Internal resistance testing, covered in our companion article, gives a fast proxy for battery health in seconds. Capacity testing answers a more direct but slower question: how much usable energy can this battery actually deliver, right now, compared to when it was new?
What Capacity Testing Actually Measures
A capacity test discharges a battery under controlled, known conditions — typically a constant current — from full charge down to a defined cutoff voltage, measuring the total charge delivered. Comparing that against the battery's rated nameplate capacity gives a direct state-of-health figure: a battery delivering 80% of its rated capacity has degraded to 80% state of health, a figure widely used as an end-of-useful-life threshold.
Discharge Rate Affects the Result
A battery discharged slowly (a low C-rate) typically delivers a higher measured capacity than the same battery discharged quickly (a high C-rate), because internal resistance losses scale with current. Meaningful capacity comparisons need to specify the discharge rate the test was performed at.
Lab-Grade vs Field Capacity Testing
Lab-grade discharge testing uses programmable electronic loads for precisely controlled discharge, typically integrated with data logging across the full discharge curve — the reference method for genuine capacity verification, but requiring multi-hour test duration.
Field-capable capacity testers use faster methods — internal resistance measurement, partial discharge profiling, or algorithms correlating a shorter sequence against expected behaviour — trading some precision for a test completed in minutes.
| Factor | Lab-Grade Discharge Test | Field Capacity Tester |
|---|---|---|
| Test duration | Hours (full discharge) | Minutes |
| Precision | High — direct measurement | Good — often correlation-based |
| Best suited for | Manufacturing QC, certification, R&D | Fleet maintenance, field troubleshooting |
Multi-Cell and Pack-Level Considerations
For battery packs made of multiple cells, pack-level capacity alone can hide a problem: overall capacity is often limited by the single weakest cell. Per-cell internal resistance screening combined with periodic full pack capacity testing gives a more complete picture.
Temperature's Effect on Results
Battery capacity is genuinely temperature-dependent — lithium batteries typically deliver measurably less usable capacity when discharged at low temperature, independent of any actual degradation. Meaningful comparisons need a controlled test temperature or explicit correction.
Practical Guidance
For manufacturing QC, R&D, or warranty disputes requiring a defensible capacity figure, lab-grade discharge testing remains the reference method. For fleet maintenance and routine screening across many batteries, a field-capable capacity tester gives a practical balance between speed and diagnostic value.