I'll bet, if plotted out like Andy says there'll be a dip beginning mid October, a slight bulge in the curve at the opposite part of the calendar (Mar/Apr) and gradual climb from there till late September again?
Also, I'll bet the considerable current taken by heated grips will indeed make a difference. The alternator being driven directly i.e. before the benefit of any gearing (apart from pully to pulley) will contribute a significant loading on what is a relatively small capacity engine when they're on?