The Tylt Energi is a very convenient way to charge up any phone or tablet with a USB or MicroUSB port. But although the size, price, and options are right, our ideal backup battery holds a bit more energy.

The Energi is a little rectangle (3 by 2.6 by .9" HWD, 3.8 oz) with a pop-out AC plug on the back, one of its better features. There's a full-sized USB port on the bottom, and a rubber arm that pulls out from the side with a MicroUSB plug on it. On the front, a single button is ringed by four LEDs which show how much charge the Energi has left.

You can't beat this device for flexibility. It'll charge two devices through its two ports simultaneously. When it's plugged in, it acts as a travel charger; you don't need to carry a separate charger for your gadget. The Energi charges up its 1800mAh built-in battery in about four hours.

The only problem is, that 1800mAh battery is outpaced by other, admittedly more expensive chargers on the market. In our tests, the Tylt gave us 1 hour and 48 minutes of solid LTE streaming on a Motorola Droid RAZR ($199, 4.5 stars) and 1 hour and 38 minutes of video playback on an iPad 1 (4.5 stars). That'll help you make it through a day, but it doesn't let you entirely forget about conserving your charge.

Tylt offers a bigger battery, the 5200mAh Powerplant, for $59.99. It requires a separate AC adapter, but we like the higher capacity. Our Editors' Choice for backup batteries is the similarly higher capacity, 5000mAh Sanyo eneloop Mobile Booster ($79.99, 4 stars). That will get you more like 5-6 hours of playback time on an iPad.

If you're just looking for a bit of extra charge for your iPhone, consider a battery case. The Mophie Juice Pack Air ($79.95, 4 stars) and Juice Pack Plus ($99.95, 4 stars) are both excellent options. Those will double your iPhone's life while protecting it, and prevent you from having to carry a separate battery around.