Powerocks' Magic Cube power bank is a portable back-up power supply for smartphones and other devices. It's not the smallest such product around -- that would be the Powerock Tarot -- but it can charge two devices simultaneously, which makes it handy.

Let's say your iPhone is running out of juice. Just plug your Apple smartphone into the Magic Cube and it will charge your phone for emergency power, immediately turning on the phone as soon as you plug it in. After charging, the power bank can be re-charged when you get home using the included USB cable.

Besides smart phones and tablets, the Magic Cube can also charge some e-readers, GPS devices, MP3 players, LED lights, and digital cameras that use a 5V charge. It comes in 6000 mAh ($74.99), 9000 mAh ($89.99) and 12000 mAh ($109.99) models. I'd get the most powerful one you can afford, especially as the last one comes with enough juice to re-charge your iPhone six times. Of course, that won't be the case if you're re-charging two devices at the same time.

Powerocks claims the Magic Cube can be recharged more times than competing products: about 500 times. It's available in silver, blue, purple, pink, and black. For more info go to www.powerocks.com .