charging probs

hi there. iv go a tzr 125 ty 1989

i think i have a charging issue.

when the engine is running at idle the headlight brightens like normal, but when i rev it above 2500rpm

it goes dim and can hardly see it put a new bat on it and still the same. the headlight to get brighter if i put my foot on the brake or use indicators.

any ideas?

thanks