wire alt.....if so, just make sure all connections are tight, otherwise it wont regulate the voltage as it should do.
the capacity of the battery is irrevlevant, it doesnt really matter if you got a 100ah bat, it would just be topped up as far as the alt can go.
double check the voltage again, and check the wires going to the alt are secure.