Jump-Starting 12V Battery Charger PSA

[Slipstream]

This is just a public service announcement regarding what to use to jump start the Model 3 12V battery if it dies. Most of you know if the 12V battery dies, all windows and frunk and trunk closes and you cannot get into the car with your phone or key card, nor be able to access the car with your Tesla app. Most of you also know how to open the frunk using the tow hole cables and hooking up a 12V battery to them.

This happened to me recently, and while I was able to open the frunk using the method above (using a trickle charger), I did not want to jump start the battery due to reports of potentially damaging any car that you use to jump start it (or damage to the Tesla), or physically taking out the battery and replacing it due to the complexity of taking off the rear passenger seat to uncouple the 12V from the main battery. I also wasn't sure which portable or wheeled jump charger to use, for the same potential damage reasons.

So I called Tesla Roadside and a tow truck came out. The tow driver used a TopVision jump charger on the 12V, and the car instantly charged up, doors opened, and systems came back on.

Here is the model he used. Note that I have no proprietary interest in any of the products or companies, I just wanted to share:

Amazon.com

One question: I would love to store the above jump charger in the frunk for any potential recurrence of the above situation, but the irony is that one needs a 12V charger (it used to be 9V, but no more) to open the frunk first. Any ideas on a work-around to make this idea work?

-Slipstream

 

[rrolsbe]

If the 12V battery is dead, no need to pull the connector under the seat because the contactors will probably be already open. Just install a new or charged battery then connect positive cable first then the negative cable.

 

[mrau]

Get a small A23 battery (12 volt). Put in plastic bag and tie wrap it to the wires behind the front bumper tow hook plug.

Amazon.com

You can only use the tow hook jumper wires to open frunk when the battery is dead or the car is unlocked. So no security issue keeping the small battery behind the plug cap.

 

[Slipstream]

If the 12V battery is dead, no need to pull the connector under the seat because the contactors will probably be already open. Just install a new or charged battery then connect positive cable first then the negative cable.

OK, good to know, thanks for the info! I assume one removes the dead battery by uncoupling the negative (black) terminal first, then the positive (red) terminal last. Red first on, last off is how I remember it

 

[Slipstream]

Get a small A23 battery (12 volt). Put in plastic bag and tie wrap it to the wires behind the front bumper tow hook plug.

Amazon.com

You can only use the tow hook jumper wires to open frunk when the battery is dead or the car is unlocked. So no security issue keeping the small battery behind the plug cap.

This is brilliant - thank you! In fact, those batteries are so small that one could probably just keep in a pocket or on a keychain. Much appreciated!

 

[Gauss Guzzler]

There's really no need for any of this.

Out on the road just flag someone down with jumper cables. With their engine off, use the cables to open the frunk. Then with their engine still off, connect the cables to your 12V battery and turn on your car. Once your car is awake it will start charging the 12V system.

At home just use a trickle or standard charger the same way or use another car or motorcycle (even a Tesla) with jumper cables. The nice thing about using a non-running car to jump start it is that there's no possible way it could deliver more than 12.5V so it's super safe, whereas running cars, fast battery chargers , and portable jump start packs may supply 15-20V.

It's not like a fossil car which requires a huge battery to be well charged to start the engine, the Tesla barely needs 8 or 10 volts to boot the computer and get the HV contactors engaged so even a non-running moped can supply enough power to boot it up within seconds.

 

[Slipstream]

Carefully connect the red positive (+) cable to the red positive (+) terminal on the 12V battery. Then, connect the black negative (-) cable to the black negative (-) terminal. Turn on external supply. Wait about 10 minutes, and then the car should be able to power up its computers. You’ll see the touchscreen turn on. Give it some time to charge. Disconnect the cables. The Black first, then red.

The waiting 10 minutes is important. The car systems need enough charge to open doors/screen on/etc. When the tow person charged my 12V battery for a minute, it was just enough to get the car doors open and the screen on. Another jump allowed me to turn the car around to load it on the flatbed. So letting the 12V battery charge longer is important.
My main batteries for the car were at 200+ miles, so OK there.

 

[Slipstream]

There's really no need for any of this.

Out on the road just flag someone down with jumper cables. With their engine off, use the cables to open the frunk. Then with their engine still off, connect the cables to your 12V battery and turn on your car. Once your car is awake it will start charging the 12V system.

