This post was done in partnership with The Wirecutter, a list of the best technology to buy. Read the full article below at TheWirecutter.com.
Smartphone batteries don't always last through a busy day, but a pocket-size USB battery pack can give your handset enough of a boost to survive the evening. After 40 hours of research and 65 hours of testing, the one we like the most is Anker's 2nd Gen Astro 6400. It fits in any pocket or purse, and it charges phones and small tablets about as fast as any pocket-friendly pack out there. At 6,400 mAh, it has a larger capacity than most, too.
The Anker 2nd Gen Astro 6400 can slide into a relaxed-fit pants pocket alongside a smartphone, though a jacket pocket or purse will be a more comfortable fit.
How we decided
We started by looking for packs that could slide into a relaxed-fit jeans pocket without bulging too much. We also wanted a pack that could fully charge power-hungry phones like the Apple iPhone 6, Motorola Moto X, and Samsung Galaxy S6 at least once, and at full speed. From there, we favored packs with the best cost-to-capacity and size-to-capacity ratios and higher-current power output (up to a point).
We settled on eight finalists and put each pack through a timed charging test on a few different devices so that we could measure charging speed. Wirecutter staff also evaluated each battery.
The Anker 2nd Gen Astro 6400 ($20) is the best pocket-size USB battery for most people, because it comes closest to hitting the sweet spot for price, charging speed, capacity, and portability. Thanks to its smart-charging circuitry, it works well with any current smartphone, and it should be able to handle new models that come out in the next couple of years, too. It's sturdy enough for the daily grind and small enough for a pair of relaxed-fit jeans.
Runner-up, with pass-through charging
If you need pass-through charging (the ability to connect the battery pack to both a wall charger and a device), and if you want to have two gadgets charge in sequence (or simultaneously)—the 6,400 mAh Zendure A2 is the best pack we found. It performed just as well as the Anker and AmazonBasics packs, but it's bulkier and more expensive than the Anker.
A flatter pack
If you don't like the chunkier shape of the Anker 2nd Gen Astro 6400, the AmazonBasics Portable External Battery Charger 5600 ($25) might be more your style. It offers the lowest price and the best overall price-to-capacity ratio among our finalists. We didn't name it our top pick because a slim majority of Wirecutter staffers preferred the candy-bar shape of the Anker over the smartphone-like slab shape of the AmazonBasics. The capacity is a bit less than that of other packs, but it's as good a battery pack as the Anker.
A really, really small and portable USB pack
Our top picks are small enough to fit into most relaxed-fit pockets, but if you're looking for a very small battery, consider the Go Design TravelCard. It's small enough to fit into most wallets (or slender makeup cases, or tiny clutches, or slim pockets), and it sports a built-in cable (micro-USB or Lightning, depending on the model).
At 1,500 mAh, its capacity is quite a bit smaller than that of our top picks, but it can charge about 50 percent of the battery in a typical 4.7-inch smartphone. This pack is the "Always have it with me just in case I need a small boost to get through the rest of the day" option.
For reliable backup battery power that can fit in your pocket, we recommend the Anker 2nd Gen Astro 6400 or the AmazonBasics Portable External Battery Charger 5600—whichever is your preferred shape. If you want something even smaller, the TravelCard is a great alternative.
This guide may have been updated. To see the current recommendation, please go to TheWirecutter.com.