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How to get main battery to charge 12V?

teethdood

Member
Jan 30, 2017
393
524
Visalia, California
My 12V died. I was able to jump the 12V and start the screen by pressing the jumper battery repeatedly (jumper stops the charge after 30s but screens need longer than 30s to start up). I scheduled for mobile service tomorrow at my office to get the 12V replaced. I'm hoping to be able to drive the car tomorrow to the office by jumping it. While the screens started up fine, as soon as the last push +30s jumper ends, the car would die again. Is there any way to get the main battery to push juice to the 12V to maintain the charge for the electronics after jumping? Thanks
 

Les236

Member
Jan 20, 2018
158
87
Kissimmee, FL, Boston, MA
It was my understanding that the main battery DOES charge the 12v battery. Maybe if the electronics determine there is an issue with the 12v it disables this; not sure. My jumper battery pack does not have a 30 sec timeout so not sure how I would handle this. Are you doing this with the car plugged in? That may help.
 

scottf200

Active Member
Feb 3, 2013
3,838
3,406
Chicagoland ModelX S603
It monitors your 12v by some criteria and then starts the DC-DC converter. Maybe your criteria (voltage) was too low so I didn't charge your 12v for fear of a dangerous charging situation (ie. what problems caused the 12v to die as it could be multiples apparently).

I charged from a booster pack OK that I 'borrowed' from a regular auto mech garage while I was on vacation. Info and thread about a monitoring tool I use: I monitored the 12v battery voltage during an OTA update
 

ShawnA

Supporting Member
Nov 13, 2017
906
666
Edwardsburg, MI
Hi @teethdood ,

I'm not sure which of your cars '17 or '18 requires the battery...

My X from March 2018 needed a new 12 volt battery in November 2020.
I got the message that software updates would not complete with the
battery in the depleted state.
I tried one of the jumper packs but it was depleted immediately.
The mobile service technician said that the remote diagnostics
showed a short in the battery...
I did not try to drive the car.
The mobile tech also said that the smell of sulfur (which I did not have)
indicated a short in the battery.

Good luck,

Shawn
 

teethdood

Member
Jan 30, 2017
393
524
Visalia, California
Thank you for your replies.

It's my 2018 car that just died. It does have a sulfur smell. My 2017 12V battery also had the sulfur smell when its 12V battery died. I was able to change the appointment location from the office to my home and will be staying home tomorrow. What's a bummer is that I always keep my car jumper pack in the car at all times for the dreaded dead 12V occasion, so I'd be able to jump it and get back home. Now that I found out that I can't jump and drive the car, I will have to rethink this. It's better than nothing that I'd still be able to get into the car though by jumping.
 

cobra

Member
Feb 8, 2021
106
19
Lansdale, PA, USA
When you jumped it, have you tried to press the brake to turn the car on? It may not enable the DC-DC until the car 'starts.'

This assumes you can keep the screens alive long enough to do the brake pedal press for 'On.'

What was the level of charge of your main battery?
 

teethdood

Member
Jan 30, 2017
393
524
Visalia, California
@cobra
I did press the brake while the screens were still on. The driver door closed shut. Then the jumper pack made a beeping sound indicating that the +30s timeout was going to happen, so I immediately opened the driver door to get out because I didn't want to get trapped inside. I didn't try to change gear to D. I'm going to get the wife to jump the car so I can get out should I get stuck inside. Wish me luck ;-) My car's SoC is over 200 miles.

OK so I was inside, screens working. I could not change gear. Instrument cluster said "Unable to start, car needs service" "Unable to charge, car needs service", and "Parking brake not engaged, car may free roll", etc.

It looks like if your 12V dies while on a road trip, even with a battery jumper, you will probably need to have roadside assistance tow you to a service center. The battery jumper pack will not save you from a tow, but does allow you to get back inside your car so you're not exposed to the elements while awaiting roadside assistance.
 
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scottf200

Active Member
Feb 3, 2013
3,838
3,406
Chicagoland ModelX S603
@cobraIt looks like if your 12V dies while on a road trip, even with a battery jumper, you will probably need to have roadside assistance tow you to a service center. The battery jumper pack will not save you from a tow, but does allow you to get back inside your car so you're not exposed to the elements while awaiting roadside assistance.
That is not true per my post that I pointed you to. See:

I was in Lincoln, NH. There are *no* service centers in the whole state of NH. We could not get mobile service to come to our rescue even tho they service the whole state but who knows where all they come from to do that (ie. probably some locals and some from other states).

