I thought I’d better test the characteristics of the PTC elements in a scientific manner as opposed to just applying 750V DC from the PFC and seeing what happens.
When measured with a multimeter at room temperature, each element read about 600 ohms. This would imply a rather low power output if that was the minimum resistance.
For my next experiment I connected one of the elements to my old Xantrex 600V bench power supply. The 600 ohm cold resistance would imply a draw of no more than 1A at 600V if the element was purely PTC. But to my surprise, the current draw actually began to increase as the element got hotter, eventually hitting the PSU’s 1.7A current limit at only 200V.
This means that the elements must actually start off as NTC, and transition to PTC at a higher temperature. That kind of makes sense, as a car heater matrix has to be able to start up from very low temperatures. Purely PTC elements would presumably exhibit an immense current surge when the heater is turned on after leaving the car parked overnight in Canada. 🙂
I was wondering why Tesla bothered to implement individual control of the 6 elements, and I guess the turn-on surge is the answer to this too: by turning them on sequentially the surge can be made 6x smaller. It’s not like a Tesla traction battery (or even an IGBT) would care if the turn-on surge was 10A or 60, but maybe it allows them to use a lower rated fuse to connect the heater to the battery. Fuses rated for high voltage DC are expensive so the savings made here might outweigh the cost of 5 IGBTs and drivers.
Anyway, I couldn’t test above 200V due to the limited output current. The next larger PSU I have is the PFC, and it has a minimum output voltage of 400V (600 on 3 phase) so I will just have to “send it” as the kids say nowadays.
Is there any data on the meantime before failure for a Tesla PTC heater that you are aware of? Any idea under what circumstances the PTC heater activates/is in use? For example, in summer weather with the AC on, I’m assuming the unit is not activated and only comes on in cold weather to warm the batteries but I’m trying to find out at what exterior ambient temperature or maybe it’s more relevant to ask at what battery temperature does the PTC heater activate even when cabin heating is not set/off.?
Hi Steven, I believe this heater is only used to warm the cabin, the Model 3 uses waste heat from the motor for battery heating and will run the motor inefficiently to make extra heat if needed. I have no data on the MTBF.
Hi. Are you tested a damaged one or one removed from your car, is it working?
I bought this heater from a car breaker and removed the CAN control board to use the heating elements directly.
It would interesting to find out why so many of these units fail in few year. Strange if Tesla did not research this. If it was just the bin heating dying fine..but the problem the car just stops and you have to tow to the service center
If it stops the car, it has to be a problem with the insulation causing the heater to leak high voltage to ground. All EVs have ground fault protection on the high voltage side that works in a similar way to a household RCD/GFCI outlet, and it will shut the car down because it’s quite a serious safety issue, a ground fault could cause fire or electric shock.