Somewhere in a test lab, a battery cell is being crushed, overcharged, punctured, and heated until it fails. That destruction is the point. Before any rechargeable lithium battery can power a passenger aircraft in the United States, it must survive a gauntlet of abuse tests defined by RTCA standard DO-311A, the benchmark the Federal Aviation Administration uses to judge whether a battery is safe enough to fly. And right now, a new class of cells is lining up for that gauntlet: semi-solid-state batteries that already ship in electric cars and promise energy densities around 350 watt-hours per kilogram, the threshold most electric air taxi designers say they need for commercially useful range.
The problem is that no public record yet confirms any semi-solid-state battery has completed the full DO-311A certification sequence for installation in a U.S.-certificated aircraft. The technology is real, the performance numbers are compelling, and the regulatory pathway exists. But the gap between a spec sheet and a signed-off flight battery remains wide, and closing it will take more than chemistry breakthroughs. It will take hundreds of destroyed cells, mountains of safety documentation, and a regulator satisfied that these batteries will not burn, explode, or fail at altitude.
The regulatory framework is already in place
The FAA has not been caught flat-footed by the rush toward high-energy batteries for electric vertical takeoff and landing (eVTOL) aircraft. The agency’s lithium battery certification page lays out the design approval process and points manufacturers toward DO-311A, which covers thermal runaway propagation, short-circuit behavior, vibration, altitude simulation, and other conditions unique to flight. Any battery maker hoping to power an air taxi must run this course.
Complementing that standard is draft Advisory Circular AC 20-184A, titled “Guidance on Testing and Installation of Rechargeable Lithium Battery and Battery Systems on Aircraft.” The draft spells out how DO-311A test data translates into the evidence an applicant must present before the FAA will approve a battery installation. It functions as the bridge between raw lab results and a regulator’s sign-off, detailing what fire and explosion risks must be ruled out before a battery earns its place on a certificated airframe.
Crucially, AC 20-184A remains a draft as of mid-2026. The FAA has not published a final version with a fixed compliance date, which means the exact documentation requirements could still shift. For battery makers and eVTOL developers alike, that uncertainty adds a layer of planning risk on top of the already demanding technical work.
Where the technology stands
Semi-solid-state batteries occupy a middle ground between today’s conventional liquid-electrolyte lithium-ion cells and the fully solid-state batteries that remain largely in development. By replacing most or all of the liquid electrolyte with a gel or polymer-based alternative, semi-solid designs reduce leakage risk and can improve thermal stability, two properties that matter enormously in aviation. Several Chinese manufacturers, including WeLion New Energy and Ganfeng Lithium, have already shipped semi-solid-state cells exceeding 350 Wh/kg into electric vehicles, proving the chemistry works at production scale on the ground.
On the higher end of the density spectrum, Amprius Technologies announced a 500 Wh/kg battery platform in early 2023, targeting aviation and defense applications. That figure, if validated at the pack level under DO-311A conditions, would blow past the 350 Wh/kg mark. Since that announcement, Amprius has shipped high-density silicon-anode cells to defense customers, but no subsequent FAA compliance filing tied to the 500 Wh/kg platform has surfaced publicly. The claim remains a commercial milestone, not a regulatory one.
Meanwhile, the eVTOL companies closest to certification have disclosed battery partnerships and performance targets without publicly tying any semi-solid-state cell to a completed DO-311A test campaign. Joby Aviation, which has been working toward FAA type certification and conducted extensive flight testing through 2025, has discussed battery performance in investor materials but has not named a semi-solid-state supplier in its public filings. Archer Aviation, building toward production at its Georgia manufacturing facility, faces the same battery qualification hurdle. Neither company has published the kind of DO-311A compliance summary that would confirm a specific cell chemistry is ready for installation approval.
Why the car-to-aircraft leap is harder than it looks
A battery that performs well in an electric sedan is not automatically suited for an air taxi, and the reasons go beyond regulatory paperwork. The physical demands are different. Aircraft batteries endure rapid altitude changes that cause pressure differentials across cell casings. They experience vibration profiles distinct from road surfaces. And in an eVTOL, power draw during vertical takeoff can spike far above anything a car motor demands, stressing cells in ways that accelerate degradation and heat buildup.
