Robert wrote:

Merside Fire are quoted on the BBC site as saying:
"We urge people to always use electrical equipment in accordance with
the manufacturer's instructions and guidance, always ensure that no
electrical items are left charging overnight or left unattended for a
long period when being charged, and do not mix parts from different
e-cigarettes.

Bigclive has done a few teardowns of disposable e-fags on UTube, I think
he found they have unprotected Li-Ion cells in them, all it takes is the
manufacturer to use those batteries in a rechargeable fag instead of
ones with a charge protection circuit, I suppose