milliampere-hour
Electric chargeexact by definition
The milliampere-hour is the charge delivered by one milliampere flowing for one hour, equal to exactly 3.6 coulombs. It is the standard rating for phone, laptop and tool batteries, and the conversion is exact because both the ampere and the hour are exactly defined. A 3000 mAh cell can supply 3000 mA for an hour, 300 mA for ten hours, or any other product that comes to the same charge.
Watch out: A milliampere-hour is a charge, not an energy. Energy is charge times voltage, so a 3000 mAh cell at 3.7 V holds about 11.1 Wh while a 3000 mAh pack at 11.1 V holds three times that. Comparing mAh ratings across devices with different pack voltages, such as a phone against a power bank against a cordless drill, is meaningless without the voltage.
| 1 mAh | 3.6 C |
About the milliampere-hour
Battery capacity in milliampere-hours answers "how many charges can this move", and the energy question needs one more number. Multiply by the nominal voltage to get watt-hours: \(E = Q \ imes V\). A 5000 mAh power bank is usually quoted at the cell voltage of 3.7 V, giving about 18.5 Wh, but it outputs at 5 V through a converter that is maybe 85 to 90 percent efficient. That is why a 5000 mAh bank does not fill a 5000 mAh phone twice, or even once and a half.
The rating also depends on how fast you draw it. Cell capacity is quoted at a specified discharge rate, often \(C/5\) or \(C/20\), and pulling harder returns less total charge because internal resistance drops the terminal voltage to the cutoff sooner. This is the Peukert effect, most pronounced in lead-acid and much milder in lithium-ion. A 100 Ah lead-acid battery rated at the twenty-hour rate will not deliver anything close to 100 Ah if you draw it flat in two.