Aquion Energy pulled in another $33 million to support its saltwater batteries for long-duration storage, according to a filing with the SEC yesterday. Aquion hopes to raise $60 million in total.

The latest round brings the company's venture funding -- which includes equity investments and venture loans -- to $190 million. Aquion CEO Scott Pearson would not comment on investors who participated in the round, but he did say that it includes "existing and new investors."

Aquion's long list of past investors is impressive. It includes Bill Gates, Gentry Venture Partners, Kleiner Perkins Caufield & Byers, Foundation Capital, Bright Capital, Advanced Technology Ventures, Trinity Capital Investment and CapX Partners, Yung’s Enterprise, and Nick and Joby Pritzker.

"The Aquion business and the energy storage market continue to develop well. The round of funding Aquion is currently engaged in incorporates both existing and new investors. The funds will support continued scaling of the business, the expansion of our product offerings, and new customer deployments worldwide.”

Aquion's sodium-ion battery is designed for multi-hour applications. According to the company, its batteries can deliver a round-trip efficiency of 85 percent and perform 5,000 cycles. The company's cost target is $250 per kilowatt-hour, with the goal of getting to $160 per kilowatt-hour when its manufacturing facility in Pennsylvania is at scale.

While short-duration lithium-ion batteries are dominating the market, investors are increasingly interested in batteries that can discharge over long periods of time. 

"But utilities and others know that there ought to be other technologies for long-duration storage. That's where a lot of the current investments are going on," said Ravi Manghani, GTM Research's senior storage analyst. 

Last September, Primus Power raised $25 million for its flow batteries, bringing the company's total investment to $60 million. Another flow battery firm, ViZn, just raised $10 million for its long-duration energy storage. And Lockheed Martin has entered the long-duration storage business with the flow battery technology it acquired from Sun Catalytix. Long-duration non-electrochemical energy storage such as ARES' railcar scheme are also being developed.

So where does $190 million of funding get a battery startup with a new technology?

For Aquion, it gets you to very early commercial deployment.

As Jeff St. John reported last year, Aquion supplied "what could be the biggest battery ever ordered up by a private citizen" -- a 1-megawatt-hour system, backing up the residence and working farm of Medtronic founder Earl Bakken.

And Aquion just announced a microgrid installation at an organic winery and farm in Sonoma, Calif. that combines fourteen 25-kilowatt-hour Aquion batteries (approximately 350 kilowatt-hours of energy storage) with a 30-kilowatt power conversion system from Ideal Power and a 32-kilowattsolarsystem. According to a release, the grid-tied microgrid can island and operate autonomously.

It takes big money to build big, new batteries. Aquion's early installations are good signs, but the company still has lots more scaling up, market development and fundraising to go.