Battery Capacity Testing: Ah, the C20 Rate, and How Actual Capacity Is Measured
Capacity is the "how much energy" number, and it is measured by draining the battery at a constant current and timing it. The catch — as with CCA — is that the answer depends on how fast you drain it.
The basic method
- Fully charge the battery and let it rest to a stable state.
- Discharge at a constant current (a resistive or electronic load).
- Stop at the cut-off voltage (for lead-acid, typically 10.5V for a 12V battery).
- Record the time; capacity = discharge current × hours.
A 60 Ah battery discharged at 3 A should last 20 hours. If it only lasts 15 hours, its actual capacity is 45 Ah — a 25% loss from the label.
Why the rate matters: C20 vs higher rates
Capacity is not a fixed property — it falls as the discharge rate rises, because a fast discharge wastes more energy as internal heat and cannot extract the electrolyte deep in the plates. This is why the label states a rate. The C20 rate (20-hour) is the lead-acid standard; the same battery quoted at C5 (5-hour) or C1 (1-hour) shows a lower number. Always compare capacity at the same rate.
Capacity vs CCA vs reserve capacity
| Test | What it measures | Method |
|---|---|---|
| Capacity (C20) | Total stored energy | ~20 h discharge to 10.5V |
| CCA | Cold cranking power | 30 s at −18°C to 7.2V |
| Reserve capacity (RC) | Minutes at a fixed load | 25 A at 27°C to 10.5V |
These are independent dimensions. A battery can hold strong CCA but degraded capacity (it cranks fine but runs down quickly), which is why a health check often runs both.
How to read a measured capacity
A new battery usually delivers at or slightly above its label. A reading below about 80% of rated capacity is the conventional end-of-life signal for many applications — the battery still works, but its usable energy has fallen materially. For a used battery, the measured C20 capacity is the honest number to compare against the label.
Common misconceptions
- "Ah is the same at any discharge rate." No — capacity falls at higher rates, which is why the rate (C20) must be stated.
- "Capacity testing is the same as CCA testing." They measure energy vs power — independent, not interchangeable.
- "A battery passes if it holds 12.6V." Voltage is a state-of-charge signal, not a capacity test — a weak battery can still read full voltage.
Key takeaways
- Capacity (Ah) = constant-current discharge × time, to the cut-off voltage.
- The label rate is C20 (20-hour); capacity falls at higher rates.
- Capacity, CCA and RC are three independent tests — energy, power and endurance.
- Below ~80% of rated C20 capacity is the common end-of-life threshold.
The Author's Take
Position: The capacity label is a promise made at one specific discharge rate, and the most useful skill is asking "at what rate?" — because the same battery is a 60 Ah battery at C20 and a notably smaller one at C5.
Reasoning: Capacity is quoted at the most flattering slow rate, which is why two "60 Ah" batteries can behave differently in a real, faster-discharge application. A buyer who understands the rate — and asks for the measured C20 capacity on a used battery — knows what they are actually buying, rather than what the label once promised.
This is the author's editorial view, not a purchasing guarantee.
Sources
Cross-verified from battery engineering references and manufacturer test documentation. The C20 rate and 10.5V cut-off are standard lead-acid test conventions.
- EN 50342 / JIS D5301 — capacity test methods (C20 rate).
- Battery Council International (BCI) — capacity and reserve-capacity definitions.
- Manufacturer test documentation — discharge test procedures.
Last reviewed: 2026-09-14