A new water-based battery could provide a cheap way to store wind or solar energy for later, researchers say. The battery stores energy generated when the sun is shining and wind is blowing so it can be fed back into the electric grid and redistributed when demand is high.

The prototype manganese-hydrogen battery, reported in Nature Energy, stands just three inches tall and generates a mere 20 milliwatt hours of electricity, which is on par with the energy levels of LED flashlights that hang on a key ring, futurity.org reports, quoting a Stanford University paper.

Despite the prototype’s diminutive output, the researchers are confident they can scale up this table-top technology to an industrial-grade system that could charge and recharge up to 10,000 times, creating a grid-scale battery with a useful lifespan well in excess of a decade.

Yi Cui, a professor of materials science at Stanford University and senior author of the paper, says manganese-hydrogen battery technology could be one of the missing pieces in the nation’s energy puzzle—a way to store unpredictable wind or solar energy so as to lessen the need to burn reliable but carbon-emitting fossil fuels when the renewable sources aren’t available.

“What we’ve done is thrown a special salt into water, dropped in an electrode, and created a reversible chemical reaction that stores electrons in the form of hydrogen gas,” Cui says.

Wei Chen, a postdoctoral scholar in Cui’s lab, led the team that dreamed up the concept and built the prototype. In essence, the researchers coaxed a reversible electron-exchange between water and manganese sulfate, a cheap, abundant industrial salt used to make dry cell batteries, fertilizers, paper, and other products.

To mimic how a wind or solar source might feed power into the battery, the researchers attached a power source to the prototype. The electrons flowing in reacted with the manganese sulfate dissolved in the water to leave particles of manganese dioxide clinging to the electrodes. Excess electrons bubbled off as hydrogen gas, storing that energy for future use.

Engineers know how to re-create electricity from the energy stored in hydrogen gas so the important next step was to prove that they can recharge the water-based battery.

The researchers did this by re-attaching their power source to the depleted prototype, this time with the goal of inducing the manganese dioxide particles clinging to the electrode to combine with water, replenishing the manganese sulfate salt. Once this process restored the salt, incoming electrons became surplus, and excess power could bubble off as hydrogen gas, in a method that can be repeated again and again and again.

Cui estimates that, given the water-based battery’s expected lifespan, it would cost a penny to store enough electricity to power a 100-watt lightbulb for twelve hours.

“We believe this prototype technology will be able to meet Department of Energy goals for utility-scale electrical storage practicality,” Cui says.

The Department of Energy (DOE) has recommended batteries for grid-scale storage should store and then discharge at least 20 kilowatts of power over a period of an hour, be capable of at least 5,000 recharges, and have a useful lifespan of 10 years or more. To make it practical, such a battery system should cost $2,000 or less, or $100 per kilowatt hour.