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January 7, 2014

Beating the Heat: New Inexpensive Sensor Can Prevent Battery Fires

Recent innovations allow this sensor to monitor the amount of heat generated while a battery is in use, which allows the user to cool the battery as soon as it begins to heat up.
Credit: JHU/APL

Recent battery fires in electric cars and aircraft have helped spread concerns about lithium batteries "faster than a forest fire," says Rengaswamy "Srini" Srinivasan, a battery specialist and electrochemist in the Research and Exploratory Development Sector.

Earlier this year, for example, Boeing grounded its 787 Dreamliners because of spontaneous fires in the lithium-ion batteries that power the aircraft's auxiliary power unit. Last month, the National Highway Traffic Safety Administration launched an investigation into Tesla Motors' lithium-ion battery-powered Model S when three of them caught fire after being damaged.

Srinivasan and his colleagues have refined technology that could improve the safety of these power sources. The lithium-ion battery, born in 1991, has become an important energy storage technology. The rechargeable batteries are, arguably, the backbone of the wireless revolution of handheld devices, Srinivasan says. But they can sometimes generate enough heat to start a fire in a process known as thermal runaway. For example, he explains, short-circuits between the two electrodes in a battery cell can heat the electrodes, potentially triggering chemical reactions that quickly generate more heat until the electrolyte—which contains organic solvents—bursts into flame.

In 2011, APL's Srinivasan and Bliss Carkhuff developed an inexpensive sensor capable of warning of impending catastrophic failure in lithium-ion batteries. The sensor was based on Srinivasan's discovery of an intrinsic relationship between the internal temperature of lithium-ion cells and an easily measured electrical parameter of the cell.

But it wasn't enough to just sense temperature increases inside the battery. Existing solutions for battery safety focus on mitigating symptoms rather than treating the root cause, he says. So over the last two years, he has focused on measuring heat, not just temperature—an innovation that allows his device to monitor the amount of heat generation even while the battery is in use (charging and discharging).

This helps battery users in several ways, he says. They can cool the battery through forced convective heat transfer as soon as excessive heat generation starts, before temperature goes way up. They can also save costs associated with cooling by choosing the duration of cooling.

Battery scientists and engineers have long known five parameters that are intrinsic to every lithium cell that are the root cause of heat generation. Now, Srinivasan's team has designed a practical tool to track the heat generated by each of the five.

"The only heat-tracking technique practiced today is surface-mounted temperature sensors; if the numbers of incidences of lithium battery fire is an indicator, then it is safe to conclude that the surface temperature sensors are unable to ensure safety of the battery," Srinivasan says. "We now have a tool and capability to monitor each [of those parameters] separately. Some of them will never be able to go out of control, and others could. Our newly acquired capability to monitor the five independently helps ensure fast charging without the fear of fire, and forestall catastrophic thermal runaway."