Donut Lab Battery Keeps 97% Capacity at -30°C, Below Its 99% Claim
A VTT report commissioned by Donut Lab measured 26.821 Ah at -30°C against 27.77 Ah at +25°C for a 14-cell module, short of the claimed 99%. Chemistry is undisclosed.
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Donut Lab's own render of its solid-state battery. The image is not a photo of the 14-cell module VTT tested.
Image: Donut Lab
Source · Press image from the Donut Lab newsroom, used for news reporting.
VTT Technical Research Centre of Finland measured a Donut Lab battery module at 26.821 Ah when discharged at -30 °C, against 27.77 Ah at +25 °C, in a customer report dated 20 October 2026 that Donut Lab commissioned. That is about 97% of room-temperature capacity.
VTT measured 26.821 Ah at -30°C against 27.77 Ah at +25°C in a test Donut Lab commissioned; 29 Ah is the customer-provided nominal.
Illustration (AI-generated)
The module was 14 cells in series, with a battery management system and no heating or cooling elements, according to the VTT report. It sat free-standing in a climate chamber for about 5 hours before a 1C discharge, meaning a one-hour rate. CleanTechnica reported the result on 23 October.
Energy Retention Fell Further Than Capacity
Energy retention was lower than capacity retention. The module delivered 1,253.9 Wh at -30 °C against 1,353.9 Wh at +25 °C, roughly 93% by Voltgrid's calculation from the two VTT figures. Donut Lab had claimed 99% capacity retention at -30 °C, a figure CleanTechnica says dates from the January unveiling, so the measured result is a few points lower.
VTT also logged a detail that matters for reading the result. During the cold discharge the internal module temperature rose to about +38 °C while the surface read about -6 °C. The cells therefore warmed themselves under load, which is normal for a 1C discharge, but the test does not show how a larger pack with thermal mass would behave over a full cold soak.
Why Cold Performance Matters for Storage and EVs
Cold is a cost driver for batteries in vehicles and in outdoor storage. Heaters and insulation add weight, parasitic load and cost, and Donut Lab's chief technology officer Ville Piippo said the test deliberately left them out. If a cell design holds capacity without them, system designers can shrink thermal hardware. One 14-cell module, with a nominal 29 Ah rating supplied by the customer, is far from a vehicle pack or a grid container.
What Donut Lab Has Not Confirmed Yet
Donut Lab described the battery at CES in January as solid-state and ready for vehicle makers, with claims of a five-minute full charge and 400 Wh/kg. The company has not disclosed the chemistry, and the VTT report states no energy density for the module. CleanTechnica noted that a whole-system report is still to come. Voltgrid News reported earlier on EVE Energy's solid-state cell. The test was ordered and publicised by Donut Lab, and VTT's report carries a notice limiting use of its name in advertising without written permission.
The report does not cover cycle life, charging at low temperature or repeated cold soaks. Those are the figures that investors and cell buyers will want before treating the claim as production-ready.
