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HV Battery Now Powering the Vehicle when idle when 12V Battery is Disconnected? [ tested and determined as false by OP]

medphys3

Member
Feb 10, 2020
98
79
Northwest Ohio
TL/DR:

Apparently the HV battery will fully power the car if/when the 12v battery fails?

Software: 2021.4.18.2
This afternoon I wanted to see how much current the car needed to “turn on” the car in the event my 12v battery failed.
I have a really old car battery charger that has two charging settings: 2 amps & 10 amps. I disconnected the 12v battery and let the car go to sleep. About a half hour later, I went out and opened the door and expected nothing to happen (which is what I’ve read happens when the 12v battery completely fails). Then I was going to hook up the charger and see if the 2 amp setting would let the car wake up & close the HV disconnect- then the 10 amp if it didn’t work.

Surprisingly, the car woke up as normal, but with two warnings:

VCFRONT_a192

“Electrical system power reduced. Non essential features may be unavailable.”

VCFRONT_a402

“Electrical system backup power is unavailable. Vehicle will consume more energy while idle.”
Reading through the manual, it clearly states that the HV battery will now power the car while the vehicle is idle.

Is this new?

I did initially get a warning that the 12v battery was disconnected. Not sure if the system differentiates between a failing/failed battery or a disconnected battery, but this was not what I expected to see. I expected the car to just be dead, but nope.

There was a post a few days ago where someone used a backup power source to power up the car and continue on their trip when their battery died. Any idea why their HV battery didn’t take over? Maybe a 12v failure while the car is asleep is different than a battery that is completely disconnected? Or if it fails while the car is still awake, it “knows” about it and can act accordingly?

12V Battery died this weekend
 

medphys3

Member
Feb 10, 2020
98
79
Northwest Ohio
Edit to add- I’m not really sure what “idle” means. Is idle different than when the car is asleep? Maybe it’s always done this and idle is when you get out for five minutes, but before the car goes to sleep?
 

bo3bdar

Member
Feb 21, 2021
139
180
Silicon Valley
“Electrical system backup power is unavailable. Vehicle will consume more energy while idle.”
That sounds to me like the car noticed that the 12V battery was disconnected, and therefore would not turn off the HV at all. You should have heard the 'clunk' when the HV connectors fall open. And also when the door was opened.

It's my current understanding that if the 12V battery fails while the HV is disconnected, then you are completely locked out of your car.

If you feel like trying the better experiment, leave your frunk open and 12V battery connected, and then wait for the car to fully sleep, including the 'clunk'. You can PowerOff from the safety tab if you want to speed it up a bit. Once that happens, disconnect the 12V battery and I expect it'll be fully dead/cannot open door.
 
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medphys3

Member
Feb 10, 2020
98
79
Northwest Ohio
That sounds to me like the car noticed that the 12V battery was disconnected, and therefore would not turn off the HV at all. You should have heard the 'clunk' when the HV connectors fall open. And also when the door was opened.

It's my current understanding that if the 12V battery fails while the HV is disconnected, then you are completely locked out of your car.

If you feel like trying the better experiment, leave your frunk open and 12V battery connected, and then wait for the car to fully sleep, including the 'clunk'. You can PowerOff from the safety tab if you want to speed it up a bit. Once that happens, disconnect the 12V battery and I expect it'll be fully dead/cannot open door.

Yeah, that makes sense even though it seems odd that the software doesn’t monitor the 12v battery enough to automatically kick in the HV battery when it fails…

I shall test tomorrow :)
 

Idkorcare

Member
Nov 3, 2019
178
141
Atlanta
Not sure how much this would help but probably a year ago I disconnected the 12v while the car was on. Couple seconds later the car shut off after tossing a few errors. So it sounds like something changed.
 
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medphys3

Member
Feb 10, 2020
98
79
Northwest Ohio
So I did a quick test this morning. Let the car sleep, then disconnected the 12v battery. Sure enough, the car was dead.

My battery charger would NOT let me open the door or wake up the car. 12.6v @ 10 amps setting. However, it showed that the car was drawing 4 amps through the charger (so not quite sure what that means).

Reconnecting the 12v battery closed the HV disconnect and allowed me to open the door. The system came up fully about 30 seconds later.
 

cali8484

Member
Jul 8, 2018
274
152
California
So I did a quick test this morning. Let the car sleep, then disconnected the 12v battery. Sure enough, the car was dead.

My battery charger would NOT let me open the door or wake up the car. 12.6v @ 10 amps setting. However, it showed that the car was drawing 4 amps through the charger (so not quite sure what that means).

Reconnecting the 12v battery closed the HV disconnect and allowed me to open the door. The system came up fully about 30 seconds later.

Any time you disconnect the 12V battery while the car is still awake it will be seen as a 12V battery/VCFRONT fault.

As for using battery chargers, many will not enable full charging power if they do not sense some minimal battery voltage for protection purposes. So, that could be why you see only 4A. In any case, I suspect even [email protected] would not be enough as the car uses ~200W idle so you probably need to provide at least [email protected]
 

Gauss Guzzler

Member
Dec 27, 2020
453
560
Thousand Oaks, California
You should never try to power something with a battery charger, especially something as complex and expensive as Tesla's 12V power distribution system.

Battery chargers are not 12.6V power supplies, some are just crude transformers and others are sophisticated battery management systems but in either case the output can go well over 20V and can have all sorts of capacitor-frying high frequency fluctuations.

It's fine to use a charger to jump start your car but you should only do it with the dead battery still connected so that the battery can absorb/limit/filter the voltage irregularities that may come off the charger.
 
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brulaz

Member
Feb 4, 2021
100
29
Ontario Canada
So I did a quick test this morning. Let the car sleep, then disconnected the 12v battery. Sure enough, the car was dead.

...

Reconnecting the 12v battery closed the HV disconnect and allowed me to open the door. The system came up fully about 30 seconds later.
Yes, I understand the HV connector requires 12V to activate.

But the car will go to sleep with the frunk open ? Allowing you to disconnect the 12V battery when asleep?
 

medphys3

Member
Feb 10, 2020
98
79
Northwest Ohio
You should never try to power something with a battery charger, especially something as complex and expensive as Tesla's 12V power distribution system.

Battery chargers are not 12.6V power supplies, some are just crude transformers and others are sophisticated battery management systems but in either case the output can go well over 20V and can have all sorts of capacitor-frying high frequency fluctuations.

It's fine to use a charger to jump start your car but you should only do it with the dead battery still connected so that the battery can absorb/limit/filter the voltage irregularities that may come off
You should never try to power something with a battery charger, especially something as complex and expensive as Tesla's 12V power distribution system.

Battery chargers are not 12.6V power supplies, some are just crude transformers and others are sophisticated battery management systems but in either case the output can go well over 20V and can have all sorts of capacitor-frying high frequency fluctuations.

It's fine to use a charger to jump start your car but you should only do it with the dead battery still connected so that the battery can absorb/limit/filter the voltage irregularities that may come off the charger.
Great advice. That never occurred to me.
 

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