Tesla Flat 12v Battery

Last updated 12-Sep-2022

Each Tesla has 2 batteries, the large expensive battery pack with an 8 year warranty and a traditional 12v battery that runs the cars ancillary systems like any other car. While many people monitor the health of the main battery pack, the 12v battery can fail or run flat, especially if the car is left for a long period of time and the main battery has entered its low energy mode.

If the 12v battery is flat then it can feel pretty terminal for the car, the doors won't unlock, the computers won't fire up and more worrying the main car battery will not charge. Tesla charge the 12v battery from the main battery pack, but Tesla also take the view they must protect the main battery pack as much as possible and if it is running low may put the battery into a deep sleep/low energy mode. This minimises any drain on the battery preserving its life, but this equally prevents it topping up the 12v battery and effectively paralysing the car. The 12v battery can run flat within 24 hours once the main battery pack has stopped supporting it.

All is not lost however, and this guide talks you through how to get going again so long as the car is close to a charge port and you have access to a few tools and a the ability to provide a 12v jump start. The basic sequence is:

  • Gain access to the 12v battery or a 12v charge point
  • Provide a 12v jump or charger to the 12v battery to enable the 12v systems to power up. We would recommend charging the 12v battery for at least 5 mins before trying to do anything with the car including charging the car to avoid any glitches.
  • Connect the car to a car charger or the UMC. We'd advise against using a rapid charger or supercharger initially, as much as anything this is not a fast process and the charge rates will be initially slow.
  • Charge the car to a modest state of charge and then charge freely on whichever charger you wish to use.

This is a matter of last resort and the main battery pack running to empty is not advisable.

Gaining access to the battery.

To gain access to the 12v battery you need entry to the car. Each car is slightly different and the details of the steps to gain access are include in the user and first responder manuals however we cover the basic steps here.

The battery is accessible in the frunk of the car and so to gain access tot he battery, you need to open the frunk. If course with a flat battery you can't just open the frunk as normal and you have to follow the emergency frunk opening procedure. Each model has a slightly different process for access and these are detailed in the appropriate guides linked to above, however as a quick start guide:

Charging the 12v battery

Once access to the frunk has been gained, you will need to remove some of the plastic coverings between the storage area and the windscreen to reveal the 12v battery access points. If there is nothing obvious bit the 12v battery is visible then you can connect directly to this.

You should not use another car as the 12v source. By far the best way to charge the 12V battery is to use a battery charger or emergency 12v battery pack which some breakdown companies carry. The charger is the better option as you want to charge the 12v battery for a short period before trying to do any more with the car.

Lithium Ion Low Voltage Battery

From 2022, many Teslas come with a Lithium Ion Low Voltage battery pack. This has some benefits but can also be more difficult to deal with if it runs flat. This battery typically has a voltage above 12V and as a result a normal car battery charger may not work. If you get a flat battery with these cars then roadside assistance may be the only option.

Coupled with the above, cars with the LFP main battery pack, which are typically Standard Range models, need to regularily top up the mauin battery to 100% to ensure the car maintains the correct charge level in the low voltage battery.

Charging the main battery

Once you have breathed some life back into the 12V battery the car should be able to power up its computers. Open the charge port and connect a charger. When the main battery is so low, the charge will take some time to get going with any speed. We suggest not using a supercharger as the charge time may be very long and may inconvenience others.Let the cars battery management system charge the car as needed. You should be able to remove the 12v battery charger once the main battery charging has commenced.

The 12v battery may still be at a fairly low level of charge however once the main battery pack has reached a sufficient level of charge it will charge the 12v battery. Unlike internal combustion engined cars, you should not need to go for a lengthy driuve to charge the 12v battery.

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