Cell Binning

Why identical cells are not identical

Cells from the same production lot vary. Capacity spreads by a few percent, internal resistance by more. That variation is normal and unavoidable — what matters is what a factory does about it.

Binning measures each cell and sorts it into a bin, so that cells assembled into one parallel group are matched to each other rather than assigned at random.

What happens without it

A parallel group of mismatched cells does not share current evenly. The lowest-resistance cell carries more than its share, works harder, heats more, and ages faster. As it ages its resistance rises, and the burden shifts to the next cell.

Meanwhile, capacity mismatch means the group is limited by its weakest member. Because the BMS protects on the weakest group, the whole pack is limited by the weakest group in it. A pack of nominally 20Ah cells with poor matching can deliver noticeably less than 20Ah on day one, and the gap widens with age.

Cell balancing corrects drift between groups during charging, but it cannot fix a group that was mismatched internally from the start. Balancing is the safety net; binning is the design.

What good practice looks like

Three things distinguish a factory that takes this seriously:

• Measurement on incoming cells, not reliance on the supplier’s grading
• Both parameters — capacity and internal resistance, not capacity alone
• A stated tolerance you can hold them to

We measure capacity and internal resistance on 100% of incoming cells, run formation cycling before assembly, and bin within ±2% capacity variation per group. Cells outside specification are quarantined and returned rather than used in a lower-grade product.

The two questions that matter

Ask for the binning tolerance in writing, and ask what happens to out-of-spec cells. A supplier who cannot answer the second question is telling you they end up in someone’s pack. See cell brands and grades for how grading claims are made and how to check them.