Battery Life Calculator
Enter the battery capacity and the load, with an efficiency derating and an optional duty cycle for devices that sleep. Results are estimates for learning and design checks.
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Formula
- Usable capacity:
usable = capacity (mAh) × efficiency - Power loads:
current (mA) = power (mW) ÷ voltage (V) - Average current:
average = duty × active + (1 − duty) × sleep - Runtime:
hours = usable ÷ average, anddays = hours ÷ 24
Worked example
A 2000 mAh battery at 80% efficiency has 1600 mAh usable. A constant 100 mA load gives 1600 ÷ 100 = 16 hours (0.67 days).
With a duty cycle: 50 mA active for 10% of the time and 0.1 mA asleep at 100% efficiency gives an average of 0.1 × 50 + 0.9 × 0.1 = 5.09 mA, so 2000 ÷ 5.09 = 392.9 hours (16.4 days).
Frequently asked questions
How do I calculate battery life?
Divide the usable capacity by the average current. A 2000 mAh battery at 80% usable gives 1600 mAh; at 100 mA that lasts 1600 / 100 = 16 hours.
What efficiency or derating should I use?
Real batteries deliver less than the label at high loads, in the cold, or when you stop before they are fully empty. 70 to 90% is a common planning range; check the cell datasheet.
How does the duty cycle work?
Average current = duty x active current + (1 - duty) x sleep current. At 50 mA active, 10% duty and 0.1 mA sleep the average is 5.09 mA, so 2000 mAh at 100% lasts about 393 hours (16.4 days).
How do I use a power load in watts or milliwatts?
Pick the power mode: current is power divided by battery voltage. 740 mW at 3.7 V is 200 mA, so a 1000 mAh battery at 90% lasts 900 / 200 = 4.5 hours. This is an estimate for learning, not a guarantee.