I'm using a Garmin I3, it's not bike specific but it runs on 2 x AA.
It's not too expensive new and being a slightly older model there's lots of cheap s/h ones on ebay.
Screen is an ok size for a bicycle and I think I get about 5 hours on one pair of rechargeable AAs as you have suggested I turn it off when I don't need it.
I just turn it on and select 'view map' - it does tell you the name of the road you are on and of any side roads displayed.
I've made some mounts for it and I use it in the car and on motorcycles.
The mount is a 17mm ball.
Hope this is of some help.