What 12v battary do in tesla ?

[Tam]

that mean if i left car with out charge will lost 12v charge for long time . ?

Yes. If your main battery does not get charged because it is not plugged in, the main battery will run low. When it runs low enough, it will stop charging your 12V. And your 12V battery will be depleted, and it'll be most likely be dead and replaced.

 

[Tam]

that is bad ? too i using this device to bmw battary ? to save life it in winter or only bad on tesla ?

thank you

Yes. It is bad.

Tesla wants to manage the charging through its system. When you inject an unknown 12V source into a Tesla, that source is not regulated by Tesla system anymore.

So, it is a bad idea that I would not recommend it.

The best way to save your 12V battery is to keep your main battery from being depleted.

To keep your main battery depleted then you should plug your car in.