Hi,
Today I tried replacing the battery and I did it successfully. You don’t need the battery of the same dimension as the original. In my case I use lipo 1s 1,200 mAh which is 40x35x13 mm. It can be put into the battery chamber easily. The new lipo has only 2 wire, red and black. Simply cut the connector from original battery and connect the connector to the new battery. Do not worry about the third wire in the middle. I can charge via nimbus and use them without problem.