Waltersons never produced a rechargeable battery for that model. It is intended to be run off AA batteries. 6 x 1.5V batteries gives a voltage of 9V (some batteries get as high as 1.6ish versus the 1.5V here used in the equation) so you should be ok with a 6 cell NIMH battery. A 7 cell 9.6V pack is probably going to be a bit higher voltage than you need really. I would stick with the 6 cell packs as those are going to be cheaper anyways for the most part.