Having a device with enough reserve power to charge your portable devices can be very handy, especially during emergencies. Portable USB chargers are becoming smaller, lighter and higher-capacity than ever before. The best external battery chargers are not only lightweight, but also powerful enough to charge devices like smartphones, tablets and iPods nearly as fast as a wall charger. Just as important is the battery's longevity: the top portable chargers won't die too quickly, and their own internal battery can be recharged via a wall outlet (which is always the fastest option) or via USB. The ideal portable charger stores plenty of juice, is able to charge multiple devices at one time, is lightweight yet durable, is easy to use and delivers tons of power simply and reliably.
Several portable chargers are exceptional. Anker's 2nd Gen Astro3 (Est. $50) takes the top spot in reviews thanks to its pocket-sized design and powerful 12,000 mAh battery, which can charge three devices simultaneously. It weighs 10.6 ounces, which doesn't make it the lightest charger out there, but it is still impressive given that it's about the size of a deck of cards and still weighs under a pound. In tests the Astro3 charged a power-hungry iPhone 5s six times before dying; one review found it could simultaneously charge an iPad, Android tablet and iPhone. In exchange for that level of power, the battery takes time to recharge itself (about 10 hours) so it's best done overnight.
The Astro3 faces stiff competition, however. Another popular model is the 10,400 mAh Jackery Giant (Est. $40) , which boasts two ports for simultaneous charging, plus a built-in flashlight, and is great for the occasional user because it can hold a charge for up to six months. The New Trent iCarrier (Est. $40), has the same 12,000 mAh with slightly less power than the Astro3; it can charge both power-hungry tablets like the iPad 4 and smartphones simultaneously. The dual-USB port ZAGGsparq 6,000 (Est. $60) , has half the mAh power but sports a wall-socket plug in its charger. This is both good and bad: it's easier than hauling around a separate wall charger (which powers up batteries faster than a USB), but the charger itself requires electricity to recharge. The ZAGGsparq can also power up devices while it's recharging; most portable chargers don't have this feature.
One of the biggest trends in portable chargers is actually quite small -- mini chargers that can easily slip into a pocket or hang on a keychain. These chargers don't have high-capacity batteries (most range from 1,000 to 3,000 mAh) but they're ideal for charging smaller electronic devices like smartphones or iPods. (Some mini chargers are also powerful enough to juice up older tablets, at least partway.)
Mini and micro chargers are not only lightweight and highly portable, but many have fun designs. The 1,000 mAh Powerstation Reserve Micro by Mophie (Est. $40) and the tiny 220 mAh Fuel Micro Charger by Devotec (Est. $25) connect to your keychain, for example. A popular style in the mini category is the candy-colored lipstick tube; the bright colors and small shapes make them easy to spot in a handbag or carry around.
Powerocks Magicstick (Est. $30) is one such portable charger; it has 2,800 mAh, weighs just under 3 ounces and comes in blue, red, black, pink, gold, purple or silver. Like most mini chargers, the Magicstick is primarily designed for mobile phones (it can power up some smartphones twice before requiring a recharge) but can also work on tablets -- you just may not get a full charge. It takes about four hours to recharge itself, which is fairly standard with a charger this size.
Another lipstick-sized portable charger is the Anker Astro Mini (Est. $20) with 3,000 mAh. It will charge your phone at least once, as opposed to the Magicstick, which can charge it twice. (Voltage conversions and other power issues can make chargers less efficient, even if their mAh is slightly higher.) The Innergie PocketCell (Est. $40) is about the size of a lighter and comes with a 3-in-1 cable with a micro USB, mini USB and Apple 30-pin connector. (Newer Apple products require a lightning cable.) It charges at a decent, if not outstanding, speed. As with all chargers, it's crucial that you check the voltage and amp prior to plugging a device into a mini charger so as not to overheat it.
A certain kind of portable charger caters to travelers who constantly use multiple personal electronics but are not always near an outlet. These mega-chargers certainly aren't the smallest of the bunch but pack a wallop of power and can charge smartphones, MP3 players, tablets, camera batteries and more, even simultaneously. Don't expect these giants to fit on your keychain (yet) -- more power, not surprisingly, means a larger footprint and heavier weight. Still, high-capacity portable chargers that are capable of charging multiple devices aren't as hefty as one might think: most weigh around a pound and are about the size of a thick smartphone.
The clear winner in this category is the Anker 2nd Gen Astro Pro2 Quad-Port (Est. $80) . The Astro Pro2 has a 20,000 mAh battery with an impressive 4.5 amps (compared with 4 for most other high-capacity portable chargers) three USB outputs and a DC output with a 9V and 12V option -- enabling users to charge power-hungry devices like a small laptop or a camcorder battery. There aren't an overwhelming number of reviews for the Astro Pro2, perhaps because not everyone needs that much portable power. If you plan on being off the grid for a while and need to use devices that require more power, the Astro Pro2 is an excellent option -- particularly given its $80 price tag, reviewers say. (Some less powerful portable chargers retail for up to $100.) The Astro Pro2's LED screen also displays the exact percentage of battery power remaining in the charger, a rare feature for portable chargers.
The Astro Pro2 is also good if you want a very speedy charge on a single device. Smartphones, for example, charge up as fast on high-capacity battery chargers as they do on wall sockets. On that note, if power is what you're after and you're not concerned about charging many devices at once, consider Limeade's L180X (Est. $100) which has two USB ports for simultaneous charging and includes a USB, micro USB and Apple 30-pin connector, all in a single cable. If you're keen to charge more than one smaller device (like smartphones or MP3 players), consider our Best Reviewed Portable Charger, the 12,000 mAh Anker Astro3 (Est. $50) which includes three USB ports.