Interesting video of replacing dead battery cells.

[thebarber]

 

[apac]

You’d have to replace all the cells if the battery pack was old and had been cycled a lot. Otherwise you’d be mixing old and new cells that would have differing internal resistances. This would mean the cells would charge and discharge at different rates. Your battery management system would not be able to manage this and the cells would become unbalanced over time. Plus then end up with individual cells dangerously overcharging and Over discharging. Not a good practise for lithium cells. Worst case scenario would be cells overheating and causing a Runaway situation of all cells doing the same and hey presto lithium battery fire of roughly 1000 degrees that would melt the frame on your bike.

i have not watched that video but the Chinese spot welders as shown are a bit hit And miss. There are better options out there. i Have a spot welder that was made for me so I could build my own battery packs. They give out a massive burst of energy.