Iron-air batteries

Also known as Aqueous iron-air batteries, Multi-day energy storage, 100-hour grid batteries

Iron-air batteries are a long-duration storage design that uses iron, oxygen from the air and a water-based electrolyte, aimed at delivering power for days rather than hours. In a 2025 California Energy Commission report, kilowatt-scale prototype modules discharged for more than 100 hours under controlled tests, but round-trip efficiency is not public and no multi-day commercial plant has yet been shown to operate.

Research and prototypesDemonstratedReviewed 21 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

An iron-air battery stores electricity in a process that works like rusting in reverse. When it discharges, iron reacts with oxygen from the air in a water-based electrolyte, and the electrons released flow out as electricity. Charging reverses the reaction, turning the rust back into iron.

The design is aimed at duration rather than speed. A battery has a power rating (how quickly it can deliver electricity) and an energy capacity (how much it can store), and dividing one by the other gives how long it can run at full power. The developer, Form Energy, designs its system to run for 100 hours, which could help cover multi-day spells when wind and solar output is low. The results so far come from early prototypes, and the cost and efficiency of a full system are not yet confirmed in public.

Technical explanation

Iron-air batteries use metallic iron as the active material, oxygen from ambient air as the second reactant, and an aqueous electrolyte. Form Energy describes the reaction as reversible rusting (Power Engineering, 2023). The company's design is built from direct-current modules of about 30 cells in series, with roughly ten modules in a shipping-container enclosure and an inverter to deliver alternating current. The module is the smallest field-serviceable building block (CEC report, 2025).

In the California Energy Commission (CEC) project, two kilowatt-scale modules were tested at Form's engineering facility in Berkeley under constant-power discharge. The first module discharged for 117 to 126 hours at about 2.14 kW. The second module, which included manufacturing changes, discharged for 108 to 110 hours at about 2.81 kW. The report describes the modules as meeting their stated expectations for a prototype. It also records an accelerated product-intent duty cycle of about 300 to 500 hours, which the report says is equivalent to six to twelve months of product operation for six full cycles, depending on applications and seasonal conditions. That equivalence comes from an accelerated test, not from measured field data.

The CEC report also includes a grid model in which EPRI analysed a hypothetical 1 MW, 100 MWh system connected through an inverter and operating in California's 2035 electricity market. This is a modelled result, not an operating system. Round-trip efficiency is marked as proprietary, and the report does not publish an alternating-current-to-alternating-current efficiency figure, which planners would need to compare the system with other storage options.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

In controlled constant-power tests, a first kilowatt-scale prototype module discharged for 117 to 126 hours at about 2.14 kW, and a second module with manufacturing changes discharged for 108 to 110 hours at about 2.81 kW.

Sources: [1] · Tables 5 and 6

The first test module was retired after downtime following six cycles, and testing was completed on a second module; the report attributes the downtime partly to test-stand faults and partly to module manufacturing processes.

Sources: [1] · Test results section

The CEC report marks round-trip efficiency for the tested modules as proprietary, so no public efficiency figure is available.

Sources: [1] · Tables 5 and 6

In short-circuit and electrolyte-drain tests on the cells tested, the report records zero safety events.

Sources: [1] · Safety codes and standards section · medium confidence

In June 2024, the US Department of Energy awarded more than $4.3 million to the Multiday Iron Air Demonstration for planning, permitting and design work, with the aim of preparing two 10 MW, 100-hour systems at Xcel Energy sites in Minnesota and Colorado for later construction.

Sources: [3] · About this project

Reported, unverified Stated by a company or team but not independently confirmed.

The CEC report states that six full cycles of throughput on the test module were equivalent to six to twelve months of product operation, depending on applications and seasonal conditions. The equivalence is derived from an accelerated test, not from field data.

Sources: [1] · DC Battery Performance, AC Interface Modeling, and Verification section (Chapter 2) · medium confidence

EPRI modelled a hypothetical 1 MW, 100 MWh alternating-current system in California's 2035 market and found significant deployments of multi-day storage in scenarios where combustion resources are limited.

Sources: [1] · Grid-level modeling outcomes section · medium confidence

Form Energy describes the iron-air battery as using reversible rusting, with iron as the active material, air as the oxygen source and a water-based electrolyte.

