99% of the "medical" ones, are also; "ran on simple AA batteries".
I've gotten two of them, from doctors and simply took a plumber's "tubing cutter" and cut off the last 1/2" of the top end.
Take out the old batteries, (if dead), drop in two new ones and tape the end back on. You're set to go.
You can also simply cut the last 1/2" off the end, CAREFULLY, using a small hobby type hack saw, anything that will cut plastic evenly and slowly. (so you don't cut into the batteries themselves).