Credit: Kanex
External batteries for phones or tablets are widely available at low prices.
For MacBooks, there are few options, & they tend to be expensive.
http://hhg2tech.blogspot.com/2014/03/external-battery-for-macbook-now-there.html
For the MacBook 12" that has changed with the Kanex GoPower, which can completely charge a MacBook 12" through its USB-C port, & costs just $US99.95. Other USB-C laptops too.
USB-C can also be used to charge a small, but growing number of phones & tablets multiple times. Other phones & tablets can be charged through the USB-A port.
Credit: Kanex
http://the-gadgeteer.com/2016/05/25/kanex-gopower-keeps-your-macbook-running-after-the-battery-dies/
Manufacturer Web site
http://www.kanex.com/gopower-usbc / Kanex GoPower MacBook 12 inch running after the battery dies latest MacBooks have great battery life battery will drain charger outlet power point socket dead batteries battery MacBook USB-C port Kanex GoPower built in 15,000mAh battery USB 3.0A 2.4A fast charging ports GoPower enough extra battery power completely recharge your USB-C MacBook MacBook GoPower provide multiple recharges other USB-C devices Nexus 6P LG G5 charge two devices at one time extra USB-A port Kanex GoPower priced at $99.95 /
