Very interesting to read.

http://www.wired.com/2014/09/mobility-batteries/

Future progress depends not on sharp increases in battery capacity, but in a diversity of techniques to extend battery life. Microprocessors are now rated on power consumption in addition to raw speed, using part of the ongoing miniaturization to lower power consumption instead of increasing processing power. System designers everywhere are working to improve power-saving capabilities in displays, network interfaces, and protocols.

In the future, small devices might even use solar cells to gather energy during the day to be stored for operation in the dark, removing the need for charging from the electrical grid altogether.