Originally Posted by StuartV
I thought the way the controller works is that when it's set at, for example, 50%, it would cycle the gear on and off several times a second, so that the total On time is 50% of the time. If so, then that means that when it is On, it's drawing the full load of power (i.e. 90W) - but only very briefly. That seems like it means the bike would need to be able to handle a 90W load, whether you ever run the jacket at 100% or not.
It is true that at 50% the jacket will draw 90W when on and 0W when off. If your charging system cannot supply the entire 90W then the battery will supply the difference. The charging system needs to only supply the average or net power draw (plus a little more) or just over 45W in this example. The battery will smooth out the voltage changes if the changes are fast enough -- kind of like a shock absorber on small road irregularities. If you use a switch to manually turn the jacket on and off, the voltage will change -- the same way a shock absorber cannot smooth the road rising and falling over longer distances.
A PWM controller with high frequency makes the changes mostly invisible to your battery and charging system.
I use a 90W jacket at full power on bikes with 210W and 280W alternators. On a bike with a 170W alternator I can draw the battery voltage down low enough that the alternator takes a while to charge it back up.