Finnish Lab Confirms Battery Holds 97.7% Charge After 10 Days

Imagine leaving your electric car parked for 10 days and finding it still has almost the same charge you left it with.

That's exactly what happened in independent testing of Donut Lab's solid-state battery technology. VTT, Finland's Technical Research Centre, charged a battery cell to 50% capacity and let it sit for 240 hours at room temperature. When they checked back, the cell still held 97.7% of its original charge.

The testing process was straightforward but telling. VTT charged the battery with 13.335 amp-hours of energy and recovered 13.029 amp-hours after the waiting period. The battery lost most of its voltage in the first hour, which researchers say is normal relaxation after charging rather than actual power loss.

What happened next was more impressive. From hour 10 to hour 240, the voltage barely budged, dropping just 12 millivolts across 230 hours. That ultra-flat performance suggests the battery's long-term power retention is exceptionally stable.

This marks the third independent verification Donut Lab has released in three weeks. Previous tests confirmed the battery can fast-charge in 4.5 minutes and work in extreme heat up to 100 degrees Celsius. The company is showing more transparency than most battery startups ever offer.

Standard lithium-ion batteries typically lose 1 to 3% of their charge per month at room temperature. Solid-state batteries are expected to perform better because their solid materials are less prone to the chemical reactions that drain liquid batteries over time.

Why This Inspires

Every independent test brings us closer to electric vehicles that behave more like the reliable tools we need them to be. Better charge retention means EV owners won't worry about their car slowly losing power while parked at the airport or sitting unused during a vacation.

The technology could also transform how we think about energy storage for homes and businesses. Solar panels paired with batteries that hold their charge for weeks could make renewable energy more practical for everyone.

Donut Lab CEO Marko Lehtimäki has promised to deliver these batteries in Verge Motorcycles by early 2026. While researchers still want to see testing on the company's most ambitious claims about energy density and battery lifespan, each verified result builds confidence that solid-state battery technology is moving from laboratory promise to real-world product.

The steady drumbeat of positive test results suggests we're watching a genuine breakthrough unfold in real time.