Thanks guys. I think what fbonomi is suggesting is that I shouldn't mix different types on nimh in parallel. For the test I just used whatever AAs I could find around, but they would have had different charge and voltage, and that may not be good.
So I've ordered 6 identical new ones, which should all stay at the same level of discharge. They are long life ones too.
Each pack of three is wired in series to get ~3.6v to 4v. This is fine for the camera. Extra packs are wired in parallel to keep the voltage the same.
The original nb4l battery is rated at 760mah. With two packs of 3 each (6 x AA) I reckon I get about 4000mah total, and in a reasonably compact solution. I'm really pleased with it!
I'm not sure what the lump in the power cable is for, perhaps it has a fuse. Besides the canon ac/dc adapter pumps out 4.3v I think, which suggests the camera is happy with a reasonbly wide tolerance level.
The tricky bit is getting the dc adapter and dummy coupler battery, though I gather you can make your own by deconstructing an old nb4l.
