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How to get main battery to charge 12V?

scottf200

Active Member
Feb 3, 2013
3,899
3,512
Chicagoland ModelX S603
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?
During the OTA the software that manages the DC to DC may be off for part of it so the 12v battery is on its own.
I created a whole thread on this. See the graphics where I use a tool to monitor the 12v directly on the battery post here:
 

Lasttoy

Active Member
Mar 24, 2017
1,594
850
St Augustine, Fl
My car died on interstate in 2019. 400 miles from home and nearest SC. Finney after 6 calls to Roadside, a lady said my 12v was dead. I told her it had 12.5v in it. She said that was not enough, 12v battery must always be at 14 volts as regulator in car keeps it. She told me to get big tow truck to jump me. I did, the jump brought car alive, charged to 14.5. I supercharged car full. Checked 12v battery, it was 14v. Never had another problem till last month, 12v battery finally died . It was original 2012 battery. The lady explained the charge/regulator system keeps tiny 12v battery at 14v. That battery runs entire car. I was shocked she said that. Such a tiny battery to run entire electronics. Unbelievable.
 

scottf200

Active Member
Feb 3, 2013
3,899
3,512
Chicagoland ModelX S603
My car died on interstate in 2019. 400 miles from home and nearest SC. Finney after 6 calls to Roadside, a lady said my 12v was dead. I told her it had 12.5v in it. She said that was not enough, 12v battery must always be at 14 volts as regulator in car keeps it. She told me to get big tow truck to jump me. I did, the jump brought car alive, charged to 14.5. I supercharged car full. Checked 12v battery, it was 14v. Never had another problem till last month, 12v battery finally died . It was original 2012 battery. The lady explained the charge/regulator system keeps tiny 12v battery at 14v. That battery runs entire car. I was shocked she said that. Such a tiny battery to run entire electronics. Unbelievable.
You can look at the post right above yours to a thread that shows it is not 14v. ~13.4 while the car is awake it seems. Several monitoring graphs for the X and 3s in that thread.

It is not the 'tiny battery' that runs the entire car as the DC-DC converter keeps the 12v rail energized. The battery typically is to 'boot' the computer not turn a starter over :)

IMO, people shouldn't wait until it dies like I did or you did as it could be pretty inconvenient (vacation, meeting people, having others with you, etc). I'm changing mine every 2.5 years and setup a repeating calendar entry for it. It is really a minor expense and cheaper if you do it yourself. My kid's TM3 was $125 after tax installed (35+85).
 

glusc2002

Member
Jul 6, 2016
195
30
Orange County
You can look at the post right above yours to a thread that shows it is not 14v. ~13.4 while the car is awake it seems. Several monitoring graphs for the X and 3s in that thread.

It is not the 'tiny battery' that runs the entire car as the DC-DC converter keeps the 12v rail energized. The battery typically is to 'boot' the computer not turn a starter over :)

IMO, people shouldn't wait until it dies like I did or you did as it could be pretty inconvenient (vacation, meeting people, having others with you, etc). I'm changing mine every 2.5 years and setup a repeating calendar entry for it. It is really a minor expense and cheaper if you do it yourself. My kid's TM3 was $125 after tax installed (35+85).
would you suggest replacing it every 2.5 yrs or does it vary on the mileage driven? are the batteries easily replaced with out SC like an ICE car or better to have SC / mobile tech replace it? I'd like to locate one from a reputable source and store it as my 2.5yrs is approaching. thanks in advance.
 

ArizonaP85

Member
Apr 6, 2017
265
430
Phoenix
Instead of replacing every 2.5 years and constantly risking getting stranded, why not buy a lithium 12V and forget your worries?

I bought mine from Ohmmu and couldn’t be happier.

P.S. That sulphur smell is sulphuric acid which can eat wiring and metal ... if it doesn’t build up and explode. It’s devilish - fix it quick!
 
