I haven't been able to find one battery powered, all the ones I have seen have been the plug in type.
I would like to find a clock that will display the time and alarm the alarm from battery rather than just shut down. I was late for work the other morning because I didn't know the power went out. I'm about the buy a cheap one and just rip out the transformer and rectifier and put in a bettery then power and charge from a little wall wart.