Use a peltier unit and if needed, use a darlington driver! Even more effeciency and two-in-one bonus, heater and colder! Also its not just heating so that the car temperature is unaffected (generally.)
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Why on earth would you use a Peltier cell for a purely heating application ?
because a peltier will move extra heat from the surroundings into the hot zone. Thus efficiency becomes over 100%
Only at ludicrous expense for the very very slight benefit, since Joule heating dominates.
Please explain why you want an inductive heater, and not say a resistive one like most. AFAIK, induction heating only works with metallic pots and pans/ as the eddy currents in it heat up the metal. No metal->no eddy currents ->no heat. If you only want to warm a cup of coffee, go with a resistive type, or as mentioned above, a peltier type. The peltier type will be slightly more efficient as it will cool your car too when in use.
I see several ways to warm a cup of coffee. Get a coffee cup warmer, the one that plugs into the wall. Then plug it into an inverter, but make sure the inverter can put out more power than the warmer uses. Get a stop/tail light about the size of the coffee cup, the ones that are sold to replace the ones that go on the back of boat trailers. Take the lens off and replace it with a piece of flat aluminum. When you light up both the stop and tail light, it should get nice and warm, plus you get free light if there are any holes in it. Get two 15 ohm, 10 watt or more resistors, and mount them on a piece of aluminum the size of the cup. Use high temp silicone seal to affix them to the aluminum. Run a power cord from them to a cigarette plug. They should warm up the aluminum nicely.
If you want to boil water off an auto' battery you can get things that plug into your cigar/cigarette lighter. You don't want to be trying to make one really. L