My friends 1989 NC30 has a problem with the battery loosing charge. i have tryed a new rectifier and sorted out an earth and some wiring into the rectifier. i used a voltmetre to test it and it shows that the bike is charging the battery when its running and its fine when riding. but the problem is the battery drains very quick when riding the bike with the lights on (it last 10mins until the battery was totally dead last night) but is fine and seems to charge if we run it without the headlights on. Could anyone please shine some light on what may be the issue...i have been trying my hardest to fix it for him for weeks and i cant figure it out

Thanks people any advice is very welcome

Andrew