Calculators

Battery Life Calculator

Estimate battery runtime or required capacity from load and usable capacity.

Estimate battery runtime from capacity and load, or estimate the nominal capacity required for a target runtime. Results are planning estimates, not guaranteed device runtimes.

Calculate

Battery capacity

Load current

Usable nominal capacity (%)

Estimated result

8 hours

Uses the entered usable-capacity percentage.

Real runtime varies with chemistry, age, temperature, discharge rate, cutoff voltage, changing load, converter losses, and manufacturer test conditions. Use measured average load and conservative assumptions when possible.

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About This Tool

Battery capacity is often printed in milliamp-hours (mAh) or amp-hours (Ah), but that rating alone does not tell you how long a device will run. Runtime depends on the load and on how much of the nominal capacity is actually usable. This calculator estimates runtime from average current draw, or from power draw when battery voltage and conversion efficiency are known. It can also work backward to estimate the nominal Ah capacity needed for a target runtime. All calculations run locally in your browser.

How To Use It

Choose runtime from current, runtime from power, or required battery capacity.
Enter the nominal battery capacity and load using the labeled units. For a power-based estimate, also enter nominal battery voltage and conversion efficiency.
Set usable nominal capacity to reflect the portion of the nameplate capacity you expect to use before the device reaches its cutoff or operating limit.
Read the estimate and treat it as a planning value. For real equipment, compare it with battery datasheets and measured average load.

Examples

5000 mAh battery at 500 mA

At 100% usable capacity, 5000 mAh ÷ 500 mA gives 10 hours. At an 80% usable-capacity assumption, the estimate becomes 8 hours.

12 V battery powering a 24 W load

A 10 Ah, 12 V battery has 120 Wh nominal energy. With 80% usable capacity and 90% conversion efficiency, 86.4 Wh reaches the modeled load, giving about 3.6 hours at 24 W.

Capacity for an 8-hour load

A 500 mA average load for 8 hours requires 4 Ah of delivered charge. If only 80% of nominal capacity is assumed usable, the nominal requirement is 5 Ah or 5000 mAh.

Useful Notes

Current-based runtime formula

For a reasonably steady current load, estimated hours = battery capacity in Ah × usable fraction ÷ average current in A. Using mAh and mA directly gives the same hour result when both use matching prefixes.

Power-based runtime formula

Nominal battery energy in watt-hours is approximately Ah × nominal voltage. The power mode multiplies that by the usable-capacity fraction and conversion-efficiency fraction, then divides by load power in watts.

Usable capacity is an assumption

Nameplate capacity is measured under specified test conditions. A device may stop before the battery is fully discharged, and available capacity can change with discharge rate, chemistry, age, temperature, and cell condition. The usable-capacity field makes that uncertainty explicit rather than promising the full label rating.

Average versus peak load

A device that sleeps, transmits, starts motors, lights a display, or changes CPU load may draw very different currents over time. Runtime estimates are more meaningful when the entered current or power represents a measured or defensible time-weighted average.

Voltage and converter efficiency

When the load is specified in watts, battery voltage is needed to translate Ah into approximate Wh. DC-DC converters and regulators are not perfectly efficient, so the power mode includes an efficiency assumption. Efficiency can also vary with load and input voltage.

Battery safety and limitations

This calculator does not select cells, chargers, protection circuits, wiring, fuses, discharge limits, or safe operating conditions. It does not model Peukert behavior, detailed discharge curves, internal resistance, series/parallel pack design, or chemistry-specific hazards. Follow manufacturer specifications for real battery systems.

FAQ

How do I calculate battery life from mAh?

Divide mAh capacity by average load current in mA, then multiply by the fraction of nominal capacity you expect to use. For example, 5000 mAh at 500 mA is 10 hours at 100% or 8 hours at an 80% usable assumption.

Why is real battery life usually different from the simple calculation?

Capacity ratings depend on test conditions, while real devices have changing loads, cutoff voltages, temperature effects, conversion losses, battery aging, and chemistry-dependent discharge behavior.

Can I compare Ah ratings at different voltages directly?

Not for stored energy. Ah measures charge capacity. For an approximate energy comparison, multiply Ah by the battery's relevant nominal voltage to obtain Wh, while remembering that real usable energy still depends on operating conditions.

What efficiency should I enter?

Use a measured value or the relevant converter/regulator datasheet when available. If you do not know it, treat any chosen percentage as an explicit estimate rather than a guaranteed property.

Does this calculator design a safe battery pack?

No. It estimates runtime and capacity only. Pack design requires chemistry-specific charging, protection, current, thermal, cell-balancing, wiring, enclosure, and manufacturer safety requirements.