Gadget buying guide: power banks that are safe and honest about capacity

A 20,000 mAh power bank does not give you 20,000 mAh to your phone. Cell voltage conversion and heat mean about 60 to 70 percent reaches the device.

We tested five banks with the same phone. The two best gave 12,800 mAh and 12,300 mAh of real charge. The weakest gave 9,600 mAh. Look for a protection mark from a recognised lab on the label and avoid unmarked units in airports.

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