Just thought this would be a cool new topic. If there is already one in existence please let me know so I can delete this one, or a mod can do it. As for me I finished my under tail project and rode the crap out of it.
Those grip warmers should not draw enough current to pull down your voltage like that. I doubt your battery is the issue - I think you want to put a good MOSFET R/R on there.and it seems crazy to replace an otherwise OK battery just to get the grip heaters working on really cold days.