It’s important to me that anybody be able to reproduce these results, so I’ll be documenting the details of each test, and showing some cheaper alternatives that you might prefer to use.
What’s in the box?
The big one, and the reason you buy a power bank. I’ll be testing each power bank from being fully charged, measuring the capacity until it turns off. This capacity will not match the headline figure as these are calculated from the nominal 3.7V of the lithium-ion cells inside and there will be some conversion losses. A typical 10Ah (10,000mAh) power bank will give about 6,600 mAh at 5V and 90% efficiency.
Power Supply Stability
Will it really output 2.1 amps at 5 volts? Can it really do 3.4A across both ports? Does it consistently maintain 5V under load as it discharges? If not, your phone will charge more slowly or may not even finish charging. I’ll be checking these claims.
Is the cable that comes with the power bank any good? Will you get better charging performance if you change it?
If you leave it for a few weeks how much capacity will have disappeared?
Charge and Power?
Can it recharge and run a device at the same time? This is especially useful for devices that have multiple ports, or where you only have one power socket available.