Check out this Forbes article from Scott Kramer:

I recently got back from vacation. And while on the road, I took extra charging bricks for my cell phone and tablet. And honestly, it was a pain because they require different connectors – my phone a lightning cable and my tablet a micro USB. As a result, my backpack got tangled with cords and power bricks.

It can probably fully charge my device, but I haven’t tested it to the max yet. A company spokesman says that it really depends on the battery size of the phone, but that most phones will get anywhere between a 40 and 100 percent charge. That said, I was able to charge an iPhone from roughly 50 percent to completely full in a matter of about an hour. It works faster than you might expect. For me, carrying a card like this is a no-brainer. That’s because I play a lot of golf. Out on the course, there’s usually no place to plug in a phone unless the cart happens to be equipped with a USB port. But because I prefer walking, I rarely take a cart. Plus I like using an app on my phone to get GPS yardage, which drains the battery – as do weak cell phone signals out in the wilderness, on many courses. So having portable storage like this is absolutely key. Speaking of outdoors, I can see campers really benefiting from this, too. The convenience of being able to pull it out and get a quick significant charge is what has attracted most buyers thus far, according to the company.