Luckily, I decided on my own to try jumping the 12v via local auto mechanic after looked at YouTube videos to see how to get into my frunk to do it (I was locked out of my whole car).

The auto mech brought a battery booster pack and after hooking it up, my X came to life (sentry mode horns blaring) and it completed the install starting at 60% progress bar and took 45 min-1 hr. [and I left the battery booster pack on the entire time].

The next day at a supercharge I got the 'replace 12v battery or your car could shut down message'. Long story but found an open (Sat) service center that we could barely make it to before they closed. We literally made it at 2:57p and they said we'd have to be there by 3pm. It was replaced by 4pm. Worse case I would have bought the 12v battery from them (no core charge) and found a way to get it installed even if I had to buy tools at Walmart!
 

teethdood

Member
Jan 30, 2017
393
524
Visalia, California
That is not true per my post that I pointed you to. See:
Thanks scott,

What do you think is going on with my car? The DC-DC converter must not have kicked in at all, leading to the car displaying the message "Unable to start, car needs service". I guess the 12V battery is so dead that it doesn't accept a charge, or perhaps another unknown reason.
 

scottf200

Active Member
Feb 3, 2013
3,838
3,406
Chicagoland ModelX S603
What do you think is going on with my car? The DC-DC converter must not have kicked in at all, leading to the car displaying the message "Unable to start, car needs service". I guess the 12V battery is so dead that it doesn't accept a charge, or perhaps another unknown reason.
You are welcome. I'm sorry but I'm not sure about your car based on the description. Perhaps ShawnA had some clues with the 'short' or sulfur smell comments. The criteria Tesla used to test your 12v when booting up must have told the car that it was not safe.
 

cobra

Member
Feb 8, 2021
106
19
Lansdale, PA, USA
Probably limits of the battery booster you tried... there are different kinds, and the ones with 30 second limit aren’t designed to charge very long, just jump start a conventional vehicle. But it does also depend on how bad the 12V battery failed. Most do not just die outright and can be boosted for a while before really going dead.

It would be interesting to get a wall powered charger on the car and then see if it will start. It’s always possible that the DC-DC converter failed so the car cannot charge the 12V. Similar to an alternator failure in an ICE vehicle.

Also, I thought you couldn’t get stuck in a X with the driver door pull handle? Isn’t that release mechanical?
 

teethdood

Member
Jan 30, 2017
393
524
Visalia, California
Mobile tech was able to jump start the car with my battery jumper pack. Not only was he able to get in the car (I was able to also) but he was able to shift to drive without issues (I was not able to). I wasn't there to witness it, but my wife was. So I guess it was user error on my part. My wife said he placed the negative end on a different clamp point than me, but I fail to see how that made any difference (since I was able to get into the car). I'll just chalk it up to user error :cool: Hopefully I'll have better luck next time when my 12V dies again.

PS @cobra You're right, the front doors on the MX is mechanical per this article:
The FWD doors you'd need to remove the speaker grille. I knew that about the FWD, but was unsure whether front doors were mechanical or electrical.
 
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cobra

Member
Feb 8, 2021
106
19
Lansdale, PA, USA
Good to know!

I remember when you tried you said you pressed the brake and the drivers door closed on you? That actuator probably took more power to close the door. If the mobile tech thought to close the door manually, the jumper pack would have had some more power in reserve and that might have been enough to get the car to turn on before the pack went off...

I don't know when the DC-DC is enabled, but I'm assuming it's when the "Car Off" message goes away after you press the brake pedal for a couple seconds. Up until then, you are probably running on the 12V battery (and jumper pack, if attached) only.

I had similar problems with our Accord PHEV. I discovered the hard way that even though it has a high voltage battery, it would not enable if the 12V was down. I once killed the start capability running the emergency flashers on that car for 15-20min. Always carried a jump pack after that!
 

teethdood

Member
Jan 30, 2017
393
524
Visalia, California
Good to know!

