Box of 24 x Procell AA Batteries
For wireless microphones, and really anything that requires serious staying power from 1.5v AA battery, let ProCell be your first choice.
Broadcasters, Stage Managers and Theatres all around the world depend on the battery that gives them maximum output for as long (and consistently) as possible.
Grab them by the box (24 per box) - heaps cheaper than your local supermarket. Also get a box for your kid's toys, remote controls and other gadgets...