The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Battery capacity describes how much charge or energy a battery can store or deliver under specified conditions. Runtime is how long that energy powers a particular device. To estimate runtime, divide usable battery energy in watt-hours (Wh) by the device’s average power draw in watts (W). The result is an estimate—not a guaranteed time between charges.
Capacity, runtime and battery life are different things
Capacity is an amount; runtime is a duration. Amp-hours (Ah) and milliamp-hours (mAh) describe charge capacity, while watt-hours (Wh) describe energy. Watts (W) describe how quickly a device uses energy. As Battery University explains, energy is power multiplied by time.
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In ordinary conversation, “battery life” may mean runtime on one charge. It can also mean service life over months or years, or cycle life—the number of charge and discharge cycles a battery can undergo. Use “runtime” or “time between charges” when you mean how long a device runs on a charge. For background on capacity loss as batteries age, see Battery University’s battery-life discussion.
Ah alone can mislead when comparing batteries with different voltages. Wh is generally more useful for comparing their nominal energy. Battery University’s unit glossary defines these measurements and gives the relationship between voltage, Ah and Wh.
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How to estimate battery runtime
Use these relationships for a first estimate:
- Nominal energy (Wh) ≈ voltage (V) × capacity (Ah)
- Ideal runtime (hours) ≈ usable energy (Wh) ÷ average device power (W)
Convert mAh to Ah by dividing by 1,000. If you divide nameplate Wh by power without accounting for energy the device cannot use, label the result an ideal estimate. Do not assume a universal usable-energy percentage: the appropriate value depends on the battery and device.
- Find the battery’s voltage and capacity. Check its label or manufacturer specification. Note whether capacity is listed in Ah or mAh; divide mAh by 1,000 to convert it to Ah.
- Work out nominal energy if needed. Multiply voltage by Ah. For example, a hypothetical 12 V, 7 Ah battery has about 84 Wh of nominal energy. Prefer a manufacturer-stated Wh value when available, since ratings are specified under particular conditions. The U.S. Department of Energy’s battery-charger test-procedure material distinguishes voltage, Ah capacity and Wh energy in its reporting context.
- Estimate average power during the intended use. Use the average draw, not automatically the device’s maximum rating or a brief peak. Where voltage and current describe the same measurement point, power in watts equals volts multiplied by amps.
- Divide usable Wh by average W. The quotient is an estimated number of hours. If you have only nominal energy, call the result ideal and state that assumption.
- Check the estimate against representative use. A measured runtime can be more useful when the device’s load varies or the battery is aged.
Worked example
Suppose a hypothetical battery is rated at 18 Wh and a device averages 6 W during the task. The ideal estimate is 18 Wh ÷ 6 W = 3 hours. This is arithmetic, not a tested result; actual runtime can be shorter if the device cannot use all the nameplate energy.
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For a changing load, use average power over the period you care about. If that average is uncertain, calculate a range using explicit low- and high-power assumptions rather than presenting one precise-looking figure.
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- Battery condition: An aged or degraded battery may deliver less energy than its original rating. Capacity affects runtime and can decline with age, as discussed in Battery University’s capacity and battery-life article.
- Discharge conditions: A capacity rating depends on how it was measured. Discharge rate and cutoff can affect the result, and conditions described for one chemistry should not be applied automatically to another. Battery University’s capacity-measurement guide discusses discharge-based tests and specified conditions.
- Changing device load: Power draw may shift with workload, screen brightness, radio activity or operating mode. A brief peak is not necessarily representative of average use; use device documentation or measurements for its actual load profile.
- Usable energy and cutoff: Nameplate energy is not necessarily the energy available to a particular device before it shuts down. The cutoff and battery-management behavior depend on the battery and device; use their specifications or a controlled measurement instead of applying a generic correction.
How to measure a battery’s actual capacity
A discharge test measures how long a charged battery sustains a defined load or how much energy it delivers before reaching a specified cutoff. The result is meaningful only alongside its test conditions. Battery University’s capacity-testing guidance describes discharge-based measurement and why method and rate matter. In the specific context of U.S. battery-charger testing, the Department of Energy’s procedure calls for reporting battery discharge energy in Wh; that regulated testing context is not a universal test recipe for every battery.
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A battery capacity tester may help, but it must be compatible with the battery’s chemistry, voltage and connections, and provide a safe discharge method. Follow the battery maker’s instructions and applicable safety guidance. A result should include the load or discharge rate and cutoff used, rather than being presented as an unconditional capacity figure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Comparing two batteries for the same device
For a useful runtime comparison, evaluate both batteries under the same intended load and assumptions. Do not rank them by mAh alone.
Quick Recap
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- Button Function: 1.Click the button once to switch screens in a cyclical manner; 2.Double-click the button to rotate the displayed content; 3.Press and hold the button (3 seconds): Clear all capacity, amount of electricity, and time.
- Energy: Compare Wh at the relevant voltage, not just Ah.
- Compatibility: Check voltage, chemistry, form factor, connectors and device approval.
- Condition: Distinguish a new battery from an aged one; measured available capacity may differ from the original rating.
- Load capability and rating conditions: Confirm the battery can sustain the device’s draw, and note the discharge rate and cutoff under which its rating was obtained.
- Estimated runtime: Apply the same average device power and clearly stated usable-energy assumptions to each battery.
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