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A plug in block heater is usually at least 300W and it takes hours to heat up an engine. Putting a 25A load on your alternator for 5 minutes isn't gonna do squat unfortunately.
If you really wanna look into preheating the engine I'd follow what Toyota did/is doing with their coolant thermos on the Prius, or the exhaust heat exchanger. I do think the coolant thermos idea could be implemented fairly inexpensively with a bit of thought from the guys here.
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