Power for days, not hours
A high-capacity power bank is the big kind: enough for several phone charges, or a phone and a tablet, or two phones over a weekend. It suits travelling, festivals, long journeys, camping, and keeping the household's phones going in a power cut. The high-capacity power banks here today include a magnetic one that clicks onto the back of the phone, in a 20,000mAh size, with a magnetic ring for use with a case. Larger sizes are filed in 20,000mAh and 30,000mAh and over power banks.
What the size means
The mAh on a power bank is measured at its cells, at about 3.7 volts, and roughly two-thirds of it reaches the phone. So a 20,000mAh power bank gives a phone with a 4,000mAh battery about three full charges, and a 30,000mAh one about five. Busy phones, cold weather and an old power bank give less, so buy one size above what the sum says. Bigger also means heavier: a 20,000mAh power bank is more for a rucksack than a pocket, and many people end up with two, a small one for every day and a large one for trips.
What to check on a big power bank
- The input. This is the speed people forget. A 20,000mAh power bank holds about 74Wh, so on a 10-watt input it needs well over seven hours to fill, and on a 30-watt USB Power Delivery input closer to three. Fill it from a fast wall charger with a UK three-pin plug.
- The output. USB PD on a USB-C socket for fast charging. Some big power banks can charge a laptop; check the watts.
- Sockets. Several sockets let two devices charge at once, but the output is shared between them.
- The watt-hours. These decide whether it can fly. A 20,000mAh power bank at 3.7 volts is about 74Wh, well within the cabin limit. A 30,000mAh one works out at about 111Wh and needs the airline's approval. Read the figure printed on your own power bank.
Questions people ask about high-capacity power banks
Can I take a 30,000mAh power bank on a plane?
Power banks travel in your hand luggage, never in a checked bag, and the UK Civil Aviation Authority allows two per passenger. They must not be recharged on board, and each should sit in a pouch or its packaging so its sockets cannot touch keys or coins. The limits are in watt-hours: under 100Wh is allowed in the cabin, 100Wh to 160Wh needs the airline's approval, and over 160Wh is not allowed in passenger baggage. Check your airline's own rules as well, as some are stricter.
How should I keep one for power cuts?
Apple's advice for storing its own devices suits the same kind of battery: about half charged, somewhere cool, topped up every six months. Fill it before a storm is due.
Is it safe?
Never charge a swollen, damaged or hot power bank. Stop using one that bulges, cracks, smells odd, smokes or gets too hot to hold, and do not take it apart. Charge it uncovered on a hard surface, not on a bed or inside a bag, and unplug it once it is full: Electrical Safety First advises against charging power banks overnight or while you are out. An old power bank goes to a battery recycling point, never the household bin.
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