Tesla Model S Battery Heater
Overview
Except for a Philips & Temro Zerostart mode used in early Model S production, the hydronic (fluid) heater for the Model S battery pack remains unchanged from 205 and up. Originally this part was branded as LG (Tesla p/n 1038901-00-E through H), but from 2018 up (Tesla p/n 1038901-00-I through K) the unit appears the same but is now Tesla-branded.
Control
The battery heater itself does not have any onboard control circuitry, however, it is controlled by the Model S front HVJB using PWM on the HV cables. For this reason, if you want to easily control the battery heater, a front HVJB is required. As mentioned in the Tesla Model S Front HVJB article, the control pins are on the Molex MX 150 plug found on this unit. The LV connector on the bottom left side is a 12-pin Molex MX150 series (Molex p/n: 33472-1201). Only 6 of the 12 pins are populated, as follows:
| pin | function |
|---|---|
| 4 | 12V |
| 5 | PWM in |
| 6 | STATUS out |
| 10 | GND |
| 11 | HVIL in (not needed) |
| 12 | HVIL out (not needed) |
Control is done via a 12V PWM signal on pin 5. Experimentally, some measure of control has been achieved at a frequency of 50Hz and two different ranges of duty cycle. From 20-30% duty cycle, the current increases fairly linearly though the minimum (20%) still draws a bit of power. However, from 30-40% the current decreases to nearly the same level as 20% duty cycle, but you are able to continue further, all the way to 45%, where the current decreases to the point where the heater draws very little power. Ultimately, not sure how useful this finding will be, but it's here for reference.