SAN FRANCISCO — The next big smartphone, or maybe the one after that, might not have a traditional battery . Instead, it could pull energy from the air or power itself through television, cellular or Wi-Fi signals.

For years, engineers at Apple have experimented with solar charging, current and former employees said.

Batteries, long the poor cousin to computer chips in Silicon Val-ley, are now the rage.

As tech companies push their businesses into making wearable devices , the limitations of battery technology have become the big-gest obstacle . Consumers are un-likely to embrace a wristwatch computer like the one being worked on by Apple, or Google’s smart glasses, if they work only a few hours between charges .

Battery technology is largely stuck in the 20th century. Device makers have relied on incre-mental improvements to battery power, now usually supplied by a decades-old lithium-ion con-coction, in combination with more energy-efficient chips and screens.

The problem, in part, is that it is hard to ensure the safety of many new technologies. A faulty battery could turn into a minia-ture bomb. So the products re-quire exhaustive testing. Then, a change often requires adoption by major brands before consum-ers trust it.

Some in Silicon Valley, like To-ny Fadell, the former Apple exec-utive who led iPod and iPhone de-velopment, think it is smarter to focus on improving batteries and other components , rather than trying to reinvent the battery .

“Don’t wait for the battery technology to get there, because it’s incredibly slow to move ,” said Mr. Fadell, who is now the chief executive of Nest, which makes household technology and was bought by Google last month.

Mr. Fadell said Apple tried for many years to add solar charging to iPhones and iPods. But the method never proved practical, he said, because mobile devices often stay inside pockets when people are outdoors, and indoor artificial light generates only a tiny amount of energy.

For its watch, Apple has been testing a method to charge the battery wirelessly with magnetic induction, according to a person briefed on the product. A similar technology is used in some Nokia smartphones: when a phone is placed on a charging plate, an electrical current creates a mag-netic field, which then powers it.

Another experiment at Ap-ple has involved charging the battery through movement, a method used in many watches. A person’s arm swinging could power the device while walking, according to a patent filed by Ap-ple in 2009.

Apple also has a patent for a flexible battery that could fit in a wristwatch or tablet. Although the battery would be traditional, it would have a thin and curved form that could easily couple with a flexible solar panel layer.

Samsung, too, has been de-signing new types of batteries with wearable computers in mind. The company has intro-duced compact curved batter-ies that can be installed inside wristbands.

Universities and start-ups are also making their own efforts .

Researchers at the University of Washington in Seattle have been working on a method for wireless devices to communi-cate without using any battery power. The technique involves harvesting energy from TV, cellular and Wi-Fi signals that are already in the air, said Shy-amnath Gollakota, an assistant professor of computer science and engineering .

“The idea is basically you have signals around you,” he said. “So why do you have to generate new signals to communicate?”