Battery Life Calculator
Convert battery capacity between mAh, Ah and Wh, check the FAA 100 Wh flight limit, and estimate runtime from a device's draw and duty cycle.
FAA limits for lithium-ion batteries
Passenger baggage. The mAh column assumes a 3.7 V cell — check the voltage printed on your own pack.
| Rating | Carry-on | At 3.7 V, about |
|---|---|---|
| 0-100 Wh | Allowed | 27,027 mAh |
| 101-160 Wh | Airline approval | 43,243 mAh |
| Above 160 Wh | Forbidden | Above that |
Where to find a draw: ESP32 power modes
Espressif's own figures for the ESP32, as an example of what a datasheet gives you. Your chip's table will differ.
| Power mode | Current |
|---|---|
| Modem-sleep, 80 MHz | 20 mA – 25 mA |
| Light-sleep | 0.8 mA |
| Deep-sleep, ULP running | 150 µA |
| Deep-sleep, RTC timer only | 10 µA |
| Hibernation | 5 µA |
Estimates assume a steady draw and ignore temperature, cell ageing, voltage sag near empty and converter losses, all of which shorten real runtime. Flight rules are the FAA's for passengers departing the United States; your airline and destination may impose stricter limits.
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How Long Does a 5000 mAh Battery Last?
That question has no single answer, and any tool that gives you one has invented the missing half. Runtime is capacity divided by draw, so a 5000 mAh pack runs 10 hours at 500 mA and 500 hours at 10 mA. What one number does settle is the battery itself: how much charge it holds, how much energy that is in watt-hours, and whether you are allowed to fly with it. Start there, then add a draw when you know one.
How Battery Runtime Is Calculated
Capacity comes in two currencies. Milliamp-hours (mAh) measure charge and only mean something at a stated voltage; watt-hours (Wh) measure energy and are comparable across any battery. The FAA gives the conversion: divide mAh by 1000 to get amp-hours, then multiply by the volts. Runtime then follows from energy and average draw. If the device sleeps between bursts, the average draw is what counts, not the peak.
Energy and runtime
Wh = mAh ÷ 1000 × V · Runtime (h) = Usable Wh ÷ Average draw (W)How to Use the Battery Life Calculator
Enter the capacity printed on the battery and pick its unit — mAh, Ah or Wh.
Enter the nominal voltage, also printed on the pack. It is what links charge to energy.
Read the watt-hours and the flight verdict straight away.
Switch to Runtime and add the current or power the device draws.
If the device sleeps, set the duty cycle and the sleeping draw.
Lower Usable capacity if you plan to stop before the pack is flat.
Common Applications
Airline carry-on check
Convert a power bank's mAh to watt-hours and see whether it is under 100 Wh, in the 101-160 Wh approval band, or forbidden.
Comparing two batteries
Put both in watt-hours before you compare. mAh figures at different voltages are not the same quantity.
Battery-powered sensors
Model a device that wakes, transmits and sleeps, using the duty cycle and a sleeping draw from its datasheet.
Power bank reality check
A 10,000 mAh bank is rated at its 3.7 V cells but delivers at 5 V. Convert to watt-hours to see what actually reaches the phone.
Why Convert Before You Estimate
mAh alone is not comparable
A 10,000 mAh pack at 3.7 V holds 37 Wh; a 10,000 mAh pack at 11.4 V holds 114 Wh and needs airline approval to fly. Same headline number, three times the energy.
Airlines measure in watt-hours
The FAA sets its carry-on rules on Wh, not mAh, and most packs print only mAh. Converting first tells you which of the three bands yours falls in.
Average draw decides runtime
A sensor that wakes for one second a minute is not drawing its peak current. Weighting the draw by duty cycle is the difference between days and months of predicted life.
Frequently Asked Questions
It depends entirely on the draw, which is why no honest calculator answers this from one number. At 500 mA it lasts 10 hours; at 50 mA, 100 hours; at 5 mA, over a month. Find the draw in the device's specifications, measure it with a USB power meter, or read it from the chip's datasheet, then use the Runtime mode.
Divide the milliamp-hours by 1000 to get amp-hours, then multiply by the battery voltage. The FAA states it with this example: a 12-volt battery rated at 8 Ah is rated at 96 Wh, because 12 times 8 is 96. A 5000 mAh phone battery at 3.7 V is 18.5 Wh.
The FAA allows lithium-ion batteries of 0-100 Wh in carry-on baggage, requires air carrier approval for 101-160 Wh, and forbids anything above 160 Wh. Spare batteries must travel in the cabin, not in checked baggage. At 3.7 V, 100 Wh is about 27,000 mAh, so most consumer power banks are under the limit.
The bank's mAh rating is measured at its internal cell voltage, around 3.7 V, but it delivers over USB at 5 V. Energy is what is conserved, not charge: 10,000 mAh at 3.7 V is 37 Wh, which at 5 V is only 7,400 mAh before any conversion loss. Comparing in watt-hours removes the illusion.
Yes, because the capacity itself falls. Apple states that iPhone 14 and earlier batteries are designed to retain 80% of original capacity at 500 complete charge cycles, and iPhone 15 batteries at 1000 cycles, under ideal conditions. Enter the aged capacity rather than the label figure if you want a realistic estimate.