Well I wanted to take out the old batteries and install some Lithium-Ion batteries so they would be easy to replace in the future. I decided on a Li-Ion 3.6V 650mAh Rechargeable RCR123A battery. 2 of them in fact. Hooking them up in parallel will equal the original voltage. 3.6 is the original voltage. These batteries will not have the memory problem of the old batteries. You will need a new charger for them because the old one will over charge and you might have a fire. You'll also need 2 battery holders that are the right size for 123A batteries.
Tools you will need:
Dremel tool with cutting wheel
small flat head screwdriver
120 grit sandpaper
Duco cement or equivalent
I think that about covers it.
Step 1: Taking the battery apart
It was suggested by easy-does-it to use boiling water to soften the plastic to remove. This idea should work better. Once the plastic starts to get soft I would think you could just pop it off.
Now that you have it off it is time to use the dremel to cut out the end so we can remove the batteries. Don't forget to use your goggles.
Try and cut as close to the edge as possible. There is a rubber washer on the inside at the end and will probably start to smoke once it heats up. You'll see it once you start cutting in deeper. Remove the rubber washer. Once that is done the batteries should fall out. Just tap it on the table and it should come out. The battery that comes out last will have a metal cup spot welded to the positive terminal. Use the screwdriver to pop it off. Don't bend it. We need it. If the screwdriver isn't working, use a putty knife or pocket knife.
Now you can start cutting out a large slot like you see in the picture. Make it wide enough so you can fit the battery holders. Make sure the battery holders are set in deep enough with a battery installed. You don't want the batteries to be sticking out. Be sure and sand all metal edges so you don't end up cutting yourself.