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12v battery issues and best practice

jcanoe

Active Member
Oct 2, 2020
3,809
3,872
Maryland
When a mobile tech was installing homelink onto the MY. He was telling me all about the components under the frunk. He mentioned to me that the 12v battery does not get charged when the MY is plugged in. He said it only charges when the vehicle is on and being driven. I thought it was strange. He said some people put a trickle charger on the 12V when they park it for an extended period. Can anyone confirm that's actually the case?
This is incorrect. The Model Y monitors the voltage of the 12V battery when the vehicle is parked. Even when not plugged in, when the voltage of the 12V battery drops below a specified voltage the Tesla will power on and charge the 12V battery. You can reduce how often the Tesla vehicle must charge the 12V battery while the vehicle is parked by leaving Sentry mode, Summon and Smart Summon turned off at your home location.

All during Covid lockdown my Model Y sat in my garage, most of the time unplugged, unlocked and with Sentry mode turned off at my home location. The Model Y would wake up about every 24 to 30 hours. I was aware of when the Model Y would wake from sleep mode because of the Alexa Auto device plugged into the 12V accessory port. Whenever the Tesla Model Y powers on the 12V systems the Alexa Auto device would communicate via BT to the Alexa App on my phone. The Alexa app then creates a notification that it is ready to drive.

If what the Mobile Technician said was true then you could never park a Tesla vehicle for any length of time before the 12V battery would discharge and you would be unable to unlock or wake up the Tesla vehicle to drive. This is not the case. If you read the Tesla Model Y Owners Manual there is no mention of connecting a 12V battery tender to the 12V battery if you leave the Tesla vehicle parked, not driven. The Owners Manual states that you should leave the Tesla vehicle plugged in so that the vehicle can charge the lithium battery pack as needed. The Tesla vehicle automatically maintains the charge of the 12V battery.
 

Coulombian

Member
Jun 12, 2021
22
30
OC
You can blame the entire auto industry for its reluctancy of moving away from 12V. It's been known for many decades that a higher voltage system (39V?) is more efficient and better in general but there's so much inertia in the OEM parts industry that only now there are some movement in changing, due to HV system in EV.
It really has little to do with the voltage. Yes, a higher voltage system will allow more efficiency by allowing for smaller wire sizes in the vehicle harnesses (true of the low voltage and high voltage buses by the way), thus reducing weight but it doesn't effect the reasons we need the independent bus in the first place.

It more has to do with safety/ADAS, range/energy, and use of the low voltage bus as the independent power source for the main BMS controller, battery contactors, and all the other ECUs. Sure, you can say why not just use the DC/DC converter(s) with a constant connection to HV, but you would lose a level of redundancy that the LV batteries provide.
 
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T-Mom

Member
Jul 27, 2019
64
85
Tahoe City & SF bay
When a mobile tech was installing homelink onto the MY. He was telling me all about the components under the frunk. He mentioned to me that the 12v battery does not get charged when the MY is plugged in. He said it only charges when the vehicle is on and being driven. I thought it was strange. He said some people put a trickle charger on the 12V when they park it for an extended period. Can anyone confirm that's actually the case?
was it the mobile tech first day on the job after he gave up his career as a gardener?
 

Watts_Up

Active Member
Mar 4, 2019
3,754
2,728
In a galaxy far, far away
When a mobile tech was installing homelink onto the MY. He was telling me all about the components under the frunk. He mentioned to me that the 12v battery does not get charged when the MY is plugged in. He said it only charges when the vehicle is on and being driven. I thought it was strange.
This is totally B.S.

You should install a 12 V Battery Monitor to check the status of your battery using a phone App.

This is what I noticed using such battery monitor:

The 12 V battery has its own charger connected to the Tesla propusion battery. This charger is independant of the fact that the car is plugged or is driven.​
When the car is parked, plugged or not, if the battery voltage value get beloww 12.55 V,​
the charger get activated and generates a volatge of about 14.50 V for about two hours to charger the battery.​
This occurs every 36 hours to 48 hours, depending of the ambiant temperature and how the 12 V battery keeps its charge when the battery get older....​
After two hours when the charger shutdown, the battery is around 13 to 13.5 V and the car goes back to sleep mode.​
When the car is parked, after about 5 minutes the car goes to sleep, unless you use Sentry or set the Camp mode...​
In this case the charger get activated and generates a volatge of about 13.50 V to avoid the 12 V to get discharged.​
When car is driven, the charger is activated and generates a volatge of about 13.50 V to avoid the 12 V to get discharged.​
However if the car notices a high level of 12 V usage (high beam, radio amplifier...) the charger generates a volatge of about 14.50 V to keep the battery charged.​




He said some people put a trickle charger on the 12V when they park it for an extended period. Can anyone confirm that's actually the case?
This is completely useless since the 12 V battery has already its own charger.

A trickle charger is only needed on an ICE car because the only way to charge the 12 V battery is when the alternator is running.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,083
8,702
Boise, ID
This is completely useless since the 12 V battery has already its own charger.

A trickle charger is only needed on an ICE car because the only way to charge the 12 V battery is when the alternator is running.
It's not totally useless, but those are two completely separate issues.

Yes, with the gas car, the 12V will not have any way to recharge itself while the car is off, so it can go dead.
With the Teslas, yes, the cars can recharge them.....over and over and over and over and over and over and over and over. It runs the 12V battery down, supporting all of those electronics loads in the car until the voltage in the battery gets low, and then the car will wake up enough to use the main battery pack to refill it. And then the 12V is on its own running down again until the next refill cycle. It will be putting a lot of cycles and wear on that 12V if it sits for months or years like that. So the point of the external charger is just to supply that electronics energy externally, so the battery isn't getting run up and down over and over, shortening its life.
 

Watts_Up

Active Member
Mar 4, 2019
3,754
2,728
In a galaxy far, far away
It's not totally useless, but those are two completely separate issues.

Yes, with the gas car, the 12V will not have any way to recharge itself while the car is off, so it can go dead.
With the Teslas, yes, the cars can recharge them.....over and over and over and over and over and over and over and over. It runs the 12V battery down, supporting all of those electronics loads in the car until the voltage in the battery gets low, and then the car will wake up enough to use the main battery pack to refill it. And then the 12V is on its own running down again until the next refill cycle. It will be putting a lot of cycles and wear on that 12V if it sits for months or years like that. So the point of the external charger is just to supply that electronics energy externally, so the battery isn't getting run up and down over and over, shortening its life.
I see your point, but if you have already a 110 V plug for a 12 V trickle charger, why not also (or instead) plugging the Tesla Mobile Connector to keep the propulsion battery charged?

However if you are parked for several weeks at the airport, having a solar trickle charger for the 12 V battery could avoid the car to weak up, like every 36 hours for two hours in my case,
thus lowering the phantom drain, which I noticed during COVID was around 3% for two weeks when parked unplugged in my garage.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,083
8,702
Boise, ID
I see your point, but if you have already a 110 V plug for a 12 V trickle charger, why not also (or instead) plugging the Tesla Mobile Connector to keep the propulsion battery charged?
Because if you are supplying those 12V electronics loads externally, there is almost no drain from the main propulsion battery AT ALL. It stays with the contactors opened up, disconnected from the car and can sit for a couple of years like that with very little loss. That only runs down from having to transfer energy to refill the 12V battery. So if you've got a battery tender on the 12V for long term, there's no need to charge the car.
 
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