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Wall charging with dead 12V battery?

Lunares

Member
Jul 9, 2018
746
543
San Diego
Got the "your 12V battery is dying" warnings, and the car actually didn't start a few times (lights flickering/refusing to have the screen go on). Eventually (after waiting a while) everything did power up with only the "12V needs to be replaced soon" warning

I have about 65% on the main pack and a mobile service appt for wed to replace the 12V. Should I leave it unplugged from the wall during this time? Or just plug into standard 240V charging anyway to keep everything topped up.
 

Phlier

Bluebird
Jun 12, 2019
1,475
1,847
Utah
Fighting insomnia, so nothing better to do than to expand on @smatthew 's advice, in case you're wondering why he's saying to do this...

When the car is asleep, the car uses the 12v battery to power the 12v system. It also uses the 12v battery to wake up from sleep.

Once the car is awake, it uses the DC/DC convertor (which gets it's energy from the main battery pack) to power the 12v system, not the 12v battery.

So if you're able to keep your car awake, you're taking away one of the ways that a dead 12v battery can strand you; it doesn't need any energy from the 12v battery since that energy is being supplied from the DC/DC convertor.

If you want to make sure that the car is usable from now until the 12v battery is replaced, keep it awake. :D
 

danarcha

Member
Feb 9, 2020
76
109
South Bend, IN
Fighting insomnia, so nothing better to do than to expand on @smatthew 's advice, in case you're wondering why he's saying to do this...

When the car is asleep, the car uses the 12v battery to power the 12v system. It also uses the 12v battery to wake up from sleep.

Once the car is awake, it uses the DC/DC convertor (which gets it's energy from the main battery pack) to power the 12v system, not the 12v battery.

So if you're able to keep your car awake, you're taking away one of the ways that a dead 12v battery can strand you; it doesn't need any energy from the 12v battery since that energy is being supplied from the DC/DC convertor.

If you want to make sure that the car is usable from now until the 12v battery is replaced, keep it awake. :D

Would having sentry mode on also keep the car awake?
 

Phlier

Bluebird
Jun 12, 2019
1,475
1,847
Utah
It should.......
I personally would feel more comfortable with the AC on. The AC runs directly from the HV battery, so you're kinda guaranteed the HV battery will stay in use and connected.
Me too. Here's why...

I used to use a Raspberry Pi to send sentry and saved videos to my desktop computer over wifi. The Raspberry Pi would activate sentry mode so that the USB ports would stay powered, which in turn powered the Raspberry Pi while it transferred the videos.

Keep in mind that Sentry mode is active... the car should be awake. There have been times when I've gone out to the car, with Sentry mode active, and heard the familiar sound of the high voltage contactors closing when I open the driver door.

This kinda drives me nuts, as they should have already been closed, since Sentry mode was on. I have no idea why this happened. And it isn't an always thing, it's intermittent. Sometimes the relays are already closed, sometimes they're not.

IMO, it would be worth using an alternative way to for sure keep the car awake when you absolutely need to keep it awake, as is the OP's case.
 

Skione65

Active Member
May 5, 2016
1,484
729
Kentucky
Got the "your 12V battery is dying" warnings, and the car actually didn't start a few times (lights flickering/refusing to have the screen go on). Eventually (after waiting a while) everything did power up with only the "12V needs to be replaced soon" warning

I have about 65% on the main pack and a mobile service appt for wed to replace the 12V. Should I leave it unplugged from the wall during this time? Or just plug into standard 240V charging anyway to keep everything topped up.

@Lunares,

Year of manufacture and miles? We’re all tracking data points on this. Thanks. Also HW2.5 or 3.0?

Ski
 

camalaio

Active Member
May 28, 2019
1,483
2,086
Vernon, BC, Canada
If you genuinely have a failing 12V, some of the above advice is potentially dangerous (though very low risk).

Keeping the car on so that it's still powering 12V and charging the 12V battery could be bad. Failing 12V batteries that are being rapidly charged to make up for their failure can get very hot, boiling the acid inside. It shouldn't damage anything else (this isn't unique to Teslas), but it can be stinky as heck if nothing else.

Tesla could have changed the behaviour so that it doesn't try to charge a failing 12V, which would make the above moot. Not sure if that would be the case though, as you'd be nearly guaranteed to be stranded somewhere if it chooses to not charge it.

All that said, no danger in keeping it plugged into 240V. May as well have a charged car if possible, especially as many of the recommendations are to keep the car awake. Wouldn't want to end up at 0% (or 20%, where most of the above solutions would stop working anyways!)
 

Lunares

Member
Jul 9, 2018
746
543
San Diego
Camp mode turns off walk-away lock, so don't forget to use the app to lock the car.

Learned that you cannot engage camp mode or sentry mode with a dying 12V

Fighting insomnia, so nothing better to do than to expand on @smatthew 's advice, in case you're wondering why he's saying to do this...

When the car is asleep, the car uses the 12v battery to power the 12v system. It also uses the 12v battery to wake up from sleep.

Once the car is awake, it uses the DC/DC convertor (which gets it's energy from the main battery pack) to power the 12v system, not the 12v battery.

So if you're able to keep your car awake, you're taking away one of the ways that a dead 12v battery can strand you; it doesn't need any energy from the 12v battery since that energy is being supplied from the DC/DC convertor.

If you want to make sure that the car is usable from now until the 12v battery is replaced, keep it awake. :D

Thankfully it didn't fully die (or rather it did, then after sulfur smell came back to life enough to at least get frunk open easily when service showed up)

@Lunares,

Year of manufacture and miles? We’re all tracking data points on this. Thanks. Also HW2.5 or 3.0?

Ski

2018, one of the first P3Ds off the line. 25k miles, upgraded to HW 3.0 last year.

If you genuinely have a failing 12V, some of the above advice is potentially dangerous (though very low risk).

Keeping the car on so that it's still powering 12V and charging the 12V battery could be bad. Failing 12V batteries that are being rapidly charged to make up for their failure can get very hot, boiling the acid inside. It shouldn't damage anything else (this isn't unique to Teslas), but it can be stinky as heck if nothing else.

Tesla could have changed the behaviour so that it doesn't try to charge a failing 12V, which would make the above moot. Not sure if that would be the case though, as you'd be nearly guaranteed to be stranded somewhere if it chooses to not charge it.

All that said, no danger in keeping it plugged into 240V. May as well have a charged car if possible, especially as many of the recommendations are to keep the car awake. Wouldn't want to end up at 0% (or 20%, where most of the above solutions would stop working anyways!)

I ended up not plugging it in because of this risk as you said. service guy said shouldn't do anything to the 12V, except that your charger cable could get stuck if the 12V died. Knew it was only a few days so wasn't worried about HV pack.
 

smatthew

Active Member
Jun 9, 2018
1,235
2,039
CA Bay Area
Learned that you cannot engage camp mode or sentry mode with a dying 12V



Thankfully it didn't fully die (or rather it did, then after sulfur smell came back to life enough to at least get frunk open easily when service showed up)



2018, one of the first P3Ds off the line. 25k miles, upgraded to HW 3.0 last year.



I ended up not plugging it in because of this risk as you said. service guy said shouldn't do anything to the 12V, except that your charger cable could get stuck if the 12V died. Knew it was only a few days so wasn't worried about HV pack.
Thank you for reporting back!
 

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