Donut Battery cold-weather test points to resilient performance at -30°C

Independent testing by VTT found that a 14-cell Donut Battery module retained 97 per cent of its ampere-hour capacity after being chilled to -30°C, although the energy retained on a watt-hour basis was lower at around 93 per cent. The distinction matters when assessing what the result could mean for electric vehicles operating in severe winter conditions.

Cold weather remains one of the more testing environments for battery-powered transport. Low temperatures can affect electrochemical processes and increase resistance within batteries, making winter performance an important consideration as electric vehicles (EVs) move into a broader range of climates.

Against this background, Finnish technology company Donut Lab has released the results of a cold-temperature assessment of its first-generation battery technology. The test was conducted by Finland’s VTT research organisation and, significantly, used a complete module rather than evaluating an individual cell.

The result adds another piece to the developing story around the Donut Battery. As Digital Journal previously reported, Donut Lab has been attracting attention partly because the company has not publicly disclosed the detailed chemistry of its battery, while independent material analysis by Intertek found only trace quantities of lithium and sodium in materials submitted for testing.

For the latest experiment, VTT tested a battery module made from 14 Donut Lab first-generation cells. The module had a nominal voltage of 48 volts and incorporated a small battery management system. According to the test information released by the company, however, it used no thermal-management system, heating elements, insulation or cooling.

VTT first established the module’s performance at +25°C. Discharging at a 1C rate produced a measured capacity of 27.77 ampere-hours (Ah) and energy of 1,354 watt-hours (Wh). The environmental chamber was subsequently cooled to -30°C, with the module remaining there for five hours before the discharge test was repeated at the same 1C rate.

Care was also taken to assess what was happening at cell level rather than relying only upon the temperature of the surrounding chamber. One sensor was positioned on the surface of a cell inside the module and another externally. Donut Lab says the resulting measurements showed the cells had reached -30°C before the cold-temperature discharge began. Under these conditions, VTT recorded 26.8 Ah, equivalent to approximately 97 per cent of the capacity obtained at room temperature.

This is a notable result, but the additional energy measurement provides important context.

The cold module delivered 1,254 Wh compared with the room-temperature baseline of 1,354 Wh, or roughly 93 per cent. Donut Lab attributes the difference between the Ah and Wh results to the greater voltage drop occurring under freezing conditions.

For consumers interested primarily in how far an EV might travel in winter, that distinction between charge capacity and energy delivered is important. The laboratory result demonstrates strong cold-weather retention under the specified test conditions, but it should not be read as a direct prediction that a vehicle would retain precisely 97 per cent of its driving range.

A demanding test without thermal assistance

The absence of active heating is a particularly interesting aspect of the experiment.

Modern battery testing increasingly combines controlled environmental chambers with battery cycling equipment. Environmental chambers used for battery development can reproduce wide temperature ranges, with commercially available systems designed specifically for charge and discharge testing under controlled conditions.

Here, however, Donut Lab deliberately sought to demonstrate how the module performed after being thoroughly chilled without relying on thermal conditioning. During discharge, the VTT measurements also showed the cells warming despite the chamber remaining extremely cold.

Donut Lab CTO Ville Piippo argues that the stringent configuration strengthens the case for the technology’s winter performance, saying the company deliberately removed advantages that would ordinarily assist a vehicle battery. He further argues that an insulated production module would allow Donut Lab to approach its previously claimed 99 per cent capacity retention at -30°C.

The latter figure remains a company projection rather than the measured outcome of this particular test. The experimentally reported result for the uninsulated module was 97 per cent in Ah terms and around 93 per cent in Wh terms.  That distinction is important when separating laboratory evidence from a manufacturer’s expectations for a finished application.

Why cold resistance matters

Batteries are becoming simultaneously more important and more technically sophisticated. Improving energy density is only one part of the challenge. Manufacturers also need to understand degradation, temperature behaviour, charging characteristics and safety as batteries are expected to operate across increasingly varied environments.

Recent research into conventional lithium-ion systems illustrates how complex battery behaviour can be. Battery ageing can involve coupled chemical, thermal and mechanical processes, including electrolyte decomposition and increasing impedance, while researchers are consequently examining new methods of sensing battery condition and predicting deterioration.

At the same time, work on next-generation high-nickel cathodes demonstrates that apparently small factors during manufacture can influence subsequent degradation. Research covered recently by Digital Journal, for example, identified air exposure of precursor materials as a source of damaging chemical changes in certain high-nickel cathode designs. It is within this wider push for more resilient batteries that Donut Lab’s latest data should be considered.

From cell claims to system evidence

The cold test concludes a series of VTT assessments commissioned on Donut Lab’s Generation 1 cells. According to the company, these have included rapid charging, testing at 100°C, self-discharge, repeated rapid charging of a damaged cell, swelling and pressure assessments, energy-density measurement and the newly published -30°C trial.  Donut Lab CEO Marko Lehtimäki says the company’s attention is now shifting towards its next-generation technology and system-level demonstrations.

That progression will be consequential. Laboratory measurements can establish how cells and modules behave under carefully defined conditions, but system-level testing is necessary to understand how the technology performs when incorporated into the larger thermal, electrical and mechanical environment of a practical vehicle.

There are also unanswered questions around the technology itself. Previous independent analysis established that the samples supplied to Intertek contained only trace quantities of lithium and sodium, but this did not identify the precise chemistry underpinning the Donut Battery. Furthermore, accreditation of a laboratory method should not be mistaken for endorsement of every wider claim made about the technology.

The cold-temperature results nonetheless provide a useful, quantified addition to the available evidence. A module that delivered 26.8 Ah after being brought to -30°C, compared with 27.77 Ah at +25°C, produced strong capacity retention under the conditions tested. The corresponding fall from 1,354 Wh to 1,254 Wh shows why the complete dataset, rather than the headline percentage alone, needs to inform interpretation.