Batteries had a great year in 2015 - with costs going down and large scale installations occuring, primarily in the utility sector. The United States energy storage capacity grew by 221 megawatts in 2015, which is triple the capacity added in 2014.
Batteries serve to bridge the gap between renewables like wind and solar and the grid. Wind and solar are both intermittent in terms of power generation, and battery storage technology is a necessary to ensure power is available when its needed versus when it happens to be generated.
I wrote an article for Oil & Energy magazine going into depth on new developments in battery technology and what the future holds in terms of the goals of cutting edge developments at MIT, Harvard, and the Department of Energy. You can read that article here: Oil & Energy Magazine: "Persuing the Holy Grail of Battery Tech"