Finnish startup Donut Lab's third test shows its solid-state battery retains charge for 10 days, proving it isn't a supercapacitor.
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 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 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.”The latest testDesigned for Verge Motorcycles’ TS Pro, the technology boasts a 370-mile range, five-minute charging, 400 Watt-hour per kilogram energy density, and superior safety across extreme temperatures. But since its introduction earlier this year, speculation has grown over whether it is truly a solid-state battery, or rather a high-performance supercapacitor, given its extraordinary, almost too-good-to-be-true performance data. To support its claims, the firm previously released two independent tests. A fast-charging test showed the technology can charge from 0 to 80 percent in about 4.5 to five minutes.A separate thermal stability test demonstrated efficient operation at 176 and 212 degrees Fahrenheit , where the battery delivered 110 and 107 percent of its room-temperature capacity, respectively. Now, in a third independent test, scientists evaluated the battery’s self-discharge performance. The results showed the cell retained 97.7 percent of its charge after remaining idle for 10 days.The experiment followed a straightforward procedure designed to measure how well the battery retains energy when not in use. VTT experts first conducted a 1C capacity test. It confirmed the cell delivered 26.5 amp-hours of discharge capacity, slightly above the battery’s nominal 26 Ah specification.Battery durability checkFor the self-discharge test, VTT first charged the cell to about 50 percent state of charge . The battery was left idle for 240 hours at an ambient temperature of 72 to 82 degrees Fahrenheit . Its voltage was recorded every 10 seconds.Following the idle period, VTT scientists discharged the cell and recovered 12.029 Ah. This was equal to 97.7 percent of the original charge. Voltage measurements revealed that most of the drop occurred immediately after charging. The battery’s voltage dropped by about 60 millivolts in the first 10 seconds after charging. It fell further to 103 mV within the first hour. The team attributed this to voltage relaxation after charging. Once stabilized, the battery’s voltage declined by only 12 mV from hour 10 to hour 240. The 3rd independent test of the Donut Battery, conducted by VTT.Credit: Donut LabAS per VTT’s report, the testing process caused no visible damage or structural changes to the cell. This further supported the battery’s stability under the test conditions.Donut Lab said a capacity test carried out at the end of the test period confirmed that the voltage drop matched the amount of energy stored in watt-hours. “The Donut Battery behaved in the test exactly as a battery should,” the firm said. Piippo added that a supercapacitor would have lost charge much faster over the same time period. “Supercapacitors charge and discharge quickly, but they also lose their charge quickly when not in use,” he concluded in a press release. “The Donut Battery behaves like a battery and can maintain a charge for significantly longer.”
Charge Donut Lab Energy Energy &Amp Environment Finland Solid-State Batteries Supercapacitor Sustainability VTT Technical Research Centre Of Finland
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