Last week and a bit have been tinkering with heating options.
There will be two coolant loops one for the batteries, charger/dc/dc and a second for the tesla motor/inverter.
Each loop will have a valve to control coolant flow. In the batteries the valve will simply cut the loop to bypass the radiator if battery temps are low. The motor loop valve will direct the flow either to the radiator or the stock heat core. Knowing an electric won't produce the same levels of heat as an internal combustion I decided some form of auxiliary heat would be a nice add on.
I specifically am avoiding 400V PTCs as I don't want to sacrifice range. I'd rather supplement what waste heat the car does produce.
I have gone through a couple Aux heaters from Euro cars to mess around with.
I started buying a BMW 87 Aux heater unit.
You can see each of the main lugs. The board is effectively just a bunch of smart switches to turn on the number of elements requested over lin bus. You can solder a ground to pin one of each switch for manual control. See Damien Maguires video for more on this process. My main issue with the BMW unit is it is just too big to fit anywhere.
So I moved on and grabbed a Phaeton's Aux heater.
You can see this unit is much smaller.
The center top lug is ground with the 2 outer ones being 12v+
Much like the BMW it has a control board with some smarts on it to control on/off and level
A bridge between the center lug and the ground pin would be needed to turn this unit into a "dumb" one as I couldn't read the IC's part number to work out how to turn it on and off.
While this would be easy enough to do and for its size it puts out a surprising amount of heat. It was too small to really fit well anywhere. It would mean restricting airflow somewhere to force through the heater and CRX fans aren't too powerful to begin with.
This brought me to my third PTC out of a B5 Passat
It's about 2/3 the size of the bmw one and should fit perfectly (it is still in shipping) The other thing that I like is it is a "dumb" heat no linbus, canbus, kline. In the original setup it has two relays a low and high. each feeding one of the 2 12v rails.
The nice thing with all these aux heaters is they are very cheap. $20 or less on ebay and you can find them.
On the point of not wasting battery, these should be a little easier on my system as they are intended to supplement the waste heat I do have vs provide the entirety of my heat. I am rigging up a small switch so that it comes on only when heat is turned to "11" and I will have an override in my ecu based on battery state.