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To measure a battery’s usable capacity, fully charge it, discharge it at a controlled current to a specified cutoff voltage, and calculate the charge delivered. For a steady current, multiply amperes by elapsed hours to get amp-hours (Ah); multiply by 1,000 for milliamp-hours (mAh). The result is meaningful only alongside the test conditions—especially chemistry, discharge rate, cutoff voltage, and temperature.
What a capacity test measures
A capacity test measures how much charge a battery delivers between a defined starting condition and a defined end-of-discharge point. It is not a single condition-free property: change the load, cutoff voltage, temperature, or charging method and the measured result may change.
Capacity is usually reported in amp-hours or milliamp-hours. Energy is reported in watt-hours (Wh) and accounts for voltage as well as current. Because lithium-ion, NiMH, and NiCd batteries have different voltage profiles, two batteries with the same mAh rating may deliver different amounts of energy. When comparing them, record both Ah and Wh.
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Before connecting equipment, identify the battery chemistry, number of cells, rated capacity, manufacturer’s discharge limits, and whether the pack has a protection circuit. Use a charger intended for that chemistry. Never bypass a pack’s protection circuit or exceed the manufacturer’s voltage limits to reach a test endpoint.
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| Chemistry | Discharge rate in U.S. e-CFR Appendix Y method | Cutoff in that method | Important qualification |
|---|---|---|---|
| NiCd | 0.2C | 1.0 V per cell | These are the cited federal test-method values, not a substitute for the battery maker’s limits. |
| NiMH | 0.2C | 1.0 V per cell | These are the cited federal test-method values, not a substitute for the battery maker’s limits. |
| Lithium-ion | 0.2C | 2.5 V per cell | The pack protection circuit may end discharge sooner; never override its limit or the manufacturer’s guidance. |
Here, C-rate expresses current relative to rated capacity: for a 2,000 mAh battery, 0.2C is 400 mA. The U.S. e-CFR values above describe one test convention. Standards can vary with application; ENERGY STAR’s battery-charger method points to IEC 61951-11 for NiCd, IEC 61951-22 for NiMH, and IEC 619603 for lithium cells. State the standard or test convention whenever you report a result.
Equipment and safe setup
- A constant-current electronic load or battery analyzer with a programmable cutoff.
- A way to log voltage and current; a precision digital multimeter can monitor voltage directly at the battery terminals during the load test.
- A thermometer and a stable test area.
- Suitable wiring and fusing, plus a nonflammable setup appropriate to the battery pack and its protection system.
Do not test a damaged, swollen, leaking, or unusually hot battery. Follow the battery and equipment manufacturers’ instructions, and stop if the pack heats abnormally or behaves unexpectedly. Make sure the load and wiring are rated for the battery’s voltage and current.
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Measure capacity step by step
- Record the battery details. Note chemistry, cell count, rated capacity, manufacturer cutoff guidance, and any protection-circuit behavior.
- Charge correctly. Fully charge the battery with a charger designed for its chemistry and the pack configuration.
- Let it rest. Rest the charged battery for 1–4 hours at about 20 °C before discharge. The ENERGY STAR method specifies 20 °C ± 5 °C and a 1–4 hour rest.
- Set the load and cutoff. Connect a constant-current load or analyzer. Choose the discharge current and end voltage from the applicable test method and manufacturer guidance. If using the federal method shown above, the rate is 0.2C; a protection circuit or manufacturer limit takes precedence if it ends discharge earlier.
- Log the discharge. Record voltage and current at least once per minute. Stop when the cutoff condition is first reached. The U.S. e-CFR method says energy delivered after the end-of-discharge condition is reached must not be counted.
- Calculate the result. Use the formulas below to calculate delivered Ah and Wh, then convert Ah to mAh if desired.
- Repeat only for a controlled comparison. Keep the test conditions identical for each run. The ENERGY STAR method permits up to five runs and uses the best result under its procedure; that selection rule is specific to that method.
Calculate Ah, mAh, and Wh
For a constant-current discharge
Capacity in Ah ≈ discharge current in amperes × elapsed time in hours. Multiply Ah by 1,000 to express the result in mAh.
For example, a steady 1 A discharge lasting 45 minutes delivers approximately 0.75 Ah, or 750 mAh, before the test cutoff. National Instruments gives a 1,000 mAh battery delivering 1,000 mA for 45 minutes as an example of 75% reserve capacity; that figure describes the stated test conditions, not a universal runtime or capacity rating.
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For a varying current
Add the charge delivered in each logging interval: Ah ≈ Σ(Iᵢ × Δtᵢ), where current is in amperes and each time interval is in hours. For energy, add the interval-by-interval voltage and current product: Wh ≈ Σ(Vᵢ × Iᵢ × Δtᵢ). Use readings covering the discharge up to the cutoff; do not include output after the endpoint.
Why a voltage reading alone cannot tell you capacity
Open-circuit voltage is not a direct capacity meter. Under load, internal resistance causes voltage to drop; after the load is removed, voltage can relax upward. The indicated voltage also depends on the battery chemistry and its state under the particular load. National Instruments describes reserve capacity as a discharge at a specified rate until voltage reaches a specified value, with the result decreasing as load rises. Texas Instruments likewise explains the load-related voltage drop and relaxation after current removal. A voltage measurement can help monitor the test, but it cannot replace a controlled discharge and delivered-current calculation.
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Why a battery gauge can disagree with a discharge test
A fuel gauge may estimate capacity by integrating current over time, a method known as coulomb counting. Texas Instruments notes that such estimates need corrections for self-discharge and charging efficiency. Long inactivity, changing loads, or infrequent full discharges can let the estimate drift and overstate available capacity. Treat the gauge as an estimate; a controlled discharge to a defined cutoff measures delivered capacity under that test’s conditions.
What to include in a capacity report
A bare mAh result is incomplete. Record enough detail for someone else to interpret or repeat the test:
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- Chemistry, cell count, rated capacity, and pack protection behavior.
- Charging method and rest duration.
- Ambient and battery temperature.
- Discharge current or C-rate and cutoff voltage.
- Voltage/current sampling interval.
- Elapsed discharge time, measured Ah, and measured Wh.
- Number of runs and the result-selection rule, if more than one run was performed.
- The test standard or convention used.
For a fair comparison, hold chemistry and cell count, charge method, rest time, temperature, discharge rate, cutoff voltage, logging interval, and protection-circuit behavior constant. If any of these differs, report the difference rather than treating the resulting mAh figures as directly comparable.
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