I have checked the tires of my QiCycle just now. They are 16" X 1.75". You can also confirm in the bicycle's manual that the wheel is indeed 16 inches https://files.xiaomi-mi.com/files/qcycle/QCycle-EN.pdf
Still guys your answers don’t address the question. seanjoeakiller wants to know whether one can just open the case and put in 20 cells in order to replace the old ones. That would be much cheaper and probably would cost around 100$ instead of 300+ that the full battery pack costs. Update (09/20/2017): Check step 17 here. Apparently, the battery case can open with some heating and "good vibrations" but you will probably break the one-time-use clips the case has in the process. However, if your battery is dead by now, it's the best thing you can try I guess and see if the case can be closed again after being opened before buying a brand new set of batteries with case costing $300.