How to use it
- Type the Battery capacity (mAh) from the cell's label or datasheet.
- Type the Active current (mA) and the Sleep current (mA) of your device. Leave the sleep current empty if it does not sleep.
- Type the Active time share (%): the part of each cycle the device is working.
- Read Estimated life (days). Open Working, step by step to see the average current and the usable capacity.
The formula
A device that alternates between active and sleep draws a current that averages out over time. The battery supplies that average until its usable capacity is used up. The derating factor reduces the rated capacity to the share a real load gets.
I_average = d × I_active + (1 − d) × I_sleep
usable capacity = C × derating
life (hours) = usable capacity ÷ I_average
life (days) = life (hours) ÷ 24
d share of time active (duty cycle)- Battery capacity falls as the discharge current rises: the higher the discharge rate, the lower the capacity, and capacity also drops at low temperature (Wikipedia, Electric battery).
- The derating default of 80 % is a planning allowance chosen for this estimate. It is not a figure from a battery maker's data, and the real share depends on the cell, the current and the temperature.
- Without derating the same calculation gives the upper figure, shown as the life with no derating.
What this calculator does not cover
- Battery curves in the datasheet: a manufacturer's capacity-against-current chart is more accurate than the single derating factor, and should be used for a real design.
- Self-discharge, which drains a cell even when the device is off, and the cut-off voltage at which the device stops working.
- Peak currents, such as radio transmissions, that a small cell cannot deliver even when its average current is low.
Worked example: 2500 mAh, 20 mA for 2 % of the time
The average current with a sleep current of 0.01 mA is 0.02 × 20 mA + 0.98 × 0.01 mA = 0.4098 mA.
With 80 % derating the usable capacity is 2500 mAh × 0.8 = 2000 mAh, so the life is 2000 mAh ÷ 0.4098 mA = 4880 h, which is 203.4 days.
With no derating the same battery would last 6101 hours, about 254 days, so the derating takes off about a fifth of the estimate.
Frequently asked questions
Why is the estimate shorter than capacity ÷ current?
A cell delivers less than its rated capacity when the current is high or the temperature is low. The derating factor allows for that, so the estimate is closer to a real device than the simple division.
What is the duty cycle?
It is the share of time the device is active. A device active for 1.2 s every minute has a duty cycle of 2 %. Enter it as a percentage.
What value should I use for the derating?
The calculator starts at 80 %. Check the battery datasheet for its capacity at your average current and temperature, and enter that share instead.
Does the calculator include self-discharge?
No. Self-discharge takes some capacity every month even with no load, so a very long estimate is only approximate. Add the cell's self-discharge rate from its datasheet if it matters for your case.
Can I use this for a battery that is not a coin cell?
Yes, as long as you know its capacity in mAh. For a rechargeable cell, the usable capacity also falls with age, so use a lower derating for an old battery.
Sources
Figures and text last checked on Tuesday, November 10, 2026.