In theory, a smartphone battery case serves two purposes – keeping the phone intact, and keeping it charged. What distinguishes Tylt's Energi Sliding Power Case is the way it splits those duties down the middle. The case has two components: a minimal, snap-on protective bumper case and a larger one that packs a 2500 mAh rechargeable lithium-ion battery. The bumper is intended to be a relatively permanent safety measure, its slim plastic frame guarding the corners of your Cupertino-crafted device during drops without overly bulking it up. When you need backup power, however, you can slide the encased phone into the bigger power sleeve – the grooves alongside the two cases thread together for a quick, secure fit.

The double-wrapped phone is pocket-stretching, but high-powered, with 9 hours of extra talk time (according to Tylt) and all-day Web surfing and data use (according to us). It's the best of the power cases we've tried, in part because of the standout battery capacity, but mostly due to the sliding case-within-a-case mechanism. It wisely lets you decide if you want something streamlined and pocket-friendly, or something that could last for hours without depleting. 

$100, tylt.com

Field Notes
Area Tested: Indoors, outdoors, planes, trains
Days Used: 100
Conditions Encountered: Mild drops, full battery drains
Nitpick: Some headphone jacks won't fit into the case, forcing you to use the included adapter cable.