Donut Lab's Solid-State Battery Achieves 97.7% Charge Retention After 10 Days, According to Third Independent Test
Key Takeaways
- ▸Donut Lab's solid-state battery demonstrated 97.7% charge retention after 10 days of idle time at ambient temperature, with most voltage loss occurring during initial relaxation rather than true self-discharge
- ▸VTT's third independent report in three weeks continues to validate engineering-achievable claims (fast charging, thermal tolerance, low self-discharge) that solid-state batteries are theoretically expected to excel at
- ▸The two most extraordinary claims—400 Wh/kg energy density and 100,000-cycle life—remain completely untested by independent parties, with VTT reports lacking weight and dimension data needed to verify energy density claims
Summary
Donut Lab has released its third independent test report from VTT, the Finnish Technical Research Centre, confirming that its solid-state battery retains 97.7% of its charged capacity after 10 days of idle time at ambient temperature. The self-discharge test charged a cell to 50% state of charge and measured only a 2.3% loss over the 240-hour period, with most voltage drop occurring during initial relaxation rather than true self-discharge. When extrapolated, the data suggests a long-term self-discharge rate significantly lower than the initial 10-day measurement would indicate.
While the result is solid, it is not exceptional in isolation—typical lithium-ion cells lose about 1–3% per month, and the 2.3% loss over 10 days would extrapolate to roughly 5–7% per month. However, the voltage curve reveals that the cell stabilized after the first 10 hours, with only 12 mV of additional drift over the remaining 230 hours, suggesting the true steady-state self-discharge rate is very low. The report marks the third successful independent verification in three weeks, following tests of fast charging and high-temperature discharge capabilities.
- The extremely flat voltage profile from hour 10 to hour 240 suggests the long-term self-discharge rate is much lower than early-period measurements would indicate, but longer-duration testing (30–60 days) would be needed for precision
Editorial Opinion
Donut Lab's transparency with three rapid independent test reports is commendable and sets a high bar for battery startup credibility. However, the company's strategy of testing only the conventionally achievable specifications raises questions about whether it is systematically avoiding independent scrutiny of its most controversial claims. The 400 Wh/kg energy density and 100,000-cycle lifespan remain the game-changing assertions that would truly disrupt the battery industry—and they remain unverified.
