When it comes to disruptive battery startups, one of the best endorsements you can get comes from software tycoon Bill Gates.

Aquion Energy has raised $35 million in a series D round, which brings Gates as a new investor. It’s at least the third energy storage startup that he has invested in. Others include Ambri, which is making a liquid metal battery, and LightSail Energy, which is using compressed air to store energy. If there’s a common thread to them, it’s that each company has taken a truly novel approach and are targeting grid-level storage, a technology considered crucial to using wind and solar at large scale.

Well-established lithium ion battery technology is being used for specialized purposes in grid storage, but their costs are high—the main reason electric vehicles are expensive. Many people believe that creating a wider market for grid energy storage will require a breakthrough in costs to be commerciall viable, slashing prices for long-lasting and durable batteries to under $300 per kilowatt-hour.

Aquion Energy’s strategy is to use cheap materials and manufacturing process. The electrodes on its batteries are activated carbon and manganese oxide and individual battery modules are packaged in plastic containers. It uses a water-based electrolyte, which is environmentally benign if there is a spill.  (See, Building Cheap Batteries to Circumvent the Grid.) 

Its first product, due by the end of this year, will be a battery built as a stack, which is about four feet high, made of individual modules. It’s also packing the modules onto larger square pallets and, by next year, a 20-foot shipping container able to store 200 kilowatt-hours and deliver 50 kilowatts of power, Ted Wiley, the company’s vice president of business and market development told me last month.

Initially, Aquion Energy is targeting renewable energy project developers who can use batteries to store energy and deliver it at peak times, when power is more expensive, he says. The batteries should work for up to 10,000 cycles, about a 10 year usable life, which Aquion expects to extend in future versions. In terms of cost, Wiley says the price will be between lead-acid batteries—the cheapest available now—and lithium-ion batteries.

The money the company raised will allow it to move into the production of its batteries from its site near Pittsburgh and ramp up volume next year, it says. Aquion is also funded by venture capital companies, including Bright Capital, Kleiner Perkins Caufield & Byers, and Advanced Technology Ventures. 

As for Gates, he may have uncovered and invested in other promising battery startups. In 2010, he noted that battery technology doesn’t improve at the same rapid pace as Moore’s Law, making it one of the toughest scientific problems. “They haven’t improved hardly at all,” Gates said at the Techonomy conference. “There are deep physical limits. I am funding five battery start-ups and there are probably 50 out there.”