As many people learned during Hurricane Sandy, a low-battery warning on your smartphone can be panic inducing when you're in a power outage or simply away from an outlet. Rechargeable battery packs can help by adding hours of juice. Here are three options to consider.

Solio Classic2 Battery Pack + Solar Charger
Powered by the sun

This 10.1-ounce solar battery can power any USB device. To power it up, spread out the panels and angle them toward the sun on a flat surface, preferably outdoors. During our test, it took about 10 hours to charge the Solio on a sunny patio in Southern California. The smartphone-size device fully charged an iPhone 4S in 1.5 hours and charged an iPad 4's battery from 20 percent to 40 percent in about 90 minutes. You can also charge the battery, which lasts for about 500 cycles, in roughly six hours using an included USB cable. Cost: $100

HyperJuice 2 External Battery
Powered by lithium-ion cells

This 1.5-pound external battery can simultaneously power a MacBook and any two USB devices, including tablets. The device, which measures 8 inches by 4.8 inches, contains military-grade lithium-ion cells that last for about 1,000 recharges. During our test, it added 15 hours of battery life to a MacBook and extended the iPad's battery for more than 24 hours. It took about four hours to juice up the battery, which can then fully recharge an iPhone up to 19 times. One caveat: To charge a MacBook, you must use Apple's MagSafe Airline Adapter, which costs $49. Cost: $300

Eton BoostTurbine2000
Powered by hand crank

The 7-ounce Boost, which is roughly the size of a smartphone, has a turbine and retractable crank that you can turn to generate power. During our test, it took one minute of cranking, at roughly one turn a second, to create enough juice for 30 seconds of talk time on an iPhone 4S--enough for an emergency call. It would take about three hours of cranking to fully charge a phone. But you can also charge the Boost with an included USB cable and use it as a regular battery that fully powers most smartphones in about two and a half hours (sorry, no tablets). The device lasts for about 300 recharges. Cost: $60