You don't have to go as far as Alaska. "Winter plug-ins" are common in
the upper midwest and Canada. They don't heat the battery though. Instead,
a heater replaces the dipstick and keeps the crankcase oil from turning to
frozen sludge overnight.
Hi, you're right, they're called block heaters. They heat the engine
block, not the battery at all, though some people also buy battery
warmers, which looks like some type of heating pad that wraps around
the battery. Do they work? beats me, only ones I've seen usually had
the cords chewed off by the engine fan.
The block heaters, as they are known, do not go into the dipstick tube
though, they'd never fit even if you did want to sacrifice your tube.
They replace one of the blocks "frost plugs", (known differently
elsewhere)..and the heating element sits inside the blocks water
jacket, which warms the oil and allows the engine to turn over.
30 minutes on the coldest days are all it takes. You can even put one
in each frost plug hole and parallel them. If that's not enough to
start it your battery is weak and needs checking /replacing, those
heating pads are junk.
Cheers