The best EV battery packs currently are about 6kg/KWh (includin BMS, cooling, enclosure etc), so that would suggest about 100kg for the 17KWhs quoted, so I doubt that it would have a spare for redundancy as it's a low failure risk anyway.
I would guess that the duty cycle they are proposing would be a 1 minute climb to pattern altitude at full power, 2 minutes downwind at say 25HP and 60 knots and a re-gen glide through base and final which would salvage enough energy for the subsequent ground maneuvering, so that's about 2KWhs per circuit.
Even at that the battery would be down to minimum after about 8 circuits.
It would be helpful if they would give more practical examples of different mission profiles, but it looks like the needle would be on empty after about 60NM unless I'm missing something.
There's a
good technical paper about the Taurus G4 that won the Electric Flight Challenge that probably has a lot in common with the technology in the WattsUP, though that carried 90KWhs of charge.