[Arran]

Actually if it is a copper Mershon cap yes you can solder to it. Also for future reference you can apparently remove the top cap and remove the old guts through the top, and place the cap back on afterward. The original chemical inside was a borax or a boric acid solution so the remnants can be washed out with soap and water. You could leave the plastic mesh inside, that was just a insulator to prevent the negative plates from touching the inside of the can, but it doesn't do anything now of course.
Regards
Arran