That jacket will warm up even on battery with engine off. It just doesn't feel like its getting warm, for a while anyway. But when its really cold the jacket makes a world of difference!
I've had a few BMW's with heavier charging systems (800 Watts). Now on my '03 ST I need to watch the system a little more closely, as I run driving light to be seen better. Accordingly I've added a little digital voltmeter to watch the charging and battery condition. I mounted it with dual-lock on the front of the fork riser, just in front of the speedo. Give me date, I need data!
(
www.digitalmeter.com) - about $40, fully encapsulated.
Once the colder weather comes and I use my Gerbing jacket and heated gloves I may be switching them off when I get to stop-and-go traffic.
Stuart