USB battery packs / powerbanks (or whatever you call them) are great when you’re out and about and need to charge your devices but there are some features which you should look out for when buying one

1. Buy from a reputable source
   • You don’t want bad batteries / electronics, fake capacities or powerbanks filled with sand
2. Sufficient capacity
   • You need a powerbank that is capable of charging all your devices and has enough juice to get you through the day
   • Phones: ~2000mAH, 7″ tablets: ~4000mAh, 10″ tablets: ~8000mAh
     (iPhone 5S: 1560mAh / HTC One M8: 2600mAh / Lumia 1520: 3400mAh / S5: 2800mAh / iPad Air 2013: 8820mAh / Nexus 7 2013: 3950mAh)
3. Output Charge speed
   • Ensure it can charge your device at the best and fastest speed
     • Tablets usually 2A
     • Phones usually 1A
4. Passthrough ability
   • When the powerbank is being charged when connected to a power outlet, passthrough allows it to charge any device connected to powerbank, straight from the power source and not from the powerbank itself: perfect if you want to leave all your other charges behind and charges all your devices with just a single power outlet! (Note: may want to ensure that it charges at full USB charge speed)
5. Battery gauge
   • Nice quick way to know how much capacity is left in your powerbank
   • Allows rationing of your charge
6. Charges via a standard USB cable
   • Being able to charge your powerbank using a standard cable is ideal as proprietary charges can be hard to replace if lost
   • Ideally the same cable as your other devices (my powerbank can charge via miniUSB but all my devices use microUSB so it is one extra cable to bring along)
7. Multiple USB ports
   • for those with multiple devices (or for sharing with others)
8. Port positions
   • Some USB cables may be bulkier than others and thus powers that are spaced out are useful so that you can utilise all ports at the same time
   • Ideally all ports on a single side so it can be pocketable or just easily placed on a surface standing up if needed
9. Charges quickly via USB
   • If allows charging via several methods (e.g. proprietary cable as well as via USB), ensure that it can charge quickly over USB (my Sanyo powerbank charges very slowly over USB)
10. If it supports passthrough, added bonus ability to maintain proper charge, amperage and voltage when plugged/unplugged from power source
    • This use case comes from using your powerbank as the power source for devices like the Raspberry Pi: you want it to provide the power to the Raspberry Pi and if the power source gets disrupted and you do not wish for the device to get restarted, the powerbank must maintain a proper current/voltage so not to trigger a restart.

Note: a 4000mAh powerbank will not charge a 2000mAh phone battery twice due to inefficiencies, battery health and other factors.