The Anker Ultra-Slim Battery Case is one of the cheapest iPhone battery cases you can get without compromising on performance and reliability. Though it has its flaws (which I'll get to later) it's still only $40 (or £30 in the UK and AU$53 in Australia when converted). That's half the cost of the Incipio offGrid Express and a third of the Mophie's Juice Pack Plus $120 price.
The accessory features a built-in 2,850mAh battery. In comparison, that's slightly less than the offGrid Express' 3,000mAh battery and the Juice Pack Plus' 3,300mAh charger. Though it won't have as many reserves left over after a full drain compared to the other two, it's still enough to juice up a completely drained iPhone back up to 100 percent.
The case comes with a Micro-USB charging cord and a headphone extender. Both accessories are pretty common for iPhone battery cases since only Apple's own Smart Battery Case uses the proprietary Lightning port to charge itself. As for the extender, battery cases pack on extra heft around the phone so you'll need to use an extender for your headphones to reach the 3.5mm jack.
From a completely drained battery, the Ultra-Slim Battery Case took about 2 hours and 8 minutes to fully charge an iPhone 6, which was about the same time the Mophie case, while Incipio's case clocked in at 2 hours and 30 minutes -- a tad too long in my opinion.
There were however some standout issues. The case has three cutouts along the edges so I can access the phone's physical control buttons and volume rocker. But due to the slight thickness of the case, it was difficult for my finger to reach these keys. In addition, the charging indicator on the back of the case remains lit while the handset is charging. This isn't a deal breaker, but it's annoying if I'm in a dark room like a movie theater or my own bedroom at night.