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Ummmm..... using a car alternator to charge a battery to pre heat an engine isn't a very efficient proposition.
Lets run some numbers.
Alternator is about 65% efficient at best.
A lead acid battery charging and discharging is about 80% efficient.
A good inverter is about 90% efficient.
so 0.65 x 0.8 x 0.9 = 0.468
Less than half the energy you expend is going to be used to preheat the engine.
Your far better off using the fuel to heat the engine by driving.
In a typical engine at operating temp 1/3 the energy goes to the cooling system. In a cold engine this would be higher. About 25% goes to moving your down the road. By just running the engine your almost getting the same amount of energy heating it plus your using energy to move you down the road for a net benefit of greater than 50%.
I know for diesels you can get a block heater that runs directly off the fuel by burning it thus reaping close to 100% of the fuel heating energy. Not sure about gas engines though.
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