How to choose a power bank: mAh, watts and ports
Capacity: mAh
The biggest number on the box is capacity, in milliamp-hours (mAh): how much charge the pack can store. The bigger it is, the more times you can charge your phone, but the pack is also heavier and bigger.
One thing boxes do not say: some energy is lost in conversion, when the pack raises the voltage for the phone and when the phone charges its own battery. That is why a 5,000 mAh pack gives the phone less than 5,000 mAh. Allow a margin.
How much capacity you need
For a long day in town, a festival or a flight, a small 5,000 mAh pack is usually enough for a partial or full phone recharge, depending on the model. It is light and fits a pocket.
For several days away from a socket, camping or in the mountains, you need more capacity, but the pack weighs more too. Think about how often you are away from a socket, not about the biggest number on the shelf.
Power: watts
Power, in watts (W), tells you how fast the phone charges. Modern phones accept fast charging over USB-C; a pack with more USB-C power charges them faster, as long as the phone supports that power.
On magnetic packs, charging over the USB-C cable usually gives more watts than the magnetic way. Wireless charging is more convenient but slower, and it warms up more than charging by cable; when you are in a hurry, use the cable.
The ports
USB-C is today's port: it charges new phones, earbuds and many tablets and, usually, the power bank itself charges through it too. Many small packs have a single USB-C port, used both ways: to charge the pack and to charge the phone.
If you want to charge several devices at once, say the phone and the earbuds, check whether the pack has several ports. For hotels and travel, a multi-port wall charger covers the rest.
Magnetic charging
Some power banks snap magnetically onto the back of the phone and charge it without a cable. Magnetic packs have a ring that holds a compatible phone while it charges: you use it in your hand without a cable hanging.
It works with phones that have compatible magnetic charging, or with a case that adds it. A thick case or a metal ring for a stand can stop it holding.
On a plane and in luggage
Power banks go in hand luggage only, never in the hold. Most airlines accept them without approval up to 100 Wh. A 5,000 mAh pack is about 18.5 Wh, well under the usual airline limit for spare batteries.
To get Wh from a mAh figure: multiply by the nominal voltage, usually 3.7 V, and divide by 1,000.
How to keep it
Do not leave the pack in a car in the sun or in hard frost. If you will not use it for months, charge it to about half before putting it away and top it up now and then. Do not cover it while it charges. If it swells, warps or gets unusually hot, stop using it and take it to a battery collection point.
Weight and size
A power bank you leave at home because it is heavy does not help. For every day, choose one that fits a pocket or a small bag: a 5,000 mAh pack usually weighs around 100–150 g, and a magnetic one holds onto the phone, so you do not carry an extra object in your hand.
For long trips, a bigger pack goes in the rucksack, where weight matters less. Many people have two: a small one for ordinary days and a big one for journeys.
Earbuds and the watch
A power bank charges not only the phone but also the earbud case, the watch or a small speaker, with the right cable. Earbud cases have small batteries, so a 5,000 mAh pack charges them several times. A case like that of the TESNA GTA18, which also has Qi wireless charging, charges from the pack over its USB-C cable.
Safety while charging
Use cables and chargers in good condition: a frayed cable or a damaged charger can overheat. Charge the pack on a hard surface, not on a bed or a sofa, and do not leave it for hours in a car or next to a heat source.
For earbuds you charge from it, see true wireless earbuds.