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Option to only charge Traction battery during 12V battery Topoff [feature request]

rrolsbe

Member
Feb 18, 2017
239
132
Albuquerque
If we had an option to only charge the traction battery while the car was awake (usually this is because the 12V battery is being topped off) the awake overhead could be reduced. The 12V battery in my three year old Model 3 requires a top off about every 18 hours and the time to perform this function is 2 hours. I would assume a newer battery would not need to be topped off for 24 hours or longer? My charge rate at home is 21MPH so as long as my average daily miles driven is under around 39 miles per day (allowing for the phantom drain power used while the car is sleeping) no additional charging would be required. In this example, the power savings per month could approach 7+ KWH. That might not seem like much but over many years it adds up; likewise, if the current fleet of Tesla's saved even 4KWH per month it becomes more significant. On a side note, it would be nice to have a selectable option in the Tesla phone app that sends a message when the car wakes up and and another message right before entering sleep state.
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
10,428
12,681
San Diego
It might not be quite as much savings as you think because I believe the 12V is often partially topped up during the charge. Definitely some randomness to it though.

A good idea, but not sure exactly what the savings would actually end up being. It depends on the use case.

If people drive a lot, the 12V might be usually topped off while driving (and charging events would be longer so also some increased probability of top off then), for example. So nearly zero savings in that case.
 

elptxjc

Member
Dec 15, 2019
833
168
El Paso, TX
When the 12V battery needs charging, it gets charged the same, whether the car is charged daily, or once a week (or whatever the case might be), correct?
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
10,428
12,681
San Diego
When the 12V battery needs charging, it gets charged the same, whether the car is charged daily, or once a week (or whatever the case might be), correct?
The point of the original post was to not wake the car up specifically to do the charge of the TRACTION battery (not the 12V). So the idea was do have a different charging regimen that opportunistically charges the traction battery when the car happens to wake up to top off the 12V. Thus saving many 10s of Wh or maybe even over 100Wh per day.

My point was that for a driver who drives regularly, the car may never wake up to top off the 12V (because it can easily take 18 hours to discharge, and it would be topped off while driving). So there wouldn't be any opportunities to opportunistically charge the traction battery.

There are probably use cases where it would help (people who don't drive very much, or people who can tolerate a vehicle that does not fully recharge reliably overnight). Basically, you just need to be able to fully recharge the vehicle within 30-60 minutes (the approx 12V topup period). But this situation might not save much, because it's entirely possible that the car would not wake to charge the 12V, anyway. It would just top it up during the traction battery charge...
 

Durzel

Active Member
Jul 17, 2019
3,453
2,408
Bath, UK
Why would the 12v battery be topped off while driving? The car doesn't have an alternator.

Given how this self-charging system works I would presume it actually can't do it while you're driving, because the HV battery is already "connected" in a different configuration.

I think the problem with your idea is that the vehicle charging the 12v on demand is technically free, as it just uses power from the HV battery, but the car charging the HV battery at sub-optimal times (i.e. when electricity is at its most expensive) would not be welcomed by most people, and would lead them to consider not leaving the car plugged in, etc.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
6,660
7,953
Boise, ID
Why would the 12v battery be topped off while driving? The car doesn't have an alternator.

Given how this self-charging system works I would presume it actually can't do it while you're driving, because the HV battery is already "connected" in a different configuration.
It's not an alternator, but it's called the DC to DC conversion circuit. It's taking some energy from the high voltage battery and converting that to about 13-14 or so volts to recharge the 12V battery. This circuit is active when the car is on, so yes, it is basically "topping up" whenever the car is on and in use.
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
10,428
12,681
San Diego
yes, it is basically "topping up" whenever the car is on and in use.

Yes, it is either "floating" it around 13.3V or so, or stepping up the voltage to 13.8-13.9V to charge it. Depends on whether it has decided whether the 12V needs a charge or not (not sure how it tells when this is needed - I assume by monitoring the current flow into it).
 
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