By Josh Perry, Editor
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Engineers from the University of California Berkeley have developed a new thermal regulator that maintains optimal operating temperatures for lithium-ion batteries even in extreme heat or cold, which could be a boost for battery-powered vehicles or mobile devices.

Shown is a schematic and photo of a thermal regulator designed by Berkeley engineers to passively keep lithium-ion batteries within an optimum temperature range.
(Menglong Hao, UC Berkeley)

According to a report from the school, battery performance drops drastically in extreme cold (at -20°C, a battery will only deliver 20% of its capacity) and high heat will impact the battery lifetime (halving for every 13°C of excess temperatures).

Batteries perform best between 20-40°C, but that is a temperature range difficult to find in winter in New England or during the summer months in Phoenix. This has been a limiting factor in the viability of electric vehicles.

Berkeley engineers designed a passive regulator system with a shape-memory alloy, which is soft and pliable at low temperatures but hardens at higher temperatures. Using a mix of nickel and titanium alloy wires, the engineers created a material that transitions at 35°C.

“The researchers attached the wires to a lithium-ion battery pack such that the ‘on’ position was at the higher temperatures, with the stiffened wires pulling the batteries tightly into contact with a heat sink designed to cool the batteries down,” the report said. “At temperatures below 35 degrees Celsius, the softened wires were in the ‘off’ position, allowing the battery pack to lift away from the heat sink with the help of compressed springs. The resulting air gap provided insulation that helped keep the batteries warm by slowing the dissipation of their own waste heat.”

The regulator was tested in a vacuum and in real-world conditions. At an ambient temperature of -20°C, the regulator kept the battery at 20°C by using its self-generated heat and at an ambient of 45°C the regulator limited the battery temperature rise to only six degrees.

Engineers insisted that they had created a system that will work in “Lake Tahoe in January and Death Valley in August.”

The research was recently published in Nature Energy. The abstract stated:

“The poor performance of lithium-ion batteries in extreme temperatures is hindering their wider adoption in the energy sector. A fundamental challenge in battery thermal management systems (BTMSs) is that hot and cold environments pose opposite requirements: thermal transmission at high temperature for battery cooling, and thermal isolation at low temperature to retain the batteries’ internally generated heat, leading to an inevitable compromise of either hot or cold performances.

“Here, we demonstrate a thermal regulator that adjusts its thermal conductance as a function of the temperature, just as desired for the BTMS. Without any external logic control, this thermal regulator increases battery capacity by a factor of 3 at an ambient temperature (T_ambient) of −20 °C in comparison to a baseline BTMS that is always thermally conducting, while also limiting the battery temperature rise to 5 °C in a very hot environment (T_ambient = 45 °C) to ensure safety.

“The result expands the usability of lithium-ion batteries in extreme environments and opens up new applications of thermally functional devices.”