Formula
Milliamp-hours become watt-hours, and a usable share can reduce them:
hours = mAh × volts / 1000 × usable share / watts
A usable share of 100% keeps the nominal capacity. The capacity page starts from the hours you want. Charging time starts from a charge current. A power bank converts cell voltage to the output voltage.
Assumptions
- The load stays constant.
- Voltage is the battery voltage that matches the capacity rating.
- The usable share is a single percent, not a measured discharge curve.
Worked example
A 10,000 mAh battery at 3.7 V running a 5 W load lasts 7.4 hours when the full capacity is usable.
Input definitions
- Capacity
- The battery rating in milliamp-hours.
- Voltage
- The voltage paired with that rating. Many cells are labeled 3.7 V.
- Load
- What the device draws, in watts.
Limitations
Age, temperature, and a boost converter can shorten the run. This is not a rating for emergency backup.
Questions
Why ask for voltage?
Milliamp-hours are not energy until they are multiplied by voltage. A 10,000 mAh pack at 3.7 V is not the same energy as one at 5 V.
What should the usable share be?
Leave it at 100% for the nominal rating. Lower it when you already know that conversion loss or a cutoff leaves part of the pack unused.