Capacity Testing (C20)

Battery Testing Methods · Last reviewed 2026-09-15

Direct answer: Capacity (Ah) is measured by discharging a fully-charged battery at a constant current until it reaches a cut-off voltage, then computing current × time. The lead-acid standard is the C20 rate — the current that drains the battery in 20 hours to 10.5V. Capacity falls at higher rates, which is why the rate must always be stated.

The basic procedure

  1. Fully charge and let the battery rest to a stable state.
  2. Discharge at a constant current (resistive or electronic load).
  3. Stop at the cut-off voltage (10.5V for a 12V battery).
  4. Record time; capacity = discharge current × hours.

A 60 Ah battery discharged at 3 A should run 20 hours. If it quits at 15 hours, its real capacity is 45 Ah — a 25% loss from the label.

Why the rate matters

Discharge rateTypical capacity deliveredWhy
C20 (20-hour)Rated (100%)Slow, extracts the electrolyte deep in the plates
C5 (5-hour)Lower than ratedFaster discharge wastes energy as heat
C1 (1-hour)Much lowerHeavy load, significant internal loss

Two "60 Ah" batteries can differ in a real, faster-discharge application — which is why the rate is part of the spec, not an afterthought.

Reading a measured capacity

A new battery usually delivers at or above its label. Below about 80% of rated C20 capacity is the conventional end-of-life signal for many applications. For a used battery, the measured C20 capacity is the honest number to compare against the label — a battery that still holds 12.6V can nonetheless have lost much of its capacity.

The Author's Take

Position: The capacity label is a promise made at one specific rate, and the most useful question is always "at what rate?" — because the same battery is a 60 Ah battery at C20 and a smaller one at C5.

Reasoning: Capacity is quoted at the flattering slow rate. A buyer who understands the rate — and asks for the measured C20 on a used battery — knows what they are actually buying.

This is the author's editorial view, not a purchasing guarantee.

Sources

Return to Battery Testing Methods · World Battery Hub. Informational, not purchasing advice.