NiCad Pack? Type 1? Should be simple. There's some sites with instructions.

http://www.fenestrated.net/~macman/DuoBattery/

(main link to another battery is dead, lower link is there from archive.org)

https://web.archive.org/web/20011226061530/http://www.fastlight.demon.co.uk/jg/duobatt.html

Read carefully.