I remember when you tried you said you pressed the brake and the drivers door closed on you? That actuator probably took more power to close the door. If the mobile tech thought to close the door manually, the jumper pack would have had some more power in reserve and that might have been enough to get the car to turn on before the pack went off...

I don't know when the DC-DC is enabled, but I'm assuming it's when the "Car Off" message goes away after you press the brake pedal for a couple seconds. Up until then, you are probably running on the 12V battery (and jumper pack, if attached) only.

I had similar problems with our Accord PHEV. I discovered the hard way that even though it has a high voltage battery, it would not enable if the 12V was down. I once killed the start capability running the emergency flashers on that car for 15-20min. Always carried a jump pack after that!
That is an excellent observation! You're probably right that the door closing shut took a lot of energy that could've been used to activate the car. We're spoiled sometimes with the auto door closing on brake. That's a good mental note for next time for sure.

Post #7 I said "I did press the brake while the screens were still on. The driver door closed shut. Then the jumper pack made a beeping sound indicating that the +30s timeout was going to happen, so I immediately opened the driver door to get out because I didn't want to get trapped inside."

So the beeping sound immediately after the driver door closed shut is not to indicate the +30s timeout, it was most likely to signal that the voltage is too low. That explains it.
 
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ngng

Member
Jul 23, 2018
501
201
Bay Area
another psa, if you've drained your battery to a point where you need a jump you've likely done sig damage to your cells. I would put that battery on a charger and fully charge it, but also expect that it will die in the near future.

a note on the new "smart" chargers. they will not turn on if the bat voltage is below a certain point. you can get around this by connecting your low voltage battery to a good battery (like a batttery on an ice car with the alternator running). this should be enough to start the charger. or, get a dumb charger that doesn't care lol
 

scottf200

Active Member
Feb 3, 2013
3,838
3,406
Chicagoland ModelX S603
another psa, if you've drained your battery to a point where you need a jump you've likely done sig damage to your cells. I would put that battery on a charger and fully charge it, but also expect that it will die in the near future.
That draining of the battery is what happened in my case when I [foolishly] did an OTA at a BnB while on my drive home leg of our road trip. The Tesla OTA stopped on its own halfway thru. I was able to use a jump pack to finish the install (leaving the jump pack on) ... but the next day during supercharging we got the "12v service soon" message and had to reroute that day to get a new battery. We only had one days drive home after that. (Foolish, I know but the OTA had speed limit sign reading so going thru construction I thought it'd be handy ... plus I never had an OTA problem in my prior 60-some of them. Still foolish).
 

ngng

Member
Jul 23, 2018
501
201
Bay Area
That draining of the battery is what happened in my case when I [foolishly] did an OTA at a BnB while on my drive home leg of our road trip. The Tesla OTA stopped on its own halfway thru. I was able to use a jump pack to finish the install (leaving the jump pack on) ... but the next day during supercharging we got the "12v service soon" message and had to reroute that day to get a new battery. We only had one days drive home after that. (Foolish, I know but the OTA had speed limit sign reading so going thru construction I thought it'd be handy ... plus I never had an OTA problem in my prior 60-some of them. Still foolish).

i am new to teslas, so please correct me if i am wrong. if I understand correctly, the 12v is charged via a DC to DC off the high voltage pack. does this charge only happen when the car is "on" or does it happen whenever a set of parameters are met, eg, 12v below a certain voltage?
 

cobra

Member
Feb 8, 2021
106
19
Lansdale, PA, USA
I am wondering the same...

This thread (Model S) talks about both the main DC-DC and a secondary DC-DC (trickle charger? my words) in 100D packs for maintaining the 12V. Skip ahead past the first half dozen posts where the author keeps asking how the charging works and people keep telling him to just replace the battery... :oops:


Of course, Model X may be slightly different.
 

ngng

Member
Jul 23, 2018
501
201
Bay Area
I am wondering the same...

This thread (Model S) talks about both the main DC-DC and a secondary DC-DC (trickle charger? my words) in 100D packs for maintaining the 12V. Skip ahead past the first half dozen posts where the author keeps asking how the charging works and people keep telling him to just replace the battery... :oops:


Of course, Model X may be slightly different.
nobody reads on the internet, I know I sure don't 😆
 
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