The self‑discharge test of the Donut battery showed amazing results that do not disappoint.

Finnish startup Donut Lab continues to prove the reality of its claims about a solid‑state battery

At CES 2026, the Finnish company Donut Lab made a splash by unveiling its first serial solid‑state lithium battery. At the same time many experts expressed skepticism: some suspected that the “battery” was actually a supercapacitor.

To dispel doubts, Donut Lab commissioned a series of independent tests at the reputable Finnish center VTT Technical Research Centre of Finland. The third – self‑discharge assessment – was published in the recent report VTT‑CR‑00125‑26.

What was tested
1. Nominal capacity

- Discharge at 1C showed 26.5 Ah (compared with claimed 26 Ah).

2. Self‑discharge at room temperature

- Battery charged to 50 % (13.335 Ah), then left for 240 hours (10 days) in the range 22–28 °C.

- After the hold, a full discharge yielded 12.029 Ah – that is 97.7 % of the initial charge.

3. Voltage behavior

- In the first hour after charging a drop of 103 mV was observed (stabilization period).

- Over the remaining nine days the decline was only 12 mV, indicating high stability.

The test found no structural damage or cell degradation.

Why it matters
- Refutation of the supercapacitor – Supercapacitors lose energy quickly even after a few hours of standby.

- Low self‑discharge confirms the presence of real chemical capacity rather than capacitor behavior.

Donut Lab’s chief technologist noted: “The essence of the test is its simplicity – it proves that we really have a battery, not a supercapacitor. The low level of self‑discharge indicates genuine chemical capacity.”

How this fits into the overall picture
- Fast charging

- Operation at extreme temperatures

- Self‑discharge test

These three independent tests sequentially confirm the key advantages of Donut Battery and bring the product closer to commercial production.

While the most prominent claimed specifications – 100 000 cycles, energy density around 400 Wh/kg, and freeze resistance – have not yet received final independent verification, the results look promising. We expect new reports from VTT and further confirmations from third parties.