I searched for alternative block heater installs and I found everything from using starter fluid to a sterno stove placed under the oil pan. I've thought of another idea and I want some feedback on it, here's the deal:

I live in an urban area with street parking only, so running an extension cord out my front door is not the #1 choice here. I've been having more and more difficulty starting without the block heater (took 3 tries tonight, 40-45*F) so it scares me to think about the days where it doesn't get above freezing at all...

Anyways here was my thinking - has anyone tried to place an additional battery in the trunk hooked up to a power inverter, which is then hooked to a timer and the block heater? I was thinking a second battery could charge with the alternator (hooked up in parallel with current battery), then run the power inverter and block heater without draining the normal battery, which would be used to start the car. I guess I'm asking the electrical gurus - does the block heater draw too much power to run through an inverter (300W), and would that be able to run off a battery for say, 2-3 hours before charging again? If it is feasible (or has been done) I am very interested.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth