I have two medical cauteries that my brother in law, the ER nurse, got for me. The Docs throw them away after one use, it's not possible to sterilize them for another use. The batteries ran low in them and I thought it a waist to throw them away so I changed the batteries in them.
I realize I'm probably not the first person to ever do this and I suspect that it may have been posted here before. If it's been covered I apologize for using up band width. If it's been a while, someone may benefit by it.
Here's what I did.
This is what the back end of the cautery looks like. I used my belt sander to remove the rolled end that holds the end cap in.
Under the end cap is a spring and a plastic divider. Take them out....
....and it looks like this. Bend the copper tab up and remove the batteries.
Here's all the stuff. Put in new batteries.
Reassemble it the way it came apart and put on a new end cap. I use these rubber end caps that I got off eBay for ice fishing rods I make. They work perfectly for the cautery. You will need a 5/8 inch cap.
One warning though, the cautery will be much hotter than it was when you last used it so be careful. You could singe the heck out of stuff if you're not careful.
Some ice flies for this winter and soft hackle wets for this summer all made with the help of my regenerised cautery.