25.8v out of the "charger" is correct, it's actually just a 26v converter... the charger is inside the battery. maybe you could try something like this, tho it may not work on the 48v battery.
BionX Battery Recovery I too recommend some caution when charging... I would phone/send the battery to BionX myself...
EDIT; After reading the BionX battery recovery page, I see it says not to use on 48v batteries for some reason... Phone BionX support...