Sources: [4] · Technology · medium confidence

In a July 2023 article, Form Energy said its system could be deployed at costs competitive with conventional power plants and at less than one-tenth the cost of lithium-ion. The article presents these as company claims and gives no independent cost data.

Sources: [4] · Article text on cost claims · low confidence

Limitations and open problems

  • The measured results come from kilowatt-scale direct-current modules tested in a company engineering facility, not from a grid-connected plant. Combining many modules with power conversion, thermal and air management and controls can add losses and faults that a module test does not show.
  • Round-trip efficiency is marked as proprietary in the CEC report, so utilities and planners cannot compare the system's energy losses with other storage options using public data.
  • The equivalence of six cycles to six to twelve months of operation comes from an accelerated test and a developer's interpretation. Field degradation, maintenance and availability are not yet public.
  • The first test module was retired after downtime that the report attributes partly to test-stand faults and partly to manufacturing processes. Reliability across many production units has not been shown.
  • Safety testing covered short circuits and electrolyte draining in the cells tested, with no safety events recorded. These tests were small-scale and product-specific, so they do not show that large sites are free of fire, chemical or electrical hazards.
  • Grid value is a model result for a hypothetical 2035 system. Whether that value is realised depends on market design, siting, dispatch and the cost of the full installed system.

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • Covering multi-day lulls in wind and solar output in grids with high renewable shares. The CEC report describes this need, but the role has not been tested in operation.
  • Supplying power during multi-day outages, including wildfire-related public safety power shutoffs, which the CEC report cites as a use case for multi-day storage.

Timeline

  1. 24 Jul 2023Power Engineering reports that Form Energy had signed deals with utilities and broken ground on a commercial-scale plant in West Virginia, but that its CEO said the company 'has yet to put steel in the ground'. The article also reports that the Minnesota Public Utilities Commission approved Xcel Energy's planned 10 MW, 1,000 MWh demonstration, and that the company's cost claims are its own. Sources: [4]
  2. Jun 2024The US Department of Energy awards more than $4.3 million to the Multiday Iron Air Demonstration for planning, permitting and design work on two 100-hour systems at retiring coal plant sites in Minnesota and Colorado. Sources: [3]
  3. Nov 2025The CEC publishes final project report CEC-500-2025-045, which records kilowatt-scale prototype modules discharging for more than 100 hours and includes a modelled 1 MW grid case for 2035. Sources: [1], [2]

Futures this could help bring about

AI illustration
How far away?

Cheap multi-day energy storage for renewables

Pilots · 2 of 7 steps done

AI illustration
How far away?

Grid batteries that last for days

Pilots · 2 of 8 steps done

Glossary

Long-duration energy storage
Energy storage designed to discharge over many hours or several days. Definitions vary by programme and use case.
Discharge duration
How long a storage system can deliver power at a stated rate before reaching its discharge limit.
Round-trip efficiency
Energy delivered during discharge divided by the energy used to charge the system. The result depends on where the measurement boundary is drawn.
AC-to-AC efficiency
Round-trip efficiency measured from grid alternating-current input to grid alternating-current output, including power conversion and auxiliary equipment.

Sources

  1. government report
    Demonstrating an Aqueous Air-Breathing Energy Storage System for Multi-Day Resiliency
    Annie Baldwin, Mark Thompson, Nicole Vadivel. California Energy Commission, Nov 2025. Accessed 21 Oct 2026.
  2. government report
    Demonstrating an Aqueous Air-Breathing Energy Storage System for Multi-Day Resiliency (project publication record)
    California Energy Commission, 2025. Accessed 21 Oct 2026.
  3. government report
    Multiday Iron Air Demonstration: Project Fact Sheet
    US Department of Energy, Office of Clean Energy Demonstrations, Jun 2024. Accessed 21 Oct 2026.
  4. news
    Form Energy's 100-hour battery is almost here
    John Engel. Power Engineering, 24 Jul 2023. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 21 Oct 2026
Published on owner instruction (content loop iteration 3)9518996
Sources and claims re-checked against originalsfbb434c
Initial profile drafted from Codex research note and primary sources0eba374