How much money do you want to spend? All of the portable chargers reviewed here cost $50 or less. For a power bank that can charge one device at a time, $20 to $30 is typical. A mini charger is even cheaper -- around $10, give or take a dollar or two.
How much power will you need? Small portable chargers, rated at 3,000 mAh or less, store enough power to charge a typical cell phone or similar low-demand device once or twice; though some can't fully charge a more sophisticated, power-hungry device, they can bring it from dead to half power or better. Larger power banks, with capacities of 10,000 mAh or more, have enough juice to power hungrier devices like iPads. The highest-capacity portable chargers, with 20,000 mAh or more, can fully charge multiple tablets or keep a single smartphone running for a week.
What kind of connectors do you need? Most portable chargers connect to your mobile device via USB-to-micro-USB cable, which is compatible with the vast majority of modern cell phones, smartphones and tablets. If your device uses a special connector, you'll need an appropriate adapter cable (consult the device's manufacturer). Apple users, for instance, will need a USB-to-Lightning cable. If you choose a power bank with built-in cables, make sure they have the right kind of connection for your device.
How many devices will you charge at once? If you need to charge only one device at a time, there are plenty of quality, affordable options. If you're a power user who needs to charge several devices on the road, look for a high-capacity charger with multiple USB ports.
Do you want a charger you can carry in your pocket? You can carry pretty much any portable charger comfortably in a briefcase, backpack, etc. Even high-capacity chargers that will last through a week-long camping trip weigh just about a pound and are about the size of a large paperback book. But if you want something you can carry in your pocket every day, opt for a regular or mini charger that can just charge a smartphone once or twice.
What else do you need? Most portable chargers don't come with a lot of extras. For example, you'll need a wall-outlet charger to recharge your power bank quickly, but you'll rarely find one included (though you can often use the charger that came with your mobile device). Likewise, the only connecting cables provided with many portable chargers is standard micro-USB-to-USB cable that you can use either for charging the power bank or charging your devise from it. So, if you require any extras that aren't included in the box, such as a Lightning cable, budget extra for these.
Green energy solutions can be a good thing, and portable chargers are no exception. Up-and-coming within this category are solar chargers. Although plenty of solar chargers are on the market, they have a ways to go before they can compete with grid-powered devices. According to The Wirecutter.com, a solar charger can take two to three hours to recharge a smartphone, more than four times as long as some portable chargers. And that's in full sunlight; about half the solar chargers in The Wirecutter.com's test "slowed to a crawl when a cloud passed overhead." For now, editors conclude, most people who want on-the-go power are better off with a grid-charged battery pack. But that could change as solar technology continues to advance.