@
Erowbone
No relay is needed.. Just make sure and be very careful that you tap the proper cable..
You must cut the Yellow-Red Wire (normally located at the headlight).. When done, take the cable that is going to your headlight switch and tap it to the any brown wire nearby.. then properly seal with electrical tape the remaing half of the yellow-red cable that is going to the magneto/stator to avoid electrocution..

This will make all of your headlight and auxiliary lights to be battery operated.. See the post of Sir @
thon.. Thanks for his share..