How long will the battery in your flashlight, sensor or drone really last? With the Calculate battery life you get a solid estimate in seconds. This guide walks you through the formula, a concrete example and why real runtime often differs from the calculation.
The Formula
Battery life follows a simple relationship:
- t = (Capacity · Usable factor) / Current
- Enter capacity in mAh (or Ah).
- Enter current in mA (or A).
- The usable factor (default 85 %) accounts for converter losses and the usable depth of discharge.
Key unit conversion: 1 Ah = 1000 mAh. Capacity and current must always use the same unit family (mAh with mA, or Ah with A).
Worked Example
Take a power bank of 2000 mAh feeding a device with a constant draw of 100 mA. With the default usable factor of 85 %:
- t = (2000 · 0.85) / 100
- t = 1700 / 100
- t = 17 hours
Without a usable factor the math would give 20 hours – the factor brings the estimate closer to reality, because part of the capacity is lost and cannot be used.
Why Real Runtime Differs
The formula gives a good ballpark, but several physical effects cause deviations:
- Peukert effect: The higher the discharge current, the less capacity is actually usable. Under heavy load, runtime drops disproportionately.
- Self-discharge: Every battery slowly loses charge over time, even with no load attached.
- Temperature: Cold significantly reduces available capacity; heat accelerates ageing.
- Ageing: After many charge cycles, the usable capacity of a battery declines.
For a realistic figure, adapt the usable factor to your scenario: lower for high load or an old battery, slightly higher for gentle continuous discharge.
Conclusion
With capacity, current draw and a usable factor you can estimate battery life quickly and reliably. Keep the real-world influences in mind and plan a safety margin for critical applications.
