Tech
Battery Life Calculator
Estimate how long any battery will last from its capacity, voltage, and load. Supports mAh or Wh, duty cycle, efficiency, and charging time.
Battery life calculator
Common batteries
Stamped on the cell or in the device spec sheet.
Optional. Used for energy and charging time.
Average draw of the device while it's active.
Usable share of rated capacity. 80% is typical for Li-ion under continuous load.
Share of time the device is on. 100% means always running.
Leave blank to skip charging time. Includes a 1.2x Li-ion inefficiency factor.
Estimated runtime
16h
16 hours (0.6667 days)
Energy
11.1 Wh
Load
0.555 W
Capacity
3,000 mAh
Efficiency
80%
Frequently Asked Questions about the Battery Life Calculator
How do I calculate battery life?
Divide energy by load: runtime in hours equals capacity in Wh divided by load in watts. If you have capacity in mAh and a current load in mA, the math reduces to runtime = mAh / mA, no voltage needed. To mix units like mAh capacity with a watt load, multiply mAh by voltage and divide by 1000 first to get Wh.
Why does runtime go down when I add efficiency?
Real batteries never deliver 100% of their rated capacity. Internal resistance, self-discharge, voltage sag, and the cutoff voltage of the device all cut usable energy. 80% is a reasonable default for Li-ion under continuous load. Cells run cold, near end-of-life, or under heavy current may be closer to 60 to 70%; very light loads near room temperature can approach 95%.
When do I need to enter a voltage?
Whenever capacity and load are in mismatched unit families. A mAh capacity with a watt load needs voltage to convert mAh into Wh. A Wh capacity with a mA or A load needs voltage to convert that current draw into watts. If both are in the same family (mAh + mA, or Wh + W), voltage is optional and only affects derived values like energy or charging time.
What is duty cycle and when should I change it from 100%?
Duty cycle is the share of time your device is actually drawing the load you entered. A sensor that wakes for 1 second every 10 seconds has a 10% duty cycle, so its average draw is one tenth of the peak. Setting duty cycle to 50% doubles the runtime versus always-on. Use 100% for anything that runs continuously, like a flashlight on full power or a phone streaming video.
How accurate is the charging time estimate?
It is a first-order estimate. The base formula is capacity in mAh divided by charging current in mA, then multiplied by 1.2 to account for typical Li-ion charge inefficiency. Real chargers slow down during the constant-voltage (top-off) phase above roughly 80% state of charge, so a full 0 to 100% charge often takes 20 to 40% longer than the simple division suggests.