Siemens AG and AES Corp., the Virginia-based independent power producer, are forming a venture to focus on a sector that's revolutionizing electricity markets: battery-based energy storage.

The new venture, called Fluence, will market and service storage systems made by AES and Siemens. They will each own 50 percent of Fluence once the deal is finalized, which is expected in the fourth quarter, according to a joint statement. Terms weren't disclosed.

Fluence will arrive at an inflection point in the evolution of energy storage, which includes battery-based systems and other technologies. Battery costs have declined about 40 percent since 2014, and some regulators are requiring they be added to power grids. Developers are expected to build $2.5 billion in storage systems this year, and there's growing interest from banks and other prominent lenders.