Plenty of power.
The more power a portable charger stores, the more devices it can charge – or the longer it can stretch the life of a single device. However, a power bank's total capacity in mAh isn't the whole story. No portable charger is 100 percent efficient, because it always loses some of its power in voltage conversion and heat. Thus, the actual amount of juice you wring out of a portable charger is typically 60 to 85 percent of its stated capacity. In other words, a 10,000 mAh device will deliver between 6,000 and 8,500 mAh.
Quick recharging times.
Fully recharging an external power bank can take anywhere from 5 to 13 hours, depending on its capacity and quality. For the fastest turn-around, use a wall power supply, not the USB port on your laptop or computer.
In this frenetic world, the less hassle the better. A plug-and-forget portable charger starts charging the minute you plug it in and automatically stops charging when a device reaches full capacity, conserving its own power for future use.
Smart charging technology.
Most newer power banks can automatically adjust their power output to match the capacity and requirements of the device they are charging.
That lets you recharge the portable charger even while it is being itself recharged. That's especially convenient at times when power outlets are at a premium and you have multiple power-starved mobile devices to feed.
Charging times and capacities shouldn't be a mystery. The better chargers have LED lights indicating the status of the charge and of the portable charger's battery.
Portable chargers need to be rugged enough to survive life on the go. A tough outer case is a must, and a separate carrying pouch for the charger and cables is a plus. It's also a good sign if a portable charger is backed by a good warranty – at least 18 months.