Look at the device you're reading this on right now. If it's a smartphone or tablet, then you'll probably get 12 hours of use out of a full charge, more if you're mindful. If it's a laptop of any kind, you might get 16 hours out of a full charge, doing some non-Internet-related things. What's the problem with those two scenarios, though? They're limited not by the powerful hardware and efficient software, but by the physical bounds of the batteries and cells that power them. As phones get bigger and everything gets thinner with each iteration, we always are left asking 'but how's the battery life?', and that will continue for a shorter time than before recently.
Google is joining the group of companies that have begun the long stretch of research to develop improvements to the decreasingly-sufficient batteries we have today. Current Lithium-ion cell technology is fine and good for something like a flashlight or wireless mouse, but when you need to have the battery/ies recharge many, many times, problems arise. And, with the current technology, you can only fit so much battery into a certain space. Look at smartwatches and smartglasses like Android Wear and Google Glass.
Wear's set of watches only have a 2-day maximum usage because for such a small body, there's not much space to fit a huge battery, so there is as much as can be inside. Google Glass, however, faced an even greater challenge, confining all technology to one side in a space about the size of a normal Red Vine licorice. And with Google's car and Internet-carrying Loon balloons, more batteries need to fill increasingly harsher and stricter needs.
The two problems that plague batteries are these: 1, how to make a battery-less possibly-dangerous, higher capacity per area unit, and versatile, while, 2, making them affordable enough to mass produce and use widely. Companies like Tesla Motors, which is also part of the battery-focused group mentioned before, relies on batteries for their entire industry, yet many of the changes and improvements we've seen regarding batteries are ones that don't increase capacity, but reduce charging time(s). With that in mind, however, advances in different alloys, as well as solid-state, instead of liquid-state batteries have come about recently, along with the thin-film design, which utilises the solid-state material in thin layers to be both flexible and bendable. With various changes in battery technology already existing and being developed, it will be interesting to see where the next big step in battery design will come from.