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Battery Life Calculator

This battery life calculator estimates how long a battery will power your device based on capacity (mAh) and average current draw (mA). It is a practical tool for electronics designers sizing batteries for IoT sensors, hobbyists building Arduino or Raspberry Pi projects, and anyone comparing power bank options for portable devices or field equipment.

How Battery Runtime Is Calculated

The fundamental formula is:

Runtime (hours) = Battery Capacity (mAh) / Average Current Draw (mA)

For example, a 5,000 mAh battery powering a device that draws 500 mA lasts approximately 10 hours theoretically. In practice, multiply by an efficiency factor of 0.80-0.90 to account for internal resistance losses, voltage conversion overhead, and capacity reduction at higher discharge rates.

Practical Runtime = (Capacity / Current Draw) x Efficiency Factor

Worked Examples

Example 1 — Smartphone: A phone has a 4,500 mAh battery and the screen-on usage draws an average of 300 mA. Theoretical runtime = 4,500 / 300 = 15 hours. With an 88% efficiency factor, practical runtime is about 13.2 hours of mixed use.

Example 2 — Wireless IoT sensor: An ESP32-based temperature sensor wakes every 10 minutes, takes a reading (240 mA for 2 seconds), then sleeps (10 µA for ~598 seconds). Average current = (240 x 2 + 0.01 x 598) / 600 = 0.81 mA. A 3,000 mAh cell lasts 3,000 / 0.81 = 3,700 hours, or about 154 days.

Example 3 — Power bank charging a tablet: A 10,000 mAh power bank charges a tablet that draws 1,500 mA. Accounting for USB conversion losses (85% efficiency): usable capacity = 10,000 x 0.85 = 8,500 mAh. Runtime = 8,500 / 1,500 = 5.7 hours of continuous charge delivery.

Battery Capacity Reference Table

When to Use This Calculator

• Selecting a battery size for a new electronics project before you commit to a enclosure
• Comparing whether a 2,000 mAh versus 3,000 mAh battery meets your target runtime
• Estimating field deployment life for remote sensors, data loggers, or wildlife cameras
• Checking whether a power bank has enough capacity to fully charge your laptop on a flight
• Sizing backup battery capacity for a device that must survive a specific outage window

Common Mistakes

1. Using peak current instead of average current. A device that spikes to 1,500 mA during Wi-Fi transmissions but idles at 80 mA will not drain at 1,500 mA continuously. Measure or calculate the time-weighted average across all operating states — runtime predictions based on peak draw alone are typically 10-20x too pessimistic.
2. Ignoring voltage conversion losses. If your 3.7V Li-ion battery feeds a 5V boost converter to power a USB device, the converter is typically 85-90% efficient. A 5,000 mAh battery does not deliver 5,000 mAh at 5V — apply the efficiency factor before dividing by current draw.
3. Forgetting the cut-off voltage. Li-ion cells stop supplying usable power below about 3.0V per cell. At high discharge rates, voltage sag can cut runtime 15-20% shorter than the mAh-based formula predicts. Always test under realistic load conditions before finalizing a battery selection.
4. Treating rated mAh as delivered mAh. Alkaline batteries at high drain rates (above 200 mA) deliver significantly less than their rated capacity — a 2,700 mAh AA cell may only provide 1,500-1,800 mAh at 500 mA. Li-ion chemistry performs more consistently across discharge rates.

Real-World Applications

Battery life calculations drive decisions in product development, field deployment, and everyday purchasing. Medical device engineers calculating the runtime of a portable monitor use the same formula as a backpacker deciding which battery pack to bring for a five-day trip. Remote weather stations in locations without power infrastructure are sized to run for months between maintenance visits — the average-current calculation accounting for sleep modes is what makes that feasible. EV battery pack designers scale the same math to kilowatt-hours and hundreds of amps, but the underlying ratio of energy stored to power consumed remains identical.

Tips

1. Measure actual current draw with a USB power meter or series ammeter rather than relying on datasheets, which often list only peak or typical values.
2. For IoT and embedded projects, calculate a duty-cycle-weighted average current across all operating modes — active, transmitting, idle, and deep sleep — weighted by the fraction of time spent in each state.
3. Use a boost or buck converter rated at 90%+ efficiency to minimize power conversion losses between the battery and your circuit.
4. Never discharge a lithium-ion cell below 2.5V per cell — a protection circuit (PCB or BMS) is required to prevent damage and potential thermal runaway.
5. Store Li-ion batteries at 40-60% charge when not in use for extended periods to reduce calendar aging.
6. For parallel battery packs, match cells by brand, model, age, and internal resistance to prevent current imbalance during charging and discharging.