The reason for a separate 12V battery is to power critical systems in the event of a high-voltage battery disconnect. This keeps key systems operational such as emergency blinkers, airbags, seat belt pretensioners, brake booster, and a host of other systems. The high-voltage battery might be disconnected due to several rare events – such as a high-impact crash, internal main battery pack problems such as some types of cell failures, a contactor failure, and a few other unlikely situations.
Many parts of the Tesla vehicles today are powered by the 12V system, including:
- All lights, such as headlights, turn lights, fog lights, taillights, backup lights, license plate, and interior lights
- LCD Display –Instrument Cluster (S/X) and the main display
- The MCU and many modules with processors
- Non-traction motors, such as windows, liftgate (S/X), wipers, washer fluid, seat motors, side mirror adjusts, sunroof (if equipped) charge port door, steering assist, fans, coolant valves, and coolant pumps
- An audio system including tuners, antenna amplifiers, and audio amplifiers
- Safety systems, such as airbags, brake booster, and seat belt pretensioners
- Autopilot systems including cameras, sensors, and radar (if equipped)
- Heated items, if equipped such as seats, steering wheel, washer nozzle, side mirrors, rear defrost, and camera heaters.
- Other items like the alarm, mirror dimmers, USB ports, horn, Home Link, air suspension (if equipped), and various latches (doors, trunk, frunk, glovebox)
- High-voltage battery pack contactors
Only a few items are directly powered by the high-voltage battery pack. This includes the traction motor(s), the A/C heat pump, and the DC to DC converter for 12V power and charging the 12V battery.