My issue with battery banks is that they are the same technology as the batteries in your phone (is same voltage) and charging is bottlenecked at the USB standard of 5 v resulting in both conversion and charging efficiency losses. Ankur has started in their manuals that this is roughly a 20-30% loss from the stated capacity.
This means that on top of carrying a USB cable, you're carrying an extra 20-30% of capacity to just throw it away. On a 10000mah bank, this loss would be the equivalent capacity of my phone.
Aside from being required out of necessity, using a battery to charge a battery is a poor engineering decision.
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