Formula
Energy is conserved aside from the efficiency, so the output milliamp-hours shrink when the voltage rises:
charges = bank mAh × cell volts / output volts × efficiency / device mAh
The runtime page never changes voltage. This page exists because a bank rated at the cell voltage does not deliver that many milliamp-hours at 5 volts.
Assumptions
- The bank rating is at the cell voltage.
- The device rating is at the output voltage.
- Efficiency covers the conversion.
Worked example
A 20,000 mAh bank at 3.7 V, outputting 5 V at 85% efficiency, fills a 3,000 mAh device 4.19 charges.
Input definitions
- Cell voltage
- The voltage the milliamp-hour rating refers to, often 3.7 V.
- Output voltage
- The voltage leaving the bank, often 5 V.
Limitations
This is a planning figure from the numbers you type. It is not a measurement of a network, a camera, a printer, or a power supply.
Questions
Why not divide 20,000 by 3,000?
That would ignore the step from 3.7 V to 5 V and the conversion loss. Both reduce the charges.
Is this a phone runtime?
No. It counts full device batteries. Hours at a watt load stay on the runtime page.