When shopping for a portable power bank, one of the main features to look out for is the capacity for fully charging your devices, especially your smartphone. If you have long travel days with little opportunity to plug your phone in and your backup charger can only offer half a. All of the numbers referring to capacity in mAh and watt hours can be a little tricky to understand when deciding what you'll need. The term mAh. One of the most important aspects of choosing a power bank is to make sure it will be compatible with your devices. The last thing you want is to purchase a back-up charger that's only suitable for an iPhone when you have an Android. There are numerous.