What the 12-volt battery does in a Tesla
The big battery pack drives the car, but a separate 12-volt battery runs the computers, the screen, the door handles and locks, the lights, and the relays that connect the pack to the rest of the car. In normal driving the pack keeps the 12-volt battery topped up. When the 12-volt battery is weak, the car can act as if it has no power at all, however full the pack is.
That is why owners sometimes meet a car that shows plenty of range in the app and still won't open its doors. The range number belongs to the pack. Opening the doors and waking the car belongs to the small battery.
What the warning looks like
The car may put a message on the screen about the 12-volt or low-voltage battery, often well before it fails. A warning that appears while the car still works is your chance to deal with it on your own schedule, before the car is parked somewhere and won't wake up. When it gets worse, you may see:
- The phone key or key card no longer unlocks the car.
- The screen stays black, or keeps restarting.
- The car won't shift out of Park.
- Several error messages at once, or none at all because nothing wakes up.
Why it wears out
It is a battery like any other. Age and heat wear it down, and a Texas summer is hard on it. The warning can come in the days beforehand, but it can also arrive with very little notice.
Which kind of 12-volt battery you have
Tesla has used different kinds of 12-volt battery: lead-acid in some cars and lithium-ion in others. According to the Model 3 owner's manual, the jump procedure differs depending on which one the car has, and the screen shows it under Controls, Software, Additional Vehicle Information. Read the manual for your model. The jump points and the way to open the car when power is out vary from one Tesla to another.
One more thing from the manual: don't use a Tesla to jump start another vehicle.
Also don't force a door or the front trunk when the car has lost power. Cars like this usually have a manual release, and forcing a panel can cost far more than the battery does.
What we do and don't do
We help with the 12-volt system only. We do nothing with the high-voltage battery. A jump start for the 12-volt system is $49–$75, plus the $19 dispatch fee, added once to the job. If the 12-volt battery itself is worn out, ask about battery replacement, which is $239–$255 for eligible vehicles and batteries, installation included. Dispatch confirms whether your car's battery is eligible, and your total, before a driver is sent.
When you request help, say that it is a Tesla, give the model and year, and describe what the screen says. Put those details in the notes field so the driver comes with the right equipment.
Common questions
No. The main pack moves the car. The 12-volt battery wakes the car up and powers its electronics. They fail for different reasons.
Often there is a message in the days before, but it can also fail with little notice. Don't ignore a low-voltage message.
No. The main pack can be full and the car can still be dead, which is what makes it confusing.