The next breakthrough smartphone, or maybe the one after that, might not have a traditional battery as its sole source of power. Instead, it could pull energy from the air or power itself through television, cellular or Wi-Fi signals. Engineers at Apple even tried for many years to build a smarter battery by adding solar charging to iPhones and iPods, a former Apple executive said. And they have continued to experiment with solar charging, two people who work at the company said. Batteries, long the poor cousin to computer chips in research-obsessed Silicon Valley, are now the rage. As tech companies push their businesses into making wearable devices like fitness bands, eyeglasses and smart watches, the limitations of battery technology have become the biggest obstacle to sales and greater profits.