12V Electric Water Kettle

My reading of the Model 3 manual states the 12 volt port is rated at max of 16 Amps so using the port would not be a good idea (this may be related to the gauge of the wires). It does look like the pot does draw about 21 amps when drawing 250 watts. Also the port is only active when the screen is illuminated.

Usually the main traction battery has a special DC to DC charger for keeping the 12 volt battery charged. It works intermittently by my guess.

I tried something similar to this on a Ford C-Max Energy. I made a direct connection to the 12 V battery (large gauge wires) however and I had to keep the vehicle in accessory mode (radio works etc) to keep the battery being charged from the larger traction battery.

Tesla however is VERY good at spotting unexpected current drains and this approach would likely not work but I have not tested it.

Some device not using more than 10 A would be a better bet IMO.

 

Not going to get hot if it draws 20A, the car will shut it down.

And 20A on a lighter plug is pretty much well beyond the capacity of anything.

 

How about something like this. It is 15A and looks like it fits in the cupholder. If you keep it hot, then you avoid the frequent large draw and it probably runs at minimal power.

https://www.amazon.com/Spardar-Insulated-Stainless-Automatic-Cigarette/dp/B06XVR831N/ref=sr_1_3?keywords=water+warmer+12v&qid=1569685798&s=home-garden&sr=1-3