well like has been said, you'll cook a 6v battery in no time (keep an eye on the cell fluid levels)
Check if your bike has a regulator and that it looks ok...hasnt become disconnected etc
Time and again we seem to come back to charging issues on theses old single cylinder yamahas, I see some of them make no reference at all to a VOLTAGE REGULATOR, but they have a VOLTAGE RECTIFIER.
Then I see that the RECTIFIER has a sturdy bolt fixing and 'COOLING FINS' !!!?
Why? why does it have cooling fins if it's only a rectifier?
Because maybe, just maybe it is more than just a rectifier...maybe it's a regulator/rectifier (reg/rec)...regulators need cooling fins.
I wish someone would get to the bottom of this issue!
http://www.ebay.co.uk/itm/Yamaha-DT125LC-RSX100-Rectifier-/380765719622?ViewItem=&item=380765719622&nma=true&si=CzRQzZZ1CJrmFwwiRMDLxFKhnac%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557#ht_22wt_958
Its about time you had that haynes book fella