When we test-drove our Chevy Volt last year, we didn't experience any side-impact accidents. We also didn't experience any instances of the battery pack catching on fire. The National Highway Traffic Safety Administration has found that sometimes both of these things can occur back-to-back, so GM has had to come up with a fix.
To be clear, there haven't any reports of battery fires in any Volts that weren't deliberately crashed by the NHTSA, and it's not like you could be in a side impact accident and not notice, making the likelihood of a spontaneous fire with no warning practically nil. But better safe than charcoal, as they say, and GM (to its credit) decided to be proactive about the whole thing.
The issue was that a side impact had a risk of puncturing the battery compartment, leading to a coolant leak and eventual short circuit. To keep this from happening, GM says that they're going to do a couple different things: first, they're adding some structural support underneath the battery pack to help protect it in a collision, and they're also installing a sensor to monitor the battery coolant levels to at least let you know if you do have a leak.
Crash testing has shown this fix to be entirely effective, and it only adds about three pounds to the overall weight of the car. The 8,000 Volts on the road can come in for a free upgrade starting in February, and GM will also be fixing all of the cars currently in showrooms.