At home just use a trickle or standard charger the same way or use another car or motorcycle (even a Tesla) with jumper cables. The nice thing about using a non-running car to jump start it is that there's no possible way it could deliver more than 12.5V so it's super safe, whereas running cars, fast battery chargers , and portable jump start packs may supply 15-20V.

It's not like a fossil car which requires a huge battery to be well charged to start the engine, the Tesla barely needs 8 or 10 volts to boot the computer and get the HV contactors engaged so even a non-running moped can supply enough power to boot it up within seconds.

Good info, thanks! I did use a trickle charger (that works fine on my other vehicle's 12V battery - 2004 Scion xB but it did not work to charge up the Model 3 12 V battery - even after 48 hours+ of having the trickle charger hooked up to the battery. So a true battery jump charge is likely needed.

The areas I travel (remote country roads) do not always have people around to flag down. At least not unless I wait for some time.

FWIW, all this battery issue started during a download of the latest software (Holiday update 2021.44.25.2). It took hours in the download process (even though it said it should take about 25 minutes to download), the car made strange buzzing sounds at 30%, and then it died. After a 12V battery replacement the car said that it did not load the update, and provided a chance to re-load it. So far I have declined - happy to use the prior UI

 

[roblab]

I've always found that if one ever has ANY problem with the 12v. battery, it's time for a new one. They're headed down hill. If you have to jump it, you're done.

 

[Gauss Guzzler]

Interesting to know that the trickle charger didn't work. I'd bet that the computer uses more power than your trickle charger can supply so every time the battery voltage got high enough, the computer would boot and immediately drain the battery back down. This makes sense as trickle chargers are usually only around 15W and the computer probably needs 100W or more to boot up enough to close the contactors. You could circumvent this endless cycle by disconnecting the negative terminal so that your battery can charge uninterrupted.

And @roblab is correct that these batteries don't die without reason. Get a (warranty?) replacement asap and expect to be stranded often in the meanwhile.

 

[android04]

Connect the 12V jump starter’s red positive cable to the car’s red positive cable. Then, connect the 12V jump starter’s black negative cable to the car’s black negative cable. Apply power with your external jump starter to pop the frunk open. Once open, disconnect cables. The black first then Red

Just wanted to comment on this step of the process. I myself have an A23 battery (a tiny 12V battery) tucked behind the tow hook cover so I can pop open the frunk if I need to.

Your suggested method of using a portable jump starter to pop open the frunk might not work for everybody, especially if they are unfamiliar with how jump starters and how Tesla's frunk opening circuit works. If the jump starter has a bypass mode (usually a button you press or hold down) to force it to output voltage, and you know how to use it and why, then it will work to open the frunk. Most jump starters are "smart" and will not output voltage if they can't sense a small amount of voltage from a weak battery on their jumper cables. They are designed to only output voltage if there's at least 5V (for example) at the jumper cables to avoid the dangerous mistake of someone putting the cables on with reverse polarity. So a "smart" jump starter or charger will not open the frunk because the two wires behind the tow hook cover are not connected to the 12v battery and therefore do not have any voltage on them.

 

[Slipstream]

Interesting to know that the trickle charger didn't work. I'd bet that the computer uses more power than your trickle charger can supply so every time the battery voltage got high enough, the computer would boot and immediately drain the battery back down. This makes sense as trickle chargers are usually only around 15W and the computer probably needs 100W or more to boot up enough to close the contactors. You could circumvent this endless cycle by disconnecting the negative terminal so that your battery can charge uninterrupted.

And @roblab is correct that these batteries don't die without reason. Get a (warranty?) replacement asap and expect to be stranded often in the meanwhile.

Excellent points, thank you. I had the leads on each terminal; probably should have attached the negative to the car body, to allow uninterrupted charging of the 12V battery.

And yes, had the battery replaced with a new one at the service center that same day.

 

[Willy’sEV]

I was wondering whether to carry a 9-volt battery in the car.
I read about the 12-volt battery going dead and having a 9-volt batter to open the trunk thru the tow port on the front bumper is one of the ways.
Any thoughts?

 

[rrolsbe]

I tested the 9V battery over a year ago and it worked; however, I read on this forum that, after a prior firmware update, that it now requires 12V. I purchased an A23 12V battery several month ago but never tested it because the 9V battery worked. If anyone has verified the A23 battery works, please reply to this thread.

