A large battery pack, of any type represents a great deal of stored energy, which can potentially cause a fire.
Todays typical car drives around with a 12 volt lead-acid battery, which under the right conditions can explode. (One way to do this is to overcharge them.)

There have been a small number of notorious battery fires linked with Electric Vehicles, (for example the Volt fire where a car was crashed, as part of a safety test, the battery pack left connected, and three weeks later a fire started.) Does anyone seriously think the odds of an EV catching fire are higher than the odds of a gasoline-powered car doing the same?
http://abcnews.go.com/Business/Autos/story?id=1207557&page=1
[font face=Serif][font size=5]New Warning to be Issued About Deadly Car Fires[/font]
ORLANDO, Fla., Oct. 12, 2005
[font size=3] AAA and the National Fire Protection Association will issue a warning tomorrow about car fires, ABC News has learned. Last year, 266,000 car fires resulted in 520 deaths, the organizations say.
"It was a horrible explosion," said car fire victim Bob Aymar, who, in less than a minute, suffered third degree burns on his face, hand and arm. He was sprayed by a gasoline fireball during a violent traffic accident on a Southern California freeway.
"The Bronco behind me was hit so hard that it ruptured the gas tank," said Aymar, who, after seven surgeries, was finally able to play the piano again.
He is just one of more than 1,300 car fire victims every year. According to the NFPA, cars catch fire on American highways once every two minutes.
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