Batteries may sometimes pose vexing problems for the automotive and aircraft industries, but for mobile devices they seem to be improving.
To wit, the Jackery Air, which claims to be the thinnest 5,000 milliamp-hour (mAh) auxiliary battery made. Its 5,000 mAh rating means it should have enough power to charge an iPhone 5's 1,440 mAh battery twice, and then some. Since the battery connects using a USB port, it can supply backup electricity to any device that charges in that manner. It sells for $80.
If you are mathematically oriented, you might wonder why it doesn't charge an iPhone more than three times, as it should based on the mAh rating of the two batteries. It's the little secret of the auxiliary battery business that you lose about 15 percent of the battery's power transferring the charge. That spare battery is never quite as big as it seems.
In terms of physical size, though, the Air is just a shade larger than an iPhone 5, smaller than a Samsung Galaxy S4, and fairly lightweight at 5 and a half ounces.
The orange aluminum case has an LED indicator that changes color to give you an idea of how much charge the Air has left. In a test, the Air charged up pretty quickly, taking less than an hour to fill when attached to a USB power block.
Transferring power from the Air to a phone was considerably slower, but because of the size and the weight of the Air holding it while making a phone call is no hardship.
This article originally appeared in The New York Times.