Power bank: Capacity, Wh and fast charging

In short: A power bank is a mobile battery with which you can charge a smartphone, earbuds or laptop on the go; the energy actually usable is smaller than the figure on the packaging.

In more detail: The capacity is usually stated in mAh (for example 20,000 mAh). That is the amount of charge at the cell voltage (about 3.6 to 3.7 V). When charging a smartphone the voltage is converted to 5 V or higher, and losses occur. That is why you never get the full stated figure usable.

In Depth

Calculating with Wh

More meaningful is the energy in watt-hours (Wh): Wh = mAh × voltage (V) ÷ 1000. Example: 20,000 mAh × 3.7 V = 74 Wh. Of this, depending on converter losses, about 60 to 70 % is usable (around 45 to 50 Wh). A smartphone battery has about 15 to 20 Wh, so a phone can be charged about two to three times. A laptop often needs 40 to 70 Wh.

Ports and fast charging

USB-C with Power Delivery (PD) supplies 18, 30, 45, 65 W or more and also charges laptops and tablets. Important: power bank, cable and device must support the same protocols (PD, QC, PPS). Cheap cables limit the power. Several ports share the power when used simultaneously.

Cells and safety

Power banks contain lithium-ion or lithium-polymer cells. Quality products have protection circuits against overcharging, overheating and short circuits. Do not use damaged, swollen or very hot devices, and dispose of them properly (collection points, battery take-back). Do not leave them in the sun in a car.

Travel and everyday use

On aircraft power banks belong in hand luggage, not in checked baggage. The limit without approval is usually 100 Wh; up to about 160 Wh only with the airline’s consent; above that prohibited. Observe new rules of individual airlines (use in flight partly restricted). Power banks should be stored about half charged after use and used regularly.

See also: Battery and charging correctly, USB-C, Qi: Wireless charging