Donut Lab’s solid-state battery keeps 97.7% charge after 10 days in third test

Finnish startup Donut Lab has just released the third independent test of its solid-state “Donut Battery,” showing it retains 97.7 percent of its charged capacity after remaining idle for 10 days.

The technology firm released the new report from VTT Technical Research Centre of Finland (VTT-CR-00125-26) on March 9, 2026. The test puts to rest speculation that the battery, considered the world’s first production-ready solid-state battery for electric vehicles, behaves like a supercapacitor.

A supercapacitor (or ultracapacitor) is reportedly a high-capacity energy storage device that bridges the gap between conventional capacitors and batteries. This unit stores charge electrostatically, enabling fast charging and discharging.

“Since we unveiled the Donut Battery, there has been a lot of speculation and theories about whether it is a supercapacitor,” Ville Piippo, Donut Lab CTO, said. “In all its simplicity, this test proves that it is a battery.”

Boom. This is a very big deal.

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