Milliamp-Hour

A milliamp-hour (mAh) is one thousandth of an amp-hour. Phone batteries, power banks and other small lithium cells are usually rated in mAh.

To compare a mAh rating with a charger's watts, convert it to watt-hours: multiply mAh by the cell voltage and divide by 1,000. A 5,000 mAh battery at 3.7 volts holds 18.5 Wh. On a 20 watt charger at 85% efficiency it needs 21.8 Wh, or about 65 minutes before the slow top-off.

The battery charging time calculator has a mAh and watts mode for phones and power banks. The FAQ on how long a power bank takes to charge walks through the same example.