Volt vindicated

The Chevy Volt’s T-shaped battery pack includes more than 200 lithium-ion cells, a coolant-circulation system, and electronics. It also acts as a structural element.
Date:29 September 2012 Tags:, ,

I’m interested in the potential of extended-range vehicles such as the Chevy Volt, but the news stories about battery fires have freaked me out. Can you supply a definitive answer about what’s going on?

A Of the 15 054 Chevy Volts sold as of this printing, exactly zero have caught fire out in the real world. I don’t want to sound like a cheerleader for the Volt programme, but it pushed a lot of boundaries in engineering and testing and in some ways exceeded the standard testing parameters the US National Highway Traffic Safety Administration (NHTSA) uses.

All the hubbub revolves around one particularly strenuous crash test – the side pole test – done in May 2011. In this series of tests, a car is run sideways into an immovable steel post at 20 mph (32 km/h), then it’s rolled 90 degrees on to its side and all the fluids are allowed to leak for 5 minutes. The car is then rolled on to its roof for 5 minutes and then on to its other side. The test looks for side intrusion into the cabin and damage or danger caused by the loss of car fluids.

NHTSA’s Volt passed these tests and was parked in an outside storage area. A strange thing happened: after three weeks, the battery caught fire, consuming the already wrecked vehicle and a few cars nearby.

I spoke to Doug Parks, GM’s global vehicle chief engineer for the Volt, who started off by saying, “I believe the Volt has always been safe, but this was an event we needed to investigate thoroughly.” After some sleuthing, they found the source of the fire was a short circuit caused by something called dendritic growth. The phenomenon occurs when energised silicon chips get wet and their metals are leached through the substrate, growing into what looks like tree roots that eventually lead to a short circuit.

The battery in the Volt is shaped like a capital T, with the top of the T under the rear seats and the remainder running up the centre tunnel between the front seats. During the test, the undercarriage crash structure successfully absorbed crash energy but also punctured the battery case and cooling system. When the car was rotated, coolant worked its way on to the top of the battery, where an important control module lives, and got its silicon chips wet. Because the testing standards at the time did not require the battery to be discharged after the crash (analogous to draining the fuel tank, which is standard practice), dendritic growth occurred and the chip shortcircuited, connecting the poles of the battery and leading to a battery fire.

After repeated testing to replicate the scenario, GM found the probability of this happening in a real-life crash to be nearly zero, but took preventive action anyway. The area of the battery tunnel around the crash beam was reinforced, and a coolant-leak sensor was added as well. All new Volts built since February come with these upgrades, and older cars can be taken in to dealers for what GM calls enhancements, since this isn’t considered an official recall.

Parks also notes, “The major automakers including GM are now working with the Society of Automotive Engineers to standardise discharge methods following an impact event.” I’ve paid close attention to this story, and after reviewing the testing, failure mode and response, I’m comfortable with the performance of the Volt in crash testing. As a final thought, consider this: traditional cars carry around 60 litres of highly flammable (and carcinogenic) petrol in an easily pierced tank under the car, yet you don’t even think about it.

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