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Amprius secures multiple purchase orders from AALTO for SiMaxx 450 Wh/kg high-energy cells

Amprius Technologies, a developer of next-generation lithium-ion batteries with its Silicon Anode Platform, has signed purchase orders from AALTO HAPS (High Altitude Pseudo Satellite) for the company’s 450 Wh/kg ultra-high-energy density SiMaxx cells.

Amprius’ 5.8 Ah, 450 Wh/kg SiMaxx cells are designed to operate in some of the most extreme flight conditions while maximizing flight range and payload capacity. These attributes make SiMaxx cells the only known commercially available batteries capable of supplying enough endurance for persistent HAPS (High-Altitude Platform Stations) overnight stratospheric flight operations in all seasons or at higher latitudes.

This is a key consideration for AALTO, with Zephyr the only fixed-wing HAPS to have demonstrated day and night longevity in the stratosphere.


The collaborative efforts between AALTO and Amprius reached a milestone in 2022 with a record-breaking 64-day cross-continental test flight of a Zephyr 8/S. During the event, Amprius’ 400 Wh/kg high-energy cells powered the flight without descending from the stratosphere, allowing AALTO to break its own previous record of more than 25 days.

Amprius’ SiMaxx cells are equipped to operate in challenging environments and are, therefore, ideally suited for aviation and military applications. Amprius began fulfilling shipments against the new AALTO purchase orders in Q1 of 2024 and expects to continue fulfilling shipments in the coming months as the Company advances its efforts to increase its SiMaxx production capacity in Fremont, CA.



Congratulations to Amprius for hitting energy densities which can enable flight applications.

There would however still appear to be significant trade offs in how fast you can charge these to have any sort of decent cycle life:


' The charging and discharging current during the test was C/10 (10-hour charging and 10-hour discharging cycle), which is relatively slow for EV applications. At C/10 current, a 100-kilowatt-hour (kWh) battery would deliver just 10 kilowatts (kW) of power.

We don't know whether there was any limitation in the charging/discharging power due to the silicon-rich anode, but this is an important thing for electric cars (specifically, the cycle life at higher currents).

Amprius' general info says that its batteries can recharge in as low as six minutes to 80 percent state-of-charge (SOC) (it was verified in 2021 for 370 Wh/kg cells). On the other hand, the same info says that the cycle life varies from 200 to 1,200. Usually, the lower numbers were correlated with higher energy density, higher charging/discharging current, and non-optimum temperatures.'

Note that these tests were carried out on the newer 500 Wh/kg version, but presumable it is comparable in charge times and life.

This sort of issue for very high energy density batteries is why I fancy, at least initially, battery swapping for planes rather than in situ fast charging to enable any sort of acceptable turn around time.


The average plane turn-around at gate is close to an hour. That would allow for a "1C" charge rate, which is typically fairly safe for a battery charge rate.

On the Amprius product page, they list three designs under the "SIMAXX" product family: High Power, High Energy and Balanced. It would appear that the main contributor to degradation is depth of discharge. They specifically market the SICORE product family for Air Transportation (EVTOL) with a "380+ Wh/kg and "extreme-fast charge" (SA03 I believe).

Their SA82 product (500Wh/kg) seems to require the C/10 discharge (and recharge?) rate and still has a low 150 cycles listed. These are marketed for High Altitude Pseudo Satellites currently.

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