How we picked and tested

Cadex devoted a corner of its lab to testing almost 60 batteries.

When it comes to USB battery packs, someone who wants a pocketable model for occasionally topping off a phone has different needs than a business traveler who wants to keep a tablet and two phones charged for back-to-back international flights. For each scenario, we set out to find the best balance of capacity, charging speed, size, and features. A hulking, heavy, cheap pack, for instance, isn't as good as a moderately priced one you'll actually use. We also looked for models that can cut down the clutter in your bag or pockets through the inclusion of convenience features such as multiple ports and integrated cables.

Once we established our test lineup, we worked with Cadex Electronics to put the packs through an exhaustive series of technical tests. We hooked up three samples of each model to Cadex's lab gear to get insight into voltage curves, amperage, and how close the batteries' actual capacities came to matching what their makers claimed on the box. With the help of Cadex engineers, we sifted through and analyzed all of that data and came up with solid recommendations for whatever amount of juice your power-hungry devices may need.

Our pick for your pocket

The TravelCard Charger comes in both Micro-USB and Lightning flavors.

If you need the absolute smallest battery to keep your phone going when you can't get to an outlet, the TravelCard Charger is the most convenient and reliable option. A little longer and wider than a credit card and roughly three times as thick, the TravelCard stands out from other small batteries because it has two integrated cables–one with a Micro-USB or Lightning-connector plug (depending on the version you buy) to charge your device, and one with a standard USB plug to recharge the battery itself.

That said, even the best batteries at this size have limitations. And although the TravelCard charges at only 1 amp, and has an average of 1,049 milliamp hours, we still think it's a better choice because of the integrated recharging cable that other similar-size models lack. An extra 100 mAh probably won't make a huge difference in how you use one of these batteries day to day, but never wanting for an extra cable is more than just a convenience—it's a relief.

The best battery for everyday use

Our top pick from EasyAcc feels like a midsize smartphone in your hand.

If you can make just a little more room in your pocket or bag, you can upgrade to a lot more power and a better value. The EasyAcc 6000mAh Ultra Slim Power Bank is about the size and thickness of an average smartphone, and it's the best-performing model in this class—standing out thanks to its integrated Micro-USB cable and high-speed charging port (at least 2 amps).

Measuring at 5,500 mAh in our tests, this EasyAcc model had the most available power, enough to fully charge a smartphone—depending on size—two or three times. You could even charge a tablet more than halfway from this unit's high-speed port.

Our pick for your briefcase or bag

Our alternate pick from AmazonBasics is about the size of a large portable hard drive.

The AmazonBasics Portable Power Bank 16,100 mAh is our pick if you have to charge a phone repeatedly, want to keep a full-size tablet and phone topped up through a week of commuting, or need to keep your gadgets going on a long flight. It had 15,280 mAh available in our tests—not the highest in the group, but enough power to charge an average smartphone almost every day of the work week or fill a full-size tablet twice.

Now that Amazon has upgraded the USB ports on its offering, this pack puts out enough juice to charge two devices at or near their full speed. Our meters found that the loads balanced to no more than 4 amps combined, or 2.4 amps per port—high enough for this battery to slide into the top spot for this size category. This battery is the most flexible of all our picks, and it's the one to get if you want something that can work for commuting and for travel.

Our pick for your luggage

The RAVPower is closer to the size of a paperback, but it packs a big punch.

If you require a lot of power, you should get the RAVPower 26800mAh Xtreme Portable Charger. Next to the other large contenders in our test group, it offers more power, more ports, and faster charging, all at a reasonable price.

After charging and draining our three RAVPower test units, we found they averaged an actual capacity of of 19,107 mAh—about 70 percent of what the company advertises, but still enough power to both fully charge a large tablet and a fill a smartphone a few times.

Its three ports are rated at 2.4 amps, 2.1 amps, and 1 amp, and the RAVPower model can output at full speed to all three ports simultaneously. You'll have to remember which port is which, though, to make sure your device is plugged into the port that supplies its maximum charging potential.

A drawback of this pack is its heft: This model weighs 21 ounces, and it takes up the space of a large paperback. If you need the power, though, this is how you should get it.

This guide may have been updated by The Wirecutter. To see the current recommendation, please go here.