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Jump-Starting 12V Battery Charger PSA

boaterva

Well-Known Member
Supporting Member
Had the battery die in my 3 (2018) and got it going with a Gooloo (sp?) jump box worked great. (Note that all this process with a jump box is now in the Tesla doc under 'car won't drive'). Some have said elsewhere that Roadside will do 12V replacements (got the car home from the 2nd level of a garage at work) and messaged them. Nope, since I can drive it now, contact the SC. Mobile: next is a week out. Gonna see if they update it to be closer or just drop by later. Got the car charging and kept awake with Sentry.

BTW, had a problem with getting the cover off, and snapped one of the plastic hooks/tabs on it. It really did not want to move no matter how I pressed, etc. Any suggestions there on what I was doing wrong?
 
Very interesting and informative series of posts. Couple of thoughts:

If the 12v battery is kept up to full charge with the motive battery pack how the heck does the 12v go flat? Age? Sitting outside when it's cold with a low SoC maybe?

Would it not be prudent to do a "preemptive strike" and replace the 12v on our Teslas every three years or so?

Rich
 
Yeah, the tow cover is easy to remove if you know the exact right angle to push it, nearly impossible if you don't.
The angle seems unnecessarily senseless and is not noted in the manual in any place you'd be able to find it, but it is buried in there somewhere:

Screenshot 2022-04-07 102557.png
 
Very interesting and informative series of posts. Couple of thoughts:

If the 12v battery is kept up to full charge with the motive battery pack how the heck does the 12v go flat? Age? Sitting outside when it's cold with a low SoC maybe?

Would it not be prudent to do a "preemptive strike" and replace the 12v on our Teslas every three years or so?

Rich
The 12v battery isn't being maintained 24/7. The car goes to sleep and disconnects the high voltage battery, so at that point the 12v battery is on it's own fire a while until the car wakes up again to top it off. This is why the majority of 12v battery failures are encountered after the car had been parked a while and the owner tries to open the door. The 12v battery is too weak to power up the high voltage system that maintains it. If the battery is weak or has a shorted cell, it may seem fine as long as the car is awake and powering the 12v system through the high voltage battery and DC to DC converter. As soon as the car goes to sleep and that lifeline disconnects, that's when you encounter issues. Tesla has updated their charging and monitoring algorithms to try and detect weak batteries before they leave you stranded and to keep the car awake if they determine the chances are high of the 12 battery not being able to wake the car again. But their algorithms are not perfect and cannot be. So sometimes people will be left with a car that won't wake up until they get it jump started or the 12v replaced.
 
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!)
More likely the person has parked somewhere and is unable to start the car. Once the car is started, the main battery powers the system through a DC-DC converter, and it should stay alive until the car is shut down. That said, I wouldn't make a habit of jumping the car for fear of damaging the electronics.
 
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dmurphy

Active Member
Supporting Member
I tried opening the frunk with my noco jumper and it didn’t work. Needs a true 12v source

That's likely the "short circuit protection" in the noco jumper. It won't activate unless it senses some level of voltage from a typical "just about dead" battery.

Thankfully the amperage needed to pop the frunk is so minimal, one of these N21/N23 style batteries would do the trick:

 
I tried opening the frunk with my noco jumper and it didn’t work. Needs a true 12v source
Like dmurphy mentioned, the "reverse polarity protection" kept the NOCO jump starter from putting power onto the two wires because it doesn't detect any voltage on them (since they aren't connected directly to the 12v battery). It will still work but you have to push the bypass button on the NOCO to force it to output power (read the manual).
 

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