Battery Life Calculator
Estimate how long a battery will power a device from its capacity and the current the device draws. Includes an efficiency factor because no battery delivers 100% of its rated capacity in practice.
How to use this calculator
- Enter the battery capacity (mAh or Ah, from the label).
- Enter the device's current draw in mA or A.
- Adjust efficiency — 85% is realistic; lower for high drains or cold conditions.
Formula used
Capacity and draw must use matching units (both in milli- or both in base). The efficiency factor accounts for the fact that batteries never deliver their full rated capacity — usable capacity drops with high discharge rates (Peukert effect), cold, and age.
Example calculation
A 3,000 mAh battery powering a 200 mA device at 85% efficiency: 3000 ÷ 200 × 0.85 = 12.75 hours.
The ideal figure would be 15 hours, but real batteries don't deliver every rated milliamp-hour.
Why batteries never last their rated time
The simple runtime formula (capacity ÷ draw) gives an optimistic ceiling that real batteries rarely reach. Rated capacity is measured under gentle, ideal lab conditions; in use, several effects eat into it. The Peukert effect means higher discharge currents yield less usable capacity — draw hard and fast and you get proportionally fewer amp-hours than a slow drain would give. Cold reduces capacity further, and every battery ages, losing capacity over charge cycles.
The efficiency factor rolls these into one adjustment, typically 80–90% for moderate loads. It's why a phone with a "20-hour" rated battery gives fewer hours under real use, and why device makers quote runtime under specific, often lenient, conditions. For anything critical, sizing with a realistic efficiency and some margin beats trusting the raw division.
Why use this calculator?
- Estimate device runtime for projects, power banks and backups.
- Includes the efficiency reality the raw formula ignores.
- Handles mixed mAh/Ah and mA/A units automatically.
Frequently asked questions
How do I calculate battery life?
Divide capacity by current draw (in matching units), then apply an efficiency factor. A 2,000 mAh battery driving 100 mA lasts about 2000/100 × 0.85 ≈ 17 hours realistically. Enter your figures above.
Why doesn't my battery last as long as calculated?
Rated capacity assumes ideal lab conditions. In reality, high current draw (Peukert effect), cold temperatures, battery age and the device's cut-off voltage all reduce usable capacity — which is why the efficiency factor (typically 80–90%) is applied.
What's the difference between mAh and Ah?
Just scale: 1 Ah = 1,000 mAh. A 2.5 Ah battery is 2,500 mAh. Small batteries are usually rated in mAh, larger ones (car, solar storage) in Ah. The calculator converts automatically.