I store my car "up north" in an unheated garage in the winter, and was always a bit leery of having anything plugged in in case of some kind of fire.
Earlier this year I met a car collector who has a large storage facility, basically unattended (but visited frequently)and in it, he has well over 100 pristine, 99-point show cars, nearly all of them Detroit iron, and also nearly all of them historically significant in some way, such as the car "Christine" from the Stephen King movie, the most expensive production US car from 1955, many "one-off" or otherwise low-production cars (i.e. Packard Caribbean) and thus all of them worth a bit of money.
Every one of these cars lives with the hood (bonnet) opened and attached to a Battery Tender.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America