A battery module developed by Finnish technology company Donut Lab retained approximately 93 percent of its energy at -30 degrees Celsius (-22 degrees Fahrenheit) without external heating or insulation, according to new test results from VTT, which tested a complete module containing 14 first-generation Donut Battery cells.
The 14-cell module delivered 1,254 watt-hours at -30°C (-22°F), compared with 1,354 watt-hours at 25°C (77°F). It also retained about 97 percent of its ampere-hour capacity, suggesting limited capacity loss under the tested conditions.
The results address a major challenge for electric vehicles, whose batteries typically deliver less usable energy in freezing weather. Low temperatures slow the chemical reactions inside batteries and can reduce their ability to supply power.
Donut Lab, which develops battery technology for electric mobility applications, had previously claimed its battery could retain 99 percent of its capacity at -30°C, although the latest test measured a slightly lower figure.
Testing without external heating
VTT tested a complete module containing 14 first-generation Donut Battery cells, rather than evaluating a single cell. The module operated at a nominal voltage of 48 volts and included a small battery management system.
It had no heating elements, insulation, cooling equipment, or other thermal management systems.
Researchers first measured its performance at 77°F by discharging it at a 1C rate, equivalent to drawing its rated capacity over approximately one hour. The module delivered 27.77 ampere-hours and 1,354 watt-hours.
They then lowered the chamber temperature to -30°C (-22°F) and kept the module inside for five hours. Sensors positioned on a cell surface inside the module and on its exterior confirmed that the internal temperature had reached the target.
The researchers repeated the discharge test at the same 1C rate. This time, the module delivered 26.8 ampere-hours and 1,254 watt-hours.
The difference between capacity and energy retention is important. Ampere-hours measure the amount of electrical charge a battery delivers, while watt-hours account for both charge and voltage.
The module retained 97 percent of its charge capacity but delivered about 93 percent of its original energy because its voltage dropped further during cold-weather discharge.
Cold performance faces scrutiny
According to Donut Lab, the cells warmed during discharge despite the chamber’s freezing temperature. The company said an insulated battery enclosure could retain more of that heat and potentially improve performance.
“We intentionally eliminated all the potential advantages a battery might normally have,” said Ville Piippo, Donut Lab’s chief technology officer.
The company also reported that the module continued charging and discharging normally after the cold-temperature tests, with no observed adverse effects.
However, the results come from a controlled laboratory test of one battery module, not a complete electric vehicle. They do not establish how much driving range an EV equipped with the technology would retain in freezing conditions.
The cold-weather evaluation concludes a series of tests conducted by VTT on Donut Lab’s first-generation cells. Previous assessments examined five-minute charging, performance at 100°C, self-discharge, swelling, pressure, and energy density.
Donut Lab said it is now focusing on next-generation battery technologies and plans further system-level demonstrations. The company has made the cold-temperature test report available through its testing website.