Then there is the weight penalty of integration. A bare cell tested at 350 Wh/kg on a lab bench will deliver considerably less energy density once packaged into a flight-ready battery system. Thermal management hardware, bus bars, structural housings, and battery management electronics all add mass. Industry estimates suggest that pack-level energy density typically falls 20 to 40 percent below cell-level figures. A 350 Wh/kg cell might yield a 210 to 280 Wh/kg pack, and it is the pack that must pass the full abuse sequence, not the bare cell.
Semi-solid electrolytes are often promoted as inherently safer than liquid alternatives, with lower flammability and better resistance to thermal runaway. That reputation may prove justified, but DO-311A does not grade on a curve. The test sequences force cells into worst-case failures, including sustained overcharge and forced internal short circuits, regardless of the electrolyte type. Until published test data shows how semi-solid-state cells behave under those specific conditions, regulators have no basis to relax existing assumptions about containment, redundancy, and fire suppression.
The two-track race that will decide the outcome
What is unfolding is essentially a two-front competition. On one front, battery chemists are pushing energy density higher. Semi-solid-state cells, silicon-rich anodes, and advanced liquid electrolytes are all converging on or exceeding 350 Wh/kg at the cell level. Progress here has been rapid, and the headline numbers are genuinely impressive.
On the other front, certification engineers are grinding through the slow, expensive work of proving those cells can survive the worst day they will ever have. DO-311A testing requires hundreds of cells to be destroyed under controlled abuse conditions. The results feed into the AC 20-184A framework, which the FAA reviews before granting installation approval. This track does not move at the pace of press releases. It moves at the pace of test reports, engineering reviews, and regulatory findings.
Progress on one front without matching progress on the other will not be enough. A semi-solid-state cell that posts record energy density but cannot pass thermal runaway propagation tests will never power a type-certificated eVTOL carrying paying passengers. Conversely, a cell that sails through DO-311A but delivers only modest energy density will not give air taxi operators the range they need to build viable route networks.
For investors and eVTOL developers watching this space, the sequencing matters. In automotive applications, manufacturers can often ship cells while safety data accumulates over millions of miles of fleet use. Aviation does not permit that approach. Safety evidence must precede deployment, and the FAA’s advisory circular framework demands that test data be available for review before any installation approval is granted. That requirement alone could add years to the timeline for semi-solid-state batteries in commercial air taxi service.
What the public record does and does not tell us
Three categories of evidence are circulating, and they carry very different weight. The FAA’s own publications, including the lithium battery hub and draft AC 20-184A, are primary regulatory documents. They confirm that the U.S. certification framework for rechargeable lithium batteries in aircraft is active and evolving, but they do not endorse any particular chemistry or manufacturer.
Manufacturer announcements like the Amprius 500 Wh/kg disclosure are primary commercial evidence. They signal where a company believes its technology is headed and where it intends to compete. They do not, on their own, prove that the technology meets aviation safety requirements. When a battery maker cites a headline energy density figure, the first question should always be: does that number reflect a bare cell on a lab bench, or a fully integrated pack that has survived DO-311A abuse?
The third category, largely absent from the current public record, is independent test data. No third-party laboratory report, no published DO-311A compliance summary, and no FAA special conditions document specific to semi-solid-state cells at 350 Wh/kg or above has appeared in publicly available sources as of June 2026. That absence does not mean testing is not underway. Certification campaigns are typically confidential until an applicant files for a type certificate or seeks public comment on special conditions. But it does mean that anyone evaluating the readiness of these batteries for commercial flight should weigh the distance between what has been announced and what has been formally demonstrated to regulators.
The defining test is still ahead
The safest reading of the evidence right now is cautious optimism grounded in patience. Semi-solid-state batteries have proven they can hit the energy density numbers the air taxi industry needs. The FAA has built a clear, if still evolving, regulatory pathway for high-energy lithium cells on aircraft. And at least one manufacturer has publicly targeted densities that would make electric air taxis meaningfully more practical than anything flying today.
But the lack of confirmed DO-311A qualification for any semi-solid-state pack at aviation-relevant densities tells us the technology is still in transition from promising prototype to certifiable product. The chemistry works. The question is whether the safety case can be closed, and that answer will come not from press releases or investor decks, but from test labs where cells are methodically pushed to failure and the data is handed to regulators who have no incentive to rush. For semi-solid-state batteries, the leap from cars to flying taxis is not a matter of if the energy is there. It is a matter of proving, cell by destroyed cell, that the energy can be trusted at altitude.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
