The Pack Is Only as Good as Its Worst Cell

A 10-cell NiMH pack can look perfect on paper. But if one cell has slightly lower capacity and another has higher internal resistance, the pack will misbehave: the weak cell hits cutoff first, the high-resistance cell heats up, and the whole pack ages prematurely. The discipline that prevents this is cell grading and matching — testing every cell and only building packs from cells that are close enough to be interchangeable. This is the hidden quality layer that separates a pack that lasts from one that disappoints.

What Automatic Grading Measures

Modern NiMH lines grade every cell through automated test equipment before assembly. The core measurements are:

• Capacity — measured by a controlled discharge, reported at a defined rate and cutoff. Cells are binned into narrow capacity windows.
• Internal resistance — AC impedance or DC resistance at a reference state of charge. Tight resistance matching is essential for current sharing in parallel strings.
• Open-circuit voltage and self-discharge — screened to reject cells with excessive leakage or premature voltage drop.
• Dimensional and visual checks — cells outside tolerance on height, diameter or with damaged cans are rejected before they ever reach a pack.

Why Matching Matters in Series and Parallel

In a series string, the lowest-capacity cell limits the whole pack: it is the first to reach the discharge cutoff and the first to be overcharged in reverse if the pack is driven hard. In parallel, cells share the load according to their internal resistance; a low-resistance cell does more work and ages faster. Matching cells on both capacity and resistance keeps every cell working at roughly the same state of charge and temperature, which is what lets a pack deliver its full rated life.

The Limits That Matter

Practical matching windows are typically tight: capacity within a few percent, internal resistance within similarly narrow bounds. The seriousness of the application dictates how tight — a safety-critical or high-rate pack demands closer matching than a low-drain consumer device. The point is not to demand perfection but to demand consistency: a pack built from matched cells behaves predictably and ages evenly.

Traceability: Knowing Every Cell's Story

Beyond matching, responsible manufacturing keeps full traceability. Each cell carries a code linking it to its production batch, coating run, formation parameters and test results. If a field issue appears, the manufacturer can trace it back to a specific batch, understand the root cause and act — whether that means improving the process or recalling precisely the affected units. For OEM buyers, traceability is evidence of a process you can audit and trust.

What to Ask Your Cell Supplier

Before you commit to a pack supplier, ask directly:

• Are cells 100% automatically graded before assembly, or only sample-tested?
• What are your matching windows for capacity and internal resistance?
• Do you keep batch-level traceability through the whole manufacturing chain?
• Do you have outgoing quality data you can share?

Weijiang Power answers yes to all four — every cell is measured, every pack is built from matched cells, and every unit can be traced. It is the reason our packs deliver consistent performance pack after pack. Talk to us about your next NiMH pack and ask for our grading and traceability data.