I bought three different power banks before I found one I would actually trust on a long trip, and the first two were a waste of money in different ways. Here is what the third one did right, and what to look for if you are shopping.

What actually matters in a power bank

The number on the box is rarely the number you get. A “20,000 mAh” bank does not deliver all of that to your phone, because some of it is lost in the voltage conversion. Expect to get maybe two-thirds of the rated capacity in real use.

• Capacity: 10,000 mAh will charge most phones twice, 20,000 will do four times
• Fast charging: make sure it matches your phone’s standard or you will wait forever
• Weight: the big ones are bricks, decide if you actually need that much

What I got wrong the first two times

The first one was a no-name brand that claimed huge capacity and delivered maybe half of it before dying. The second was heavy and powerful but so slow to charge my phone that it was basically useless in a pinch.

The one that finally worked is a 10,000 mAh slim bank with the same fast charging as my phone. It charged my phone twice on an eight-hour drive, recharged overnight in the hotel, and fit in a jacket pocket. That is really all I needed.

For a long trip, buy something with at least 10,000 mAh and true fast charging, and skip the weird brands. A power bank is the one gadget where a recognizable name is genuinely worth the few extra dollars.