How long does a power bank take to charge?

A 5,000 mAh power bank or phone battery takes about 65 minutes on a 20 watt charger before the slow top-off. At 3.7 volts it holds 18.5 Wh. At 85% charging efficiency that means about 21.8 Wh from the charger, and 21.8 divided by 20 watts is about 65 minutes.

Bigger banks take proportionally longer, and so do slower chargers. To convert any rating, multiply milliamp-hours by the cell voltage and divide by 1,000 to get watt-hours.

Real devices slow down past about 80%. Lithium-ion charges at a steady current first, then holds its voltage while the current tapers. Battery University says the battery is full when the current falls to 3% to 5% of its rating. The battery charging time calculator has a mAh and watts mode for phones and power banks.