Battery Life Calculator

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.

The efficiency setting reduces rated capacity to an estimate of usable capacity. Internal resistance, voltage sag, temperature, age, discharge rate, and the device's cutoff voltage can all change the result. The 80% default is a planning assumption, not a universal lithium-ion value. Use test data from the battery and load when accuracy matters.

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.

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.

It is a first-order estimate. The calculator divides capacity in mAh by charging current in mA, then applies a fixed 1.2 multiplier. Actual time depends on battery chemistry, charger limits, temperature, state of charge, and the charging profile. Use the battery and charger specifications for safety-critical planning.