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).