I've got one of these in my toolbox somewhere with a dead battery. I used it twice and the battery died. Is there an off switch I don't know about?
As far as rim temperature and making sure your tires are hot, when you check the pressures before you go out (which you should always do!) you feel the rim. Is it hot? Good to go! If it's mildly warm, needs more time or just be mindful you'll need a lap or two before the tires hot. If it's cold your f'd, sit back down before you hurt yourself.
Never ever have I just geared up, yanked the warmers off and rode without feeling the RIM (touching the tire isn't telling you much). Actually I check my rim before I even think of getting suited.
Plug the warmers in and make sure they're on, come back 20min later and make sure they're still on and check the pressures (if a tires just been mounted it probably has an extra 15-20psi that doesn't need to be in there), 40min feel the rim as it should be pretty warm now, 60min the rim should be very warm and tires hot. Set pressures, gear up, go.
I also use my covers all the time windy or not.