After having been convinced by Matt Lacey's Li conversion videos that a Li conversion is quite managable, I started looking for a 2nd hand VX1.
By good fortune, I ended up with a VX1 from the former Australian Vectrix importer and distributor. The bike is ex-demo/showroom and I have been told it has done less than 500km. It has a few marks and the chrome plated surfaces have brown corrosion spots. The battery could find a new application as a zero-volt reference if it were not so heavy, and since the plan always was to convert to Lithium, I did not even attempt to revive it.
My choice was to go for the largest capacity battery possible. Partly because I don't want to have to worry about making it to the next charging point and partly because the disacharge capability of the battery is proportional to the capacity. At the same discharge current, a larger capacity battery will be stressed less.
A local distributor still had stock of CALB 66Ah cells so I ordered 42 of them. As of July 2013. CALB does not make any 66Ah cells, only the 60Ah ones. The datasheet states a max discharge current of 2*C (132A) so a peak discharge with up to 200A for a few seconds should be well within the battery's capability. Internal resistance (Ri) is quoted as < 1 mOhm. At that Ri, drawing 50A would dissipate 2.5W of heat per cell or 105W for the entire pack, at 100A that would rise to 10W and 420W respectively. Average power dissipation of the battery will probably be closer to 105W than to 420W, depending on how I use the bike. In any case, with the LiFePO4 cells, I am not too concerned about battery cooling and the 2 battery compartment impellers will be permanently removed to make room for more cells.