  • Helpful
Reactions: Art_Vandelay

scottf200

Active Member
Feb 3, 2013
3,899
3,512
Chicagoland ModelX S603
would you suggest replacing it every 2.5 yrs or does it vary on the mileage driven? are the batteries easily replaced with out SC like an ICE car or better to have SC / mobile tech replace it? I'd like to locate one from a reputable source and store it as my 2.5yrs is approaching. thanks in advance.
"harder" than an ICE car because you have to take the frunk liner out and 2 steps -- 12v ground and responder loop from manual.

I'm replacing mine every 2.5 yrs regardless of miles driven. If your car "sleeps" well then the battery may go down some and need to be recharged via the DC to DC convert more. Mine doesn't sleep so it stays in a steady charge. Some charts here showing the 12v going up and down. Some from a TM3 as I recall. I monitored the 12v battery voltage during an OTA update
 

scottf200

Active Member
Feb 3, 2013
3,899
3,512
Chicagoland ModelX S603
Instead of replacing every 2.5 years and constantly risking getting stranded, why not buy a lithium 12V and forget your worries?
A reason talked about is because Tesla's software is maintaining (charging) the battery as the old style and not lithium. The newest X/S have lithium but I bet there procedure are treated differently.

From my direct monitoring Tesla is maintaining the battery just like below.


From article and quote below: Marine Battery Maintenance 101 - e Marine Systems
*** "taken to 14.2-14.4 VDC before it is fully charged"
*** "Once fully charged, marine batteries should be held at a considerably lower voltage to maintain their charge - typically 13.2 to 13.4 volts."

Why Marine Batteries Fail -- When a lead-acid battery is discharged, a soft lead sulfate material forms on the battery plates. During the battery's recharge, this material is lifted off the plates and recombined into the battery's electrolyte solution. If, however, the battery is left in a partial state of discharge for as short as 3 days, the lead sulfate material will begin to harden and crystallize, forming a permanent insulating barrier. As this barrier becomes thicker and thicker, the battery's ability to accept a charge or deliver energy is diminished, resulting in the perception that the battery is no longer usable. The accumulation of such deposits, otherwise known as sulfation, is the most destructive process in the life of any lead-acid battery.
Multi-stage Battery Charging -- A typical 12-volt lead-acid battery must be taken to approximately 14.2-14.4 VDC before it is fully charged. (For 24 volt systems, double these figures.) If taken to a lesser voltage level, some of the sulfate deposits that form during discharge will remain on the plates. Over time, these deposits will cause a 200 amp-hour battery to act more like a 100 amp-hour battery, and battery life will be considerably shortened. Once fully charged, marine batteries should be held at a considerably lower voltage to maintain their charge - typically 13.2 to 13.4 volts. Higher voltage levels will "gas" the battery and boil off electrolyte, again shortening battery life.
 

ngng

Member
Jul 23, 2018
785
332
Bay Area
i'm gonna add a tender pigtail so I can easily charge it up. i do this with all of my cars and it makes life much easiere
 

scottf200

Active Member
Feb 3, 2013
3,899
3,512
Chicagoland ModelX S603
i'm gonna add a tender pigtail so I can easily charge it up. i do this with all of my cars and it makes life much easiere
Common on ICE cars and motorcycles and riding lawnmowers. Just be aware that the Tesla cars will automatically monitor the 12v and charge it from the HV battery via the DC-DC converter.
 

scottf200

Active Member
Feb 3, 2013
3,899
3,512
Chicagoland ModelX S603
Whether you used it to trickle charge or not -
Wouldn't his pigtail offer an easier place or method to
attach your battery health meter??? Shawn
I am familiar with a "battery tender" and have used them on my previous motorcycle and gave it to my kid recently for his riding lawn mower battery.

To answer your question ... maybe. I use two testers.
* One stays connected all the time as it takes a snapshot every X minutes. Tiny tiny draw. https://smile.amazon.com/QUICKLYNKS-Battery-Monitor-Bluetooth-Device/dp/B01MT4583U -- I can open up the app and it does a download of that data via BT. Graphics: I monitored the 12v battery voltage during an OTA update
* The other I use test IR and I don't know if it would be sensitive to running thru Y gauge of wire. Product: Ancel BA101
 

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