There are 2
batteries, AA size NiCd. I bought 2 at Frys for about $15. Desolder the old ones and solder in the new ones. It’s pretty easy. Just take it slow. Remove screws from back and pull it off. Watch carefully when putting back together to maintain
water tight seal. Not a big
project.
This is what it looks like inside after removing
batteries. (Assuming attachments work)
Edit: cannot attach
photo right now. The batteries are taped together and soldered to a two pin connector. It’s easy to cut the tape off and solder the old wires into new batteries. Tape them up and put it back together.