By Peter Weiss
Electric-vehicle developers find lithium-based batteries attractive because the cells pack a lot of power into a relatively small, lightweight package. Yet carmakers are leery of the batteries because the devices are prone to runaway heating, a fire hazard (SN: 2/12/00, p. 103: Stopping batteries from starting fires).
One possible way to reduce that danger is to use any unwelcome heat to melt wax or some other substance. This strategy could keep the cells safely cool and also improve battery performance.
Using a computer model, Said Al Hallaj and J. Robert Selman of the Illinois Institute of Technology in Chicago simulated heat flows in a sealed case of lithium-ion cells. They modeled their battery pack on a prototype power module used in the Nissan Altra-EV electric car.
Unlike that module, however, the battery case in some of the simulations contained a paraffin wax that filled the space between and around lithium-ion cells. At 35ºC, the wax absorbed some of the battery’s heat and melted. Much as melting ice cubes chill a drink, the liquefying wax cooled the cells.
In the simulation, the wax keeps cells about 5ºC cooler than waxless cells. That holds the maximum temperature below 60ºC, where runaway heating won’t occur, Al Hallaj and Selman report in the September Journal of the Electrochemical Society. It also eliminates the need for cooling systems to use fans or circulate liquids, Al Hallaj says.
What’s more, he notes, the melted wax warms the battery when the car isn’t running—a performance booster in cold climes. The researchers are now testing different heat-absorbing materials so they can build a prototype.
