What 12v battary do in tesla ?

With one exception all EVs have 12v batteries. This allows the car to disconnect the main 350-400 volt battery so it can be serviced.
There is a 12v solenoid that connects the traction battery so you can drive.

 

The 12 volt system powers all the cabin electronics, lights, power windows, etc. just like in a ICE car. Then the main battery charges the 12v battery.

 

In theory, you could do away with the 12v battery by just using the DC/DC converter. The 12v battery allows the systems to boot when the main battery (aka Traction Battery) is disconnected. IIRC, Hyundai has an EV with no 12v battery.

 

One exception? Now I’m curious...

 

IIRC, one or more of the Hyundai/Kia cars uses a segment of it's traction battery for its 12v service.
Not sure that's a good design or everybody would be doing it to save money.

 

The original Roadster did not have a 12V battery until 2.0. I think some versions of Ioniq PHEV and EV also didn’t have them.

 

Hi Oliver,

Your traction battery charges the 12 volt battery when the 12 volt battery needs charging.
It charges the 12 volt whether the car is parked or in motion...

The only exception is when the traction battery is severely low on charge...

Shawn

 

The main battery sleeps when not use. 12v stay up all the time and it wakes the main battery up when needed.

 

Hi Oliver,

If you park and stay inside the car the 12 volt will still charge from the big battery when it needs to charge.

Shawn

 

As long as the big battery stays charged, the car will charge the 12V battery from it when needed, assuming everything is working as designed.

You shouldn’t need a 12V battery tender when putting the car in storage, but it would be wise to plug the standard cord in and set the charge level to ~50%.

Probably not entirely necessary for one month off, but if it ran longer it could be important.

 

Tesla Owner's Manual wants you to plug in when you are not driving your car.

The wall outlet power source recharges the main battery as needed.

That main battery, in turn, recharges your 12V battery as needed.

It's all automatic and fine as long as you keep your car plug in.

Your 12V is recharged from the main battery as needed and it doesn't matter whether you turn on the car or turn off the car, open the door or close the door, plug or unplug, driving or parking. It's around the clock as needed.

 

It would be fine as long as your car is plug in because the process is automatic.

It's only a problem when you don't plug in for a while and the main battery will be depleted. When the main battery is low, it will no longer charge your 12V and your 12V will be dead.

Your plan of saving your 12V battery by injecting a 12V source outside of the car's automatic system which is called BMS--Battery Management System is not approved.

Since it is outside of the BMS, it might harm your 12V battery instead of saving it.

 

Correct. The first 500 Roadsters did not have a 12V battery.

As others have stated, the 400V main pack in your Tesla keeps the separate 12V battery charged. And it is important to keep your Tesla plugged in, even to a 12V power outlet if that is all you have available.

If your main pack is fully charged and you leave your Tesla unplugged for a month it will probably be okay, unless it is outside in very low temperatures and/or you leave the internet connectivity on, or Sentry Mode on (which I suspect your car does not have) or you are constantly checking on the car through the Tesla app.

Plug it in.