Running headed grips will drain your battery if you ride with the lights on. Lighting (don't forget all the ancillary lamps like the dash, warning lights etc) and engine/PV control draw the best parts of 85-90W which leaves a max of 30W of capacity in the generator to charge the battery - and thats if the engine in running at over 6000rpm. Basically every time you're sitting at the lights you'll be drain the battery, and with only 5W of spare capacity in the generator it won't be able to maintain the charge.
You can use an optimate to keep the battery charged when you're parked up, but in the long term it'll shag your battery.
Personnally, I'd invest in a decent pair of gloves instead! You can always pull up if your hands get too cold and stick them on the barrel of the engine to warm up again!
Getting frosty fingers is all part of the joy of winter biking!