Going to extremes
New research by Berkeley engineers may soon make it more practical to use battery-powered vehicles and devices in extreme temperatures, such as icy-cold winters in Minnesota or stifling-hot summers in Death Valley. Led by Chris Dames, professor of mechanical engineering, the researchers developed a thermal regulator that can improve the performance of lithium-ion batteries outside of what’s now considered to be the optimal window — typically 20 to 40 degrees Celsius. Their system uses nickel and titanium alloy wires, attached to the lithium-ion battery pack, that soften below 35 degrees Celsius but stiffen and contract above 35 degrees Celsius. At higher temperatures, the stiffened wires pull the batteries tightly into contact with a heat sink designed to cool the batteries down. At cooler temperatures, the softened wires allow the battery pack to lift away from the heat sink with the help of compressed springs; the resulting air gap provides insulation that keeps the batteries warm by slowing the dissipation of waste heat. Testing showed that at minus 20 degrees Celsius, the regulator increased the battery temperature to 20 degrees Celsius just by retaining the battery’s self-generated heat. At 45 degrees Celsius, the regulator kept the batteries from overheating by limiting the temperature rise to about 6 degrees through constant heat dissipation.