Editor’s Note: Another guest post from Dr. Destler, president Rochester Institute of Technology.
By Bill Destler
The recent tests conducted by NHTSA on Chevy Volt battery packs have raised concerns about the relative safety of the Volt, and other electric vehicles, compared to conventional gasoline powered vehicles. In the tests, Volt battery packs subjected to the kind of damage that might occur is a severe accident have caught fire or smoldered. In two of the three cases in which this behavior has occurred, the fires did not start until days or weeks after the crash tests. In the third case, sparks and smoldering were observed soon after the crash test but after the battery was flipped 180 degrees to simulate a car rollover. In a fourth test, no fire occurred. In none of the tests did NHTSA drain the battery of its stored energy after the crash tests. In fairness to NHTSA, the GM de-powering protocols were not issued until after their first Volt crash test, although they were in place well before the latest tests.
To date, however, NHTSA confirms that no fire attributable to the Volt’s battery has been reported out in the real world, either as a result of collisions or other circumstances. Given the publicity that any Volt mishap seems to generate, I feel pretty sure that if a Volt caught fire in a collision, we would have heard about it. Two fires have occurred in garages where Volts were parked, but in both cases the Volt was determined to not be the cause of the fires.
To put this information in context, we need to look at several factors that should, in a rational world, inform our decision as to whether the Volt represents a greater-than-average risk of fire in a collision than do conventional gas-powered vehicles. I realize that these considerations will not matter in the non-rational world of Fox News, but nothing we say here would deter their anti-Volt propaganda campaign, so let’s not waste our time with them.
First, let’s do a non-scientific, oversimplified statistical analysis to see if there is to date any indication that Volts are at higher risk for fires in collisions than other vehicles. There are only about 6,000 Volts out in the real world, which is not a large number compared to the roughly 250,000,000 passenger cars in the U.S. The most recent NHTSA statistics I could find indicate about 6,100,000 accidents occur annually involving passenger cars, and that about 3 fires per 1,000 crashes occur in which gasoline-powered automobiles are involved. If we assume that roughly 2,500 Volt-years of driving experience have been acquired to date (I think this is a pretty good estimate), we should expect statistically that about 61 Volts have been involved in collisions to date. Although a few Volt collisions have been reported on this site, this number seems high to me. I suspect that the Volt will have a lower rate of collisions given the excellent handling, braking, and road traction characteristics of the car. If this the case, then the risks associated with driving a Volt are even lower than estimated here.
Nevertheless, if we assume that the likelihood of a fire in a Volt collision is the same as it might be for other cars, 0.2 Volts should have caught fire to date, a number significantly less than 1 but greater than the 0 fires reported to date. On the other hand, if Volts were, say, 10 times more likely to catch fire in a collision than conventional cars, than one might argue that 2 Volt crash fires should have been observed. The point of this simple analysis is that there is no evidence in the real world to date to indicate that the Volt is substantially more likely to catch fire in a collision than any other vehicle.
Interestingly, unlike gasoline powered vehicles, the Volt has two energy storage systems that might contribute to the car catching fire in a collision, the battery pack and the gasoline tank, so it’s a good sign that collision fires do not seem to be happening at a rate greater than that for other cars.
Now let’s look at the Volt battery from a stored-energy perspective. At 16 kwh, a fully charged Volt battery contains the stored energy of only a half gallon of gasoline. In addition, the stored energy density in the Volt battery is much less than in gasoline, and the energy cannot be extracted at anything like the rate at which gasoline burns. The fire danger in a Volt battery pack, therefore, is not so much in the stored energy, which is relatively modest, but rather the possibility that sparking might ignite a fire that could spread to other flammable materials, including the gas tank. For that reason, the pack should be discharged as rapidly as possible after an accident per GM protocols. The unwillingness of NHTSA to own up to their failure to discharge the batteries after the latest crash tests is, in my opinion, both deceptive and unfair. Draining a gas tank in a car after a collision is standard practice everywhere, and I am sure that NHTSA does this routinely for their other test cars. Had they followed these protocols in the latest tests, the results would almost certainly have been dramatically different.
Nevertheless, GM might consider putting an auto-discharge feature in place in the Volt that would be activated in any collision that inflates the air bags, for example. This should be relatively simple to achieve on a cell by cell basis using the cell monitoring circuitry already present. I don’t think NHTSA should require GM to do this, as it would be the equivalent of asking other car makers to provide an automatic gas tank draining feature (to be drained where?) in the event of a collision, but it would add a measure of added safety to an already safe vehicle.
I think NHTSA is doing the right thing in looking into this issue, since they need to gain experience with this new automotive technology, but I do not believe Volt owners have any more reason to be concerned about the safety of their vehicles than do the drivers of other vehicles.
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This entry was posted on Thursday, December 1st, 2011 at 5:55 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.