Battery Testing Methods · Last reviewed 2026-09-16
| Rate | Discharge time | Measured capacity | Typical use |
|---|---|---|---|
| C20 | 20 hours | Highest (the quoted Ah) | General rating, solar |
| C10 | 10 hours | Lower than C20 | Industrial / telecom |
| C5 | 5 hours | Lower again | UPS, high-rate applications |
At higher currents, more of the stored energy is wasted inside the battery — as heat in the internal resistance and through slower electrochemical access to the active material. A battery that delivers 100 Ah at the gentle C20 rate may deliver only ~85 Ah at C10 and less still at C5. The rate is part of the specification, not a footnote — see capacity testing for the full C20 method.
Compare within the same rate: C20 to C20, C10 to C10. A battery quoted at C10 will look smaller than one quoted at C20 even if it is physically similar. The same discipline applies as with CCA standards — the number's basis defines what it means.
Position: Capacity is not one number — it is a curve over discharge rate, and the single most useful question about any Ah figure is "at what rate?", because the answer changes the number.
Reasoning: The C20 number is the flattering slow rate; real applications discharge faster and get less. A buyer who compares within one rate — and knows the C20/C10/C5 ladder — reads capacity claims the way engineers do.
This is the author's editorial view, not a purchasing guarantee.
Return to Battery Testing Methods · World Battery Hub. Informational, not purchasing advice.