 

[android04]

I tested the 9V battery over a year ago and it worked; however, I read on this forum that, after a prior firmware update, that it now requires 12V. I purchased an A23 12V battery several month ago but never tested it because the 9V battery worked. If anyone has verified the A23 battery works, please reply to this thread.

Yes, the A23 battery works and is what I stored in a ziplock bag behind the tow hook cover (no need to carry it around on me). I've tried it multiple times to make sure it worked (only works if your car's 12v battery is dead, or if the cat is unlocked and the 12v is good).

 

[dvanbrunt]

There's really no need for any of this.

Out on the road just flag someone down with jumper cables. With their engine off, use the cables to open the frunk. Then with their engine still off, connect the cables to your 12V battery and turn on your car. Once your car is awake it will start charging the 12V system.

If you have to flag down someone with jumper cables, that means your car died on the road while out driving it... so either your main battery is too dead to keep that 12v charged, or doing this will won't work. Wouldn't the car just die again soon once the other car is disconnected? After all, if it died while driving in the first place... I'd love to better understand this for emergency preparedness. (thanks to OP for starting this thread!)

 

[dmurphy]

If you have to flag down someone with jumper cables, that means your car died on the road while out driving it... so either your main battery is too dead to keep that 12v charged, or doing this will won't work. Wouldn't the car just die again soon once the other car is disconnected? After all, if it died while driving in the first place... I'd love to better understand this for emergency preparedness. (thanks to OP for starting this thread!)

By "out on the road" I suspect @Gauss Guzzler means, say, in a shopping center parking lot. Not necessarily while in motion.

I've got an A23 battery tucked on the vehicle body, and then a lithium jumpstart pack in the frunk. If those two things don't get me out of a jam, I figure it's tow truck time anyway.

 

[eprosenx]

I tested the 9V battery over a year ago and it worked; however, I read on this forum that, after a prior firmware update, that it now requires 12V. I purchased an A23 12V battery several month ago but never tested it because the 9V battery worked. If anyone has verified the A23 battery works, please reply to this thread.

I am very curious how a firmware update could change this behavior?

I have always assumed that the wires were to a solenoid and you needed a source sufficient to pop the solenoid.

If they are just a signalling method, then some other power source would be required to actuate the solenoid which defeats the entire purpose of this override (for when your battery dies).

Maybe they changed the solenoid along the way such that it requires 12v?

 

[android04]

I am very curious how a firmware update could change this behavior?

I have always assumed that the wires were to a solenoid and you needed a source sufficient to pop the solenoid.

If they are just a signalling method, then some other power source would be required to actuate the solenoid which defeats the entire purpose of this override (for when your battery dies).

Maybe they changed the solenoid along the way such that it requires 12v?

The wires don't go directly to a solenoid. They go to one of the vehicle controllers. That's how they can only enable this to work if the 12v battery is dead, or if not dead and the car unlocked. The power you put in the two wires turns on part of the logic in the vehicle controller and powers the frunk release solenoids (there are 2) if the conditions are right.

 

[rrolsbe]

The waiting 10 minutes is important. The car systems need enough charge to open doors/screen on/etc. When the tow person charged my 12V battery for a minute, it was just enough to get the car doors open and the screen on. Another jump allowed me to turn the car around to load it on the flatbed. So letting the 12V battery charge longer is important.
My main batteries for the car were at 200+ miles, so OK there.

This following might have been mentioned in this this thread. The battery can fail in multiple ways shorted cell, bad cell, case cracked etc. Leaving the jumpstarter connected for 10 minutes might work but it might also discharge it to the point where you are stranded. Not knowing what the problem is, I would disconnect the negative battery cable and attach the jumpstarter. After the contactors close and the DC-to-DC converter is enabled, the car does not require a 12V source. Remove the jumpstarter after the car is awake/operational. Turn on sentry mode so the car does not go back to sleep until you get home/replace the battery. I responded to another thread in this forum a while back with the suggestion to remove the negative cable and it worked for the owner. If anyone sees a flaw using this procedure or something that might cause further damage please respond.

A23 battery in pocket, jumpstarter and 10MM wrench in frunk. Note: most jumpstarters require a minimum voltage they use to detect reverse polarity; therefore, you need to know how to manually enable the voltage output on the unit you own/purchase.