Everyone here is pretty much spot on - I would suppose it would depend on what you had electrically running when you were at idle. If you didn't have anything running then I'm with 'Twilight' - except I'm for taking in the battery to a local shop to have it tested.
Your Gerbings heated gear thermostat is actually cycling at 100% load, the dial on the thermostat is controlling the actual time duration when your heated gear is literally fully on or fully off. It's not a variable resistor anymore - so at idle I would expect to see your headlight dimming in sequence with your heated gear. In fact on my 650 Dakar I could see the headlight varying a little when running down the road at night with my heated gear on at near max.
For me any battery over 3 years old is suspect and when they go they go all at once - usually after stopping at a service station to fill up. Yep, that last two did just that...