Real batteries rarely deliver their full rated capacity, especially at high currents or low temperatures.
How to use the Battery Life Calculator
- Battery capacity
- Average current draw
- Efficiency
Results update as you type, and every field accepts the units shown in its menu.
How it works
Divide capacity by current draw, then multiply by the efficiency factor.
Tips
- Use the figures printed on your product or its manual when you have them.
- Round quantities up so you are never short.
- Measure twice before you buy anything cut to size.
- Everything runs in your browser – try different values to compare options.
Common mistakes to avoid
- Mixing units (inches and centimetres, mAh and Ah).
- Ignoring efficiency losses and safety margins.
- Treating rules of thumb as exact.
Inputs explained
| Field | What to enter |
|---|---|
| Battery capacity | In mAh |
| Average current draw | In mA |
| Efficiency | In % |
What each result means
- Estimated runtime = [Battery capacity]÷[Average current draw]×[Efficiency]÷100
- In hours = [Battery capacity]÷[Average current draw]×[Efficiency]÷100
Formula
Runtime = capacity ÷ load current × efficiencyWorked example
Inputs — Battery capacity: 3,000 mAh; Average current draw: 150 mA; Efficiency: 85 %.
Results — Estimated runtime = 17 h 0 min; In hours = 17.
Frequently asked questions
How do I convert Wh to mAh?
mAh = Wh × 1000 ÷ voltage.
Why use efficiency?
Converters, temperature and battery age reduce usable capacity.
What formula does the Battery Life Calculator use?
Runtime = capacity ÷ load current × efficiency.