Battery Life
Enter your battery's capacity, your device's average current draw, and a derating factor to account for real-world losses. The calculator returns runtime in both hours and days.
Watch: How to estimate battery runtime correctly
The formula is one line, but a video is the fastest way to see why the "naive" answer is almost always wrong. Watching a real cell get discharged under load, and seeing where the voltage actually falls off a cliff instead of where the math says it should, makes the derating factor click.
Suggested video topics to film or source for this page:
- Discharging a Li-Ion cell under constant current load and logging voltage over time
- Comparing measured runtime against the calculator's estimate for the same setup
- Showing the Peukert effect: same battery, two different discharge rates, two different usable capacities
- Sizing a coin cell for a year-long IoT sensor node, including sleep current
Estimating battery runtime
"How long will my device run on this battery?" is one of the most common questions in electronics, and one of the easiest to get wrong with a back-of-envelope calculation. A simple formula gives a rough baseline, but real battery behavior involves discharge curves, temperature, and cut-off voltages that a single division doesn't capture.
Whether you're powering a Raspberry Pi from a USB power bank or flying a drone on a LiPo pack, this calculator converts everything into common units and applies a safety derating factor to give you a number closer to what you'll actually see.
How to calculate battery life
Theoretical runtime is capacity divided by load current. A derating factor corrects for the gap between that theoretical number and reality: heat loss, voltage converter efficiency, and the fact that no battery can be drained to 0% and still power a device.
Time (Hours) = (Capacity (mAh) / Current (mA)) × Derating Factor
Worked example
| Parameter | Value |
|---|---|
| Battery capacity | 2000 mAh |
| Load current | 50 mA |
| Derating factor | 0.85 |
| Result | (2000 / 50) × 0.85 = 34 hours |
Without derating, the naive answer is 40 hours. In practice, the voltage drops below the device's minimum operating level before the battery is technically "empty," so the real runtime lands closer to 34.
Practical applications
| Use case | Why runtime estimation matters |
|---|---|
| IoT devices | Determines whether a sensor node can run for a year on a coin cell |
| Backup power | Sizes a UPS to keep servers running through an outage |
| Wearables | Balances battery size against runtime for smartwatches and fitness trackers |
Frequently asked questions
Why does a battery drain faster at high currents?
This is the Peukert effect. As discharge current rises, internal resistance wastes more energy as heat (I²R loss), which cuts into usable capacity. A battery rated for 100Ah might only deliver around 60Ah if drained quickly.
What is a good derating factor?
For most consumer chemistries (Li-Ion, alkaline), 0.85 is a safe standard, accounting for about 15% loss. Lead-acid batteries shouldn't be discharged below 50% without risking damage, so use 0.50 for those instead.
How do I convert watts to amps?
If your device's power is listed in watts, use I = P / V. A 5W device running on 12V draws 5 / 12 = 0.41A, or about 410mA.
Does temperature affect battery runtime?
Yes. Cold temperatures raise a battery's internal resistance and lower its usable capacity, sometimes significantly for Li-Ion cells below freezing. Heat generally improves short-term discharge performance but accelerates long-term degradation. The derating factor here assumes moderate, room-temperature conditions.
Should I use average current or peak current for this calculation?
Average current, if your device's draw varies over time (like a sensor that sleeps between readings). Using peak current instead will give an overly pessimistic runtime estimate. For devices with a sleep mode, calculate a time-weighted average of the sleep and active currents first.