What charges the 12v battery and when?

[fwgmills]

We know that most of the car's regular systems (screens, radio, amps, lights, etc.) run off of the 12v electrical system and that Tesla has had issues with reliability on the 12v battery due to it being undersized and/or defective, but has Tesla ever stated under what conditions the 12v battery is "maintained" or charged? For an ICE the battery is only charged when the engine is running because that is the sole source of energy for recharging it. In a Tesla there is a DC to DC converter that can charge the 12v system from the high voltage battery but when does this happen? I would think it would/could charge the 12v system under any one or a combination of the following:

1) When the car is "on"
2) When the car is plugged in
3) When the "accessory" is active (screen/climate/radio is on)
4) Anytime the vehicle system sees fit to charge it

It seems important to know under what conditions the 12v system is maintained because the high voltage battery can charge the 12v battery, but the 12v battery is required to close the contactors that allow the high voltage pack to do it's thing. Seems like a sort of catch-22 from an engineering standpoint. I would assume it to be #4 since the vehicle can be left for a long time plugged in but off and still be functional after months.

 

[Tam]

...when does this happen?

Lead Acid battery does not like low voltage so there is no advantage to deplete it before charging.

I would choose 4) Anytime the vehicle system sees fit to charge it.

That is part of the reasons of why you got vampire drain even when you are not using your car.

I heard ICE modern cars use 12 V battery more intensively so they don't last as long as in the good old days either (I don't have references.)

 

[David99]

Any time the 12 Volt battery needs a top off, the car will do so. When the main battery is critically low, it will disconnect itself from any power draw from the car and as a result the 12 Volt battery will also not be charged.

 

[Lbkmxp100d]

Fwgmills, I'm glad you asked the question as I have often wondered this as well.

 

[Lbkmxp100d]

Any time the 12 Volt battery needs a top off, the car will do so. When the main battery is critically low, it will disconnect itself from any power draw from the car and as a result the 12 Volt battery will also not be charged.

How does it convert DC to AC and is there a certain percentage at when the vehicle charges the 12V battery? Ex: when 12V battery drops down to 10V the vehicle charges the battery.

 

[ecarfan]

How does it convert DC to AC

Both the 12V battery and the main battery are DC. All batteries are DC.

 

[Lysol]

What you would have is a DC (high voltage) to DC (12v) charger.

A good way to test when it's charging would be to measure the current going into the battery when the car is sitting in various configuration over a period of time.

 

[strider]

Some interesting info here:
Syonyk's Project Blog: Tesla Model S 12V Battery Analysis

And a long thread but lots of good info here:
Near annual replacement of 12V battery is typical according to Tesla Service Tech

 

[David99]

There is a powerful DC to DC converter in the car, that provides a lot of power to the entire 12 Volt system, not just the battery. The 12 Volt battery is just used as a small buffer so the DC to DC converter doesn't need to run all the time. The 12 Volt runs everything except the traction motors and HVAC.

How does it convert DC to AC and is there a certain percentage at when the vehicle charges the 12V battery? Ex: when 12V battery drops down to 10V the vehicle charges the battery.