Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.

12v battery is now charging 24 hours / day


Well-Known Member
Feb 28, 2015
Merced, CA
Any time in the past when I've disconnected the 12v battery, everything shuts down. Even if the inverter is humming along charging the battery like it does every 4 hours or so for a minute, disconnecting the battery instantly results in everything shutting down. If the contactors are engaged, they click off.

Well no more. My 12v battery is getting a constant 13.8 volts 24 hours a day. If I disconnect it, the MCU stays and the doors remain functional. Odd thing is even though the 12v battery is constantly being charged, the inverter hum is completely gone. Even stranger, disconnecting the 12v battery results in no alert of any kind. Super strange.

If I pull the high voltage loop with the 12v battery connected, the contactors click off and the 12v battery reads 12.7 volts.....as you would expect.

I just did the MCU2 upgrade. Did they add a small charge inverter to keep the battery always charging and leave the contactors engaged all the time?

Can anyone else with MCU2 measure their 12v voltage? I measured mine at the battery with the frunk tub out which it has been for the last 24 hours.

Full disclosure, this weekend I started getting BMS contactor alert errors. Could this be related? Although they're not stuck on. If I pull the high voltage loop the contactors click off.
Forgot to mention my vampire drain is up slightly after the MCU2 upgrade but not drastically. The 12v battery is pulling 540 CAs which is right in spec with a new battery even though it is 18 months old so the battery is fine. Amazing CCA spec for such a small battery.
Hi @sorka ,

How often is your high voltage battery charging???

I ask because my wife's S 85 D is charging about 10 times per day sometimes for less than a minute...
Her car was upgraded to MCU2 in August 2020. New 12 volt battery was added at the same time.
Its first new 12 volt battery.

I am curious if something bad may be going on here...

Hi @sorka ,

How often is your high voltage battery charging???

I ask because my wife's S 85 D is charging about 10 times per day sometimes for less than a minute...
Her car was upgraded to MCU2 in August 2020. New 12 volt battery was added at the same time.
Its first new 12 volt battery.

I am curious if something bad may be going on here...


Mine is no longer charging periodically but being charged continuously. If it's not a malfunction and intentional, it means some piece of hardware has been updated to allow continuous charging which would actually a be a good thing as it would prevent the cycling. There's a constant 70 watts or so being drawn all the time which is why the inverter would kick in every few hours and top the battery off.

My only thought is that this was somehow part of the MCU update. With the current behavior, the 12volt battery would last many years.
Today it's still showing 13.7 volts to the 12v battery. There is zero inverter hum although technically you wouldn't need an inverter, just a dc-dc converter which would be completely silent.

Either this is an intentional change which will dramatically extend 12v battery life or there's an actual problem keeping my contactors closed 24 hours / day even though they do release if I pull the HV disconnect loop.

For anyone with TM-SPY, can you check your 12v bus voltage when the car is off? Do you see 13.7 or or close to that or more like 12.6 volts (not being charged).
I can hear the contactors on my P85DL close and open but TM-Spy won’t connect to the car when the contactors are open.
Running 2020.48.37.1 and MCU1.

That's interesting. TM-SPY has always connected for when the car is shutdown and the contactors are not engaged. The 12v battery has been enough to keep basic systems running including communication with the BMS.

But what I really want to know is if anyone with MCU2 is seeing their 12v battery at a constant 13.7 volts?
I just noticed my 2016 75D MCU1 contacters are opening/closing every minute or two. It’s running 2020.48.37.1. Anyone know if this is new (intentional) behavior, or something wrong (stuck in a software loop)?
Gee, that surely doesn't happen on my car. I wonder if that will wear out the contactors a lot faster. Mine only does that about twice a day that I can notice.
Uhhh...if the 12V battery is charging 24 hours per day, and the accessories work if it is disconnected, why do we need a 12V battery?

Because there are still cases when the contactors are open and you need to the 12v battery to start the system and close the contactors. Running the HV battery too low. Disconnecting the HV battery for service or safety. I'm sure there are a half dozen other reasons including various error conditions and malfunctions that result in the contactors opening but you still want a backup power supply for the onboard electrics so that you can operate locks, emergency lights, etc.
You missed the part where that is normal for all cars with MCU2 after March 2015. The reason Tesla did this was to keep 12v batteries from going bad every 18 months.

Hmm, where was this confirmed? Not in this thread at least?

Why is March 2015 a limit? I have a 2013 car with MCU2, I just popped the nosecone out so I can measure the voltage through the day. :)
Ah, found the post:

What they don't tell you about the MCU2 retrofit is the higher vampire drain on pre-Q2-2015 vehicles because the battery packs don't have the standby power supply DCDC which powers the MCU's standby power and body control module directly from the HV battery. The standby supply means the main contactors can stay open (saving ~20-30W IIRC). Without it they have to stay closed, a pump has to run to cool the DCDC, etc. (Yeah, a pump has to run any time to contactors are closed to circulate coolant through the DCDC... so... yeah).

MCU2 was only originally installed in cars with this standby supply and seems to behave... oddly... in cars that don't have it. As in, it stays awake more than it should.

I figure they'll fix that eventually.

Now, doesn't this mean that on pre-2015 contactors stay closed and 12V battery is charged with the "regular" DCDC? On post-2015 contactors are open and MCU is powered from the standby DCDC..

Anyway, I'll measure how my 12V behaves on 2013 car.. :)
Read this thread

and look for my two oscilloscope traces comparing the battery charging in 2016 and 2021.

The car has been upgraded to MCU2.

They are propping up the 12 volt battery at the expense of higher vampire drain.
I posted some post-MCU2 vampire drain and sleep behaviour observations over in this thread, which may be helpful. Since then I’ve noticed a slight reduction in vampire drain, which has settled between 51 and 57w the last few times I’ve checked. I’ve not measured voltage at the 12v battery so can’t offer any insights on charging behaviour I’m afraid.