Do you mean their batteries? As far as I know, some e-cigarettes do not have removable batteries but are recharged directly. Here is an example of what can go wrong from anoither thread on this same topic is one of the many reasons to ensure no one attempts to use an E-cigarette onboard:
Man suffers fractured neck, third degree burns and has hole blown through the roof of his mouth after e-cigarette explodes - Americas - World - The Independent
End of the article, relates to the UK:
This summer a series of “highly disturbing” fires caused by exploding e-cigarette chargers led to a nationwide safety alert in the UK, with more than 100 fires thought to have been caused by the devices in the last two years.