Voltage ≠ capacity
Open-circuit voltage tells you little about how long a rechargeable cell will run under load.
Capacity (often in mAh) is estimated by discharging a cell under controlled conditions and integrating current over time — what charger-analyzers do.
When capacity testing helps
You reuse NiMH AA/AAA in GPS units, instruments, or high-drain remotes.
You want to retire weak cells from a set so one bad cell does not pull the pack down.
When it usually does not
Single-use CR coin cells — replace with the correct code.
Devices marked alkaline-only — do not switch to NiMH just because a tester exists.
Practical tip
Label cells after a capacity run. Keep matched sets together. Follow the charger’s guidance for discharge current so you do not overheat cells.