[Rokeby]

Originally Posted by bennelson

INSULATE YOUR BATTERIES:
Batteries will be much happier if they are wrapped in a cozy blanket. If
batteries are exposed to the outside world (such as under the hood) heat can
also be lost to wind. Any insulation should be water-resistant and non-
conductive - using foil-faced foam is a bad idea - but using pink builder's
foam works great.

WARM YOUR BATTERIES:
Find some way to get a little heat into your batteries. The best way is with
something in direct contact with the batteries, either under them, or on the
sides. It will need to have some sort of automatic temperature control, to
prevent overheating.

Ben,

Very interesting reading, thanks.

I see the idea of battery temp maintenance in the same light as grill
blocking and block heaters; a driver selectable FE/MPG aid to be used as
particular conditions warrant.

Don't have an EV, but my Prius' HV battery seems to have lowered initial
power output after an overnight cold soak. I wondered about the possible
benefits of pre-warming the battery in this thread at PRIUSchat:

HV Battery Pre-Heat... Anybody tried it? - PriusChat Forums

At any rate, I still feel that pre-warming the HV battery during/after an
overnight cold soak has merit. After an awful lot if 'net searching, I've
come to the conclusion that some sort of heating pad or element under the
battery is the best way to go. (On the Prius, this wouldn't directly heat the
battery as it is inside a metal battery box that is an integral part of the HV
isolation scheme and is part of the battery cooling ducting.) Something like
this:

ClearMirror : The original fog free bathroom mirror defogger

[EDIT: Closer reading of this product's installation guide indicates it is not
effective under 50 degF or when against an exterior wall, which I suspect
would include the floor of the car:

NewHome Bath and Mirror, Inc. and its affiliates will not warranty the labor
or material costs for installation, replacement or use of a ClearMirror®
product, mirror or other affected items when installed on the inside of
exterior walls or when room temperatures on either side of the
application wall is or becomes lower than 50 degrees Fahrenheit.]

Waterbed mattress heaters, electric blankets, and water-pipe freeze
prevention tape all have automatic temperature controls, and can easily be
repurposed to warm batteries. Make sure to not set batteries directly on
heating elements. Many heaters can be easily wrecked that way...

Just how is it that the heater element gets wrecked?

Would you care to hazard a recommendation for a specific product?