Building a 48V 20Ah e-bike battery pack from scratch

Cell selection: measure internal resistance and capacity on every cell. Discard anything more than 10mOhm off the group. Mismatched cells are the main cause of swelling later on.

Grouping: split into 13 groups of 4 in parallel. Keep capacity spread within a group under 50mAh.

Spot welding: use 0.15mm pure nickel strip, two welds per joint. A good weld looks round and bright; dark means a cold joint or too much heat.

BMS: 13S protection board, connect the balance leads in order. Disconnect the main circuit first — a short will arc instantly.

Testing: rest for 24 hours, then re-measure. A spread above 0.05V means it needs balancing again.

Enclosure: heat-shrink wrap plus foam padding at the corners. Verify the protection board once more before it goes in the bike.