It happend to the C5 that I got from the fomer Owner - I have to rebuild eveything
Thr cheapest is the smallest battery - start at 0,54 € but I think you can only test the electric not the motor(Camelion A23-BP1 23 8/LR932 Elektronikbatterie Alkali-Mangan 28,5x10,3 (HxØ/mm) 12V 28,5x10,3 (HxØ/mm)
up to 220Ah 12V Batterie Fulmen Nautica Tudor
220Ah 12V Batterie Fulmen Nautica Tudor
455,00 EUR
just take jump-cables from a car (battery) that would work without buying an extra battery