Renewable energy would be a cinch if the sun shone all day and if the wind blew as reliably as the Earth spins. They don't. And even when wind turbines and solar panels are able to harness them, the sun and wind might not produce power exactly when customers want it. Without storage, the excess energy is wasted.

Engineers at Axion Power International, near Pittsburgh, may have found a solution to renewables' intermittency problem -- under the hood of a car.

Automakers have long used lead-acid batteries in vehicles because they’re reliable and cheap. Now Axion, a battery manufacturer, is adapting the 150-year- old technology to store energy generated by renewable sources and feed it back onto the U.S. electric grid on demand.

Late last month, 87-employee Axion announced the first successful commercial deployment of electricity storage onto the PJM Interconnection, a major grid that manages power transmission in 13 Eastern states and Washington D.C.

The half-megawatt system, dubbed the PowerCube, is essentially a shipping container packed with electronics and Axion’s batteries. Axion CEO Tom Granville says it “will provide power for the times when the wind isn’t blowing and absorb the power" when the grid is overloaded. A half-megawatt is enough electricity to power several hundred homes.

Storage matters, and its importance will grow with the amount of electricity generated by renewable sources. The Energy Information Administration projects that U.S. renewable energy will more than double from 2009 to 2035, led by wind and biomass. That means backup power to compensate for intermittency is crucial. “Power companies forever have been talking about storage as being an important component” to renewables' success, says Granville. “The problem in the past has been that the storage is either too expensive or that it requires too much maintenance. Our battery fits in the middle of that.”

Most of Axion’s battery technology, which it makes at its two plants in New Castle, Pennsylvania, isn’t new. “Everything is the same as a standard 12-volt AGM car battery except for the negative plate,” Granville explains. Axion replaces the traditional lead-based negative electrode with a carbon one. The battery is three to four times less expensive than its lithium and nickel-metal hydride counterparts and lasts “four to five times longer” than traditional lead-acid batteries.

Axion, which trades as a penny stock, reported revenue of $5.2 million and a net loss of $5.7 during the first nine months of 2011, and noted in its most recent 10-Q that its hybrid battery’s “adoption process, and the general path to commercial viability, has been longer than we originally anticipated.”

More than $120 billion may be invested in energy storage projects in the next decade, increasing capacity around the world 100-fold, according to a recent report  by market research firm Pike Research in Boulder, Colo.

The power industry has traditionally spent more time building supply than capturing unused electricity, says Audrey Zibelman, a former executive vice president and chief operating officer at PJM. She went on to found and run Viridity Energy, in Philadelphia, which is providing Axion with the software it needs to manage the PowerCube’s electricity intake and release onto the grid. “We’ve always assumed we have to build more generators to meet demand,” she says.

Zibelman sees Axion’s batteries as being useful at data centers, universities, laboratories, hospitals, and military installations, where they can keep energy costs down and do what car batteries do best--provide a jump when you most need one.