The FAQ's at
DC Thermal is quite interesting.
10 FAQ's
https://dcthermal.com/faq-page/
Quote:
Q:What size of heater do I need?
This is a multi-part question: First you will need to determine the nominal voltage of your vehicle, the capacity of your alternator, and you will need to estimate how much you are trying to change the ambient air temperature in your vehicle. The second part will determine the size of the heater, the first part will determine the voltage needed and maximum amperage. All of these factors combined will help in determining the proper size of heater for your particular application.
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I trickle charge my small Odyssey, might as well trickle charge a second Odyssey not hooked up to anything else for short term defrosting.
Might prove to be the cheapest most cost effective option in the long term because of eliminating risk of alternator and or starter diode damage.
If I recall correctly I got this idea from an aircraft post, one with a generator in lieu of alternator. They just used a battery for short flights. Maybe it was a sailplane, I don't remember. Somehow the 8 hour life figure stuck in my head, but that could be for a large heavy battery, not sure.