How to use the battery life calculator
- Enter the battery capacity in mAh (milliamp-hours).
- Set the average load current in mA that your device draws.
- Adjust the efficiency percentage to account for regulator or converter losses.
- If the device sleeps part of the time, lower the duty cycle below 100%.
- Read the estimated battery runtime in hours, days and minutes, plus the comparison table.
How the battery runtime formula works
The core equation behind this battery runtime calculator is straightforward:
Hours = (Capacity_mAh × Efficiency / 100) / (Load_mA × DutyCycle / 100)
For example, a 2000 mAh cell powering a 100 mA load at 80% efficiency with a 100% duty cycle gives: (2000 × 0.80) / (100 × 1.00) = 16 hours. If the same device only wakes up 10% of the time (duty cycle 10%), the denominator becomes 10 mA, extending the mah battery life to 160 hours.
Efficiency accounts for energy lost as heat in voltage regulators, wiring resistance and battery chemistry limitations. A switching buck converter might achieve 90 to 95% efficiency, while a linear regulator can drop below 50% when the input voltage is much higher than the output.
Reference table: common capacities and loads
The table below shows estimated runtimes at 100% efficiency and continuous duty for typical pairings. Use it as a quick sanity check before entering exact numbers into the calculator.
| Capacity (mAh) | Load (mA) | Runtime (hours) |
|---|---|---|
| 500 | 50 | 10 |
| 1000 | 100 | 10 |
| 2000 | 100 | 20 |
| 3000 | 200 | 15 |
| 5000 | 500 | 10 |
| 10000 | 1000 | 10 |
Real-world results will be shorter once you apply a realistic efficiency factor. For instance, the 3000 mAh row above drops to about 12.75 hours at 85% efficiency.
Understanding duty cycle in battery life estimates
Duty cycle describes the fraction of time a load is actually drawing current. Many embedded devices, sensors and IoT nodes spend most of their time in a low-power sleep state and wake briefly to take a reading or transmit data.
If a sensor draws 50 mA but only wakes for 2 seconds every minute, its duty cycle is roughly 3.33%. The effective average current becomes 50 × 0.0333 = 1.67 mA. This dramatically extends how long a battery will last compared to running the same load continuously.
When estimating battery runtime for intermittent loads, measure or calculate the true average current over one full on-off period. Enter that average directly as the load, or enter the peak current and set the duty cycle field accordingly. Both approaches yield the same result.
Practical tips for accurate estimates
Nominal mAh ratings are measured under specific laboratory conditions, typically at a low discharge rate. At high currents, the usable capacity shrinks due to internal resistance and the Peukert effect, which is more pronounced in lead-acid cells but still relevant for lithium-ion at extreme loads.
Temperature also matters. Lithium batteries lose significant capacity below freezing. If your device operates outdoors in winter, derate the capacity by 20 to 40% depending on the chemistry and temperature.
For projects involving radio transmitters, remember that transmit bursts can draw hundreds of milliamps even if the average is low. Ensure your battery can supply the peak current without excessive voltage sag. Pairing this tool with a mah to wh converter helps when comparing batteries of different nominal voltages, since watt-hours reflect total stored energy regardless of cell voltage.
Common mistakes when calculating battery runtime
A frequent error is confusing milliamps with amps. A load listed as 0.5 A must be entered as 500 mA. Another mistake is assuming 100% efficiency for circuits that use linear regulators or cheap boost converters, which can waste substantial power.
People also sometimes forget to include quiescent current: the small amount of power a voltage regulator or microcontroller draws even in sleep mode. If your main load is tiny, quiescent current can dominate and shorten the battery life far more than expected.
Finally, do not mix up capacity units. Watt-hours and milliamp-hours are related by voltage (Wh = mAh × V / 1000). Always ensure the capacity you enter matches the unit the calculator expects.
Worked examples
| Example | Input | Result |
|---|---|---|
| 2000 mAh at 100 mA, ideal | Capacity (mAh): 2000; Average load (mA): 100; Efficiency (%): 100; Duty cycle (%): 100 | Runtime: 20 hours; Breakdown: 20 hours; Effective load: 100 mA; Usable capacity: 2,000 mAh |
| 2000 mAh at 100 mA, 80% efficient | Capacity (mAh): 2000; Average load (mA): 100; Efficiency (%): 80; Duty cycle (%): 100 | Runtime: 16 hours; Breakdown: 16 hours; Effective load: 100 mA; Usable capacity: 1,600 mAh |
| IoT sensor: 1000 mAh, 10 mA, 10% duty | Capacity (mAh): 1000; Average load (mA): 10; Efficiency (%): 100; Duty cycle (%): 10 | Runtime: 1,000 hours; Breakdown: 41 days, 16 hours; Effective load: 1 mA; Usable capacity: 1,000 mAh |
| High drain: 3000 mAh, 500 mA | Capacity (mAh): 3000; Average load (mA): 500; Efficiency (%): 85; Duty cycle (%): 100 | Runtime: 5.1 hours; Breakdown: 5 hours, 6 minutes; Effective load: 500 mA; Usable capacity: 2,550 mAh |
FAQ
How accurate is this battery runtime calculator?
It provides a theoretical estimate based on the inputs you supply. Real-world battery life varies with temperature, discharge rate, battery age and the accuracy of your load measurements.
What efficiency percentage should I use?
For a direct connection with no regulator, use 95 to 100%. For a switching DC-DC converter, 85 to 95% is typical. Linear regulators can be much lower, sometimes 40 to 60%, depending on the voltage drop.
Can I use this for car batteries or lead-acid cells?
Yes, but convert the capacity to mAh first (a 50 Ah car battery is 50,000 mAh). Keep in mind that lead-acid batteries suffer heavily from the Peukert effect at high discharge rates, so actual runtime may be significantly shorter than the formula suggests.
How does duty cycle affect mah battery life?
A lower duty cycle means the load draws current for a smaller fraction of the time, reducing the effective average current. Halving the duty cycle doubles the estimated runtime.
Why does my battery die faster than the calculator predicts?
Common reasons include unaccounted quiescent current from regulators, higher-than-expected peak loads, cold temperatures reducing capacity, or an aged battery that no longer holds its rated mAh.
Is my data sent anywhere when I use this tool?
No. All calculations happen locally in your browser. Nothing is uploaded to any server.