Have a set of these
http://sitincomfort.com/bx805gloves.html
They only heat for about an hour or two, but they are quite warm, so I ussually ride for a while till my hands start to chill, turn them on for a while till my fingers are toasty, then turn them off; it takes another hour or so for my fingers to chill, so then I do the whole thing over again. Used that way, I'm good for a 6-8 hr ride.