Finnish startup Donut Lab released independent test results Monday showing its solid-state battery retained 97.7% [1] of its charge after 10 days.
This result is intended to address skepticism from industry experts regarding whether the technology is a true battery or a supercapacitor. Because supercapacitors typically lose charge quickly, this retention rate serves as a benchmark for the startup's claims of a production-ready all-solid-state battery.
The test is the third in a series of weekly rollouts following the company's announcement at CES 2026. Donut Lab has commissioned an independent lab to verify that its solid-state battery technology is real, and the results appear to support some of its bold claims, a report said [2].
Previous data released by the company indicates the battery can reach an 80% charge in four and a half minutes [3]. The company has also provided details on its 26-Ah cell size [4] and conducted safety tests to further validate the hardware. These efforts aim to prove critics wrong who said it was not a real battery, Donut Lab said [5].
While the charge retention and speed are promising, some industry observers note that the company has yet to provide comprehensive proof regarding energy density. The tests were conducted in a Finnish lab and at a public charger to simulate real-world conditions [2, 6].
Donut Lab continues to release data to bridge the gap between its prototype claims and commercial viability. The company maintains that its technology will enable faster EV charging and longer service lives than current lithium-ion standards.
“Donut Lab's solid-state battery retained 97.7% charge after 10 days.”
The ability to retain nearly all charge over 10 days distinguishes this technology from supercapacitors, which charge and discharge rapidly but cannot store energy long-term. If Donut Lab can eventually prove high energy density alongside these charging speeds, it could resolve the primary trade-off in electric vehicle design: the choice between fast charging and long driving range.
