@ecolmi the original 18650 Li-ion batteries is a 3.7V and somewhere between 1800mAh and 3500mAh (mili-amp-hours). In order to get the voltage and the amperage needed by your bike (25.2V 13.2Ah ), the batteries are connected in series and some in parallel. It’s called series-parallel. Take a look [https://learn.sparkfun.com/tutorials/series-and-parallel-circuits/all|on here] for an indepth explanation. We would have to see a picture of exactly they individually are connected to do the math :-)
So, when you rebuild yours, make sure that you absolutely follow the old one for your pattern . Li-ion batteries are finicky and you would not want anything to go off in smoke. I’ve rebuild battery packs for power tools etc. and have not had any big issues. I do not have a fancy spotwelder so I use a soldering iron :-)