Helios Horizon Flies Solid-State Battery Demonstrator

A Sarasota, Florida, nonprofit group has flown what it says is the first piloted electric airplane powered by solid-state batteries. Helios Horizon founder and chief test pilot Miguel Iturmendi completed a series of short test flights on June 5 at Zephyrhills Municipal Airport (KZPH) in central Florida, flying a modified Pipistrel Taurus motor glider that the team reconfigured to run its single motor on solid-state batteries.

“For the first time, we have a battery technology that yields the range and charging times necessary to make commercial electric aviation viable, while providing the safety the flying public will demand,” Iturmendi said in a company statement.

The reconfigured airplane previously flew on lithium-ion batteries rated at 260 watt-hours per kilogram (Wh/kg). Its solid-state cells deliver 410 Wh/kg, which the company says amounts to 60% to 80% greater energy density than the lithium-ion cells that power most electric vehicles today.

That figure marks a clear gain over the cells the airplane carried before, but it trails the strongest lithium-ion batteries already on the market for aviation. Amprius Technologies sells silicon-anode SiCore cells rated at 450 Wh/kg, while MagniX offers a 400 Wh/kg cell in its Samson line. EHang’s solid-state cell was reportedly rated at 480 Wh/kg when it supported the first uncrewed, solid-state-powered eVTOL flight in November 2024.

Solid-state cells replace the flammable liquid electrolyte in a lithium-ion battery with a solid material, which Helios Horizon says makes its cells more stable than conventional cells, even when subjected to high temperatures and structural damage.

Helios Horizon assembled the battery packs itself from solid-state cells it sourced from several resellers, a company spokesman told AIN. The organization declined to name suppliers, citing nondisclosure agreements, and said it could not specify the cells’ chemistry (i.e., the electrolyte and electrode materials).

According to Helios Horizon, the airplane can charge from almost any AC source without dedicated infrastructure, and it can recharge in flight through its solar panels and by windmilling its propeller when the motor is not driving it for thrust. Iturmendi said the cells can charge from near empty to 80% in less than 15 minutes.

“Regenerative flying, in which we glide and windmill the propeller during descents, can significantly increase the range of the airplane,” Iturmendi said.

The June 5 flights validated the airplane’s weight and balance after the battery swap and confirmed ground-test results showing the cells held up over hours of sustained operation, the company said.

Stratospheric ambitions

Iturmendi designed the aircraft around a Pipistrel Taurus airframe, into which the team fitted its own systems for power delivery, battery management, propulsion, and thermodynamic control, along with wing extensions and solar panels. The company says the airplane, in the 500-kilogram (1,100-pound) weight class, holds the world altitude record for an electric airplane of its size.

On earlier flights powered by lithium-ion batteries near Bishop, California, in 2024, the airplane expanded its envelope from 17,500 feet to 24,000 feet while using no more than 60% of its battery capacity. It now aims to fly above 40,000 feet—higher than most airliners cruise—on stratospheric flights planned for later this year.

Iturmendi told AIN in 2024 that the main technical challenge for stratospheric electric flight is dissipating heat from the propulsion system, which is hard to shed in the thin air at altitude. He expects the energy density of solid-state cells to increase by 40% in the next two years.

Solid-state batteries remain unproven at the commercial scale and uncertified for any aircraft, and researchers have cautioned that energy density alone does not guarantee that a cell can withstand the repeated high-power demands of flight.

Helios Horizon said the airplane draws modestly on its cells. In cruise, it pulls about 9 kilowatts from a 400-volt, four-battery pack holding about 60 kilowatt-hours—roughly one-seventh of its capacity each hour, or 0.15C. A climb pulls harder, a little under half the pack's capacity each hour (0.45C), roughly doubling to 0.9C when it flies on two batteries rather than four.

The company did not provide a power density in watts per kilogram, a measure of how quickly a battery can deliver energy relative to its weight. The draw is also light next to the demands of vertical-lift aircraft: researchers simulating eVTOL takeoffs have pushed cells to 15C, many times the rate of Helios Horizon's gradual climb.

Helios Horizon describes the airplane as a concept demonstrator and flies it under an experimental research and development certification. “I’m not an entrepreneur, and this is really just a technology demonstration to show what is possible in terms of range and endurance,” Iturmendi said in 2024.

The nonprofit operates on donations, which total about $1 million to date, and it is seeking roughly $200,000 more to fund the campaign to fly above 40,000 feet, the spokesman told AIN. For now, the team plans to keep refining the airplane’s systems ahead of those flights.