I can’t remember what cells I used when I did the duo battery years ago, but if I am not mistaken, it was simply tabbed NiMh AAs. I soldered them together myself, which turned out to be a mistake, as the heat damages the cells (it was my first attempt, followed closely by a 540c battery). The proper way is a quick, more or less instantaneous weld. Unless you have the equipment, it’s unrealistic to think you can do this yourself. So buying battery packs that are already made up and making minimal modifications by adding the fuses etc to duplicate what you find in the battery once opened seems to me the way to go, as has been suggested. That’s how I would go about it today. It’s what I’ve done more recently with an eMate, for instance, and that works well.
Counterintuitively, low capacity cells are better to use in such applications than high; they’ll be a) cheaper, and b) more closely matched to the originals. There weren’t 2500ma AA cells in the 90s.
