Cell Balancing

Why packs drift out of balance

Cells in a pack are never quite identical, and small differences compound. Manufacturing tolerance, slightly different internal resistance, and uneven temperature across the enclosure all cause some groups to charge and discharge marginally faster than others.

Over hundreds of cycles that drift accumulates. Without correction, one group hits the charge ceiling before the rest are full and hits the discharge floor before the rest are empty — and because the BMS protects on the weakest group, the entire pack stops there.

A pack that is 5% out of balance loses far more than 5% of usable capacity, because the mismatch eats into both ends of the range.

Passive and active balancing

Passive balancing bleeds charge off the higher groups through resistors until the lower ones catch up. Simple, cheap, reliable, and it wastes the excess as heat. It works during charging, near the top of the range where the differences are most visible.

Active balancing transfers charge from stronger groups to weaker ones instead of discarding it. More efficient and more effective on larger mismatches, at higher circuit cost and complexity.

For e-bike packs, well-implemented passive balancing handles normal drift. Active balancing earns its cost mainly on large, high-value packs or where cells are known to be poorly matched — which is a problem better solved at assembly.

The upstream fix

Balancing corrects drift; it does not rescue a badly built pack. The real defence is matching cells before they are welded together — grading incoming cells by capacity and internal resistance so each group starts from the same place.

We bin cells within ±2% capacity variation per group and record the arrangement per work order. A supplier who balances aggressively but does not match cells is treating a symptom. Ask about both, and see cell brands and grades for what cell consistency is actually worth.