Hi FastBiker,
There is some space, a whopping 2 cells xD. The casing is a Reention Dorado Max, 505mm, that can hold 80 cells maximum.
I have two variants; a 52V = 14s/5p = 70 cells 17.5Ah (910Wh) and the original 48V = 13s/6p = 78 cells 21Ah (1008Wh).
I have seen 52v 20Ah (1040Wh) versions...