Model 3

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How does the 12v battery stay charged?

edited November -1 in Model 3
Two questions:

1). How does the 12v battery stay charged?

2). I'm trying to install a 12v accessory under the frunk compartment and needed a reliable wire/connector to attach to. Any ideas?


  • edited October 2018
    I can address #1. The 12v is charged by the HVB via a DC to DC charger.
  • edited October 2018
    How adventurous are you? Under the frunk cover there are multiple spots where there are wire bundles, there may be 12v wires in there, possibly switched and unswitched depending on your need.
  • edited October 2018
    This wheel has been invented, I believe. Head over to Youtube and search on "Ingineerix". This guy has produced a number of videos on the inner workings of the Model 3, and there are one or two on where to tap 12 volt power.

    It is important that you do this right or can you can get unfortunate results. You didn't state whether your accessory needs to run when the car if off. To answer your question though, the 12v battery is charged from the high voltage traction battery via the electronics that sit over the battery pack. The 12v battery *may* also be charged via regen braking, but I'm not sure. I think we have learned though that when the car is off the 12v battery will not be charged. So you can drain it with 24 hour accessories.
  • edited October 2018
    Ok, a couple of minor misconceptions.

    1) 12V battery is never charged directly via Regen - Regen power goes directly into the main battery pack.
    2) Very hard to drain the 12v battery as it is automatically recharged by the main battery pack when it gets low.

    Now there are some issues about connecting always-on accessories on the Model 3, as the car monitors the 12 power very carefully, and an unexpected power draw can flag an error. The reason for this is to protect the 12v battery, which like all cars (EV and ICE) has a limited number of charge/discharge cycles. Drawing more power than expected means the 12v battery needs to be recharged more frequently, causing a shorter 12v battery life.

    So depending on the accessory, and how much power it draws, it may work or not. Switched 12v power should not cause any problems with loads (under 10 amps). I don't have the power limits, but I've seen some people get errors on always-on loads as little as 300 mA from a dashcam.
  • edited October 2018
    With electricity.
  • edited October 2018
    Interesting TT. You are inferring that the traction battery charges the 12v battery when the car is off. I did not know that.

    What I keep waiting for is an unswitched connection point at the DC-DC converter that steps pack voltage down to 12v. Haven't seen a reference to that as yet, and Tesla may or not be willing to have that converter be available for 24/7 current draws.

    Here is the link to the Ingineerix video for tapping a 15A Switched 12 volts

  • edited October 2018
    Regarding 12V battery recharging, from the owner's manual...

    "When left idle and unplugged, your vehicle periodically uses energy from the Battery for system tests and recharging the 12V battery when necessary."

    So yes, the 12V battery keeps charged regardless of the vehicle being plugged in or not.

    Do know that the Model 3 12V battery is rather small - not like an ICE battery. And beware of the monitoring system that TT mentioned. Really should only use switched accessories, and best if power is pulled from the 12V outlet (or associated wiring).
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