Onboard System Could Catch Bad Batteries Before They Blow

Illustration: Randi Klett; Images: iStockphoto

A major automaker is now testing an emerging technology for detecting defective plug-in-hybrid and electric car batteries that could lead to car fires, IEEE Spectrum has learned.

Described in its early stages in an Spectrum story last year, the system now has the attention of multiple car manufacturers, according to its developer. And it is currently being developed with an undisclosed automotive industry partner for possible rollout within the next five years.

Electric car fires, while statistically less prevalent than conventional gasoline-fueled car fires, can sometimes burn with staggering fury. This is at least in part due to the lithium battery’s chemically volatile electrolyte solution. As a result, news reports of devastating Fisker Karma or Tesla Model S fires have been a black mark on an industry that must overcome multiple consumer fears before it can establish itself as a viable, mainstream alternative to gasoline–powered cars.

More than a decade after conducting post-mortem studies of lithium-ion battery fires in cellphones, laptops, and other consumer electronics, Lexington, Mass.-based TIAX has developed their own suite of battery monitoring technologies that would run onboard an electric car’s computers. TIAX says their test can pick out rare but dangerous batteries prone to so-called thermal runaway fires caused by internal short circuits dozens of use-hours before they pose a threat to the car and its occupants.

Those one-in-5-million batteries prone to runaway fires are difficult but not impossible to find, says TIAX vice president Brian Barnett. Perhaps the most challenging part of the problem is that these faulty batteries cannot realistically be found in the factory. They must be detected in the field, as their internal short-circuit slowly develops during everyday driving.

TIAX’s post-mortems from actual safety incidents have often, Barnett says, found evidence of tiny metallic dendrites that appear to have expanded in size via progressive electroplating over the course of many charge-discharge cycles. The dendrites that lead to catastrophic failures are ones that grow through the perforations in a battery’s separator, ultimately short-circuiting the cathode and anode.

In the factory, such an infernally fated battery may only contain a metallic particle of an undetectable tens of microns size. As the battery is used the particle seeds the dendrite that will ultimately short the battery. But careful monitoring of the battery’s voltage, impedance, and current can begin to tease out the presence of the growing short.

Barnett says the TIAX group has verified the danger of such particles through both computer simulations and lab testing of intentionally faulty batteries that have been deliberately implanted with microscopic metal impurities.

A typical “18650” lithium-ion battery cell, for example, has an internal impedance of about 50 milliohms, Barnett says. And a typical growing short can be detected using TIAX technology when it reaches an impedance of hundreds of ohms. Current flow through a 100 ohm embryonic short circuit in such a cell would be than three-orders-of-magnitude smaller than the current flowing through the cell during everyday battery operation, which is why battery fire warning signs are so difficult to detect with conventional approaches. But, he says, clever signal processing and other technology can still find the needle in the haystack.

“In principle there’s no reason why you should ever see a vehicle progress to the point of a thermal runaway,” he says. “In our laboratories we’ve been able to detect these things at least a few orders of magnitude lower power than the power that would lead to thermal runaway.”

Barnett says TIAX’s short circuit detection technology has picked out batteries prone to thermal runaway dozens of charge-discharge cycles (representing more than 100 usage hours) before they spark a fire.

How a car would then use the information TIAX’s real time battery tests generate, he says, would be up to the car maker. Depending on the severity of the growing short, it might inform the driver to bring the car in for maintenance soon. Or, if the warnings go unheeded for too long, it might ultimately tell the driver to pull over, turn off the car and call for roadside assistance.

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