The 4000's were 137.5mm x 46.5mm x 23mm and simply by spacing out the two layshaft pullies as far as was possible, these went straight in the larger 5200's which didnt fit were 137.5mm x 46.5mm x 2
5mm and the extra 2mm made them foul on the rear bulkheads.
Info and sizes on the cells can be seen here -
http://www.vampire-racing.com/Batteries.html