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12V battery charging, voltage limits, capacity, etc.

orangezero

New Member
May 15, 2019
1
0
Illinois
I've tried and failed to search in the various similar threads already, lots of good info. I've seen someone list 230w as a minimum the tesla model 3 uses when "awake" and may be more like 25-50w when sleeping.

What I'm after is more how the 12V battery is monitored and charged by the software. This has changed several times it appears. Some seem to mention their car charges every few hours on a regular schedule. At 230w of power draw, before powering anything else, I can see why this constant charging is needed. I can also see why tesla might find a lead acid as the most appropriate per cost of the options at the the model 3 was released. And also why they fail, as lead acid really are worked differently than ICE cars where they need a large burst to start the engine.

I imagine the computer recognizes the voltage drops below 11v (or whatever the trigger is) and then it charges until the voltage gets above 14.2v (again, random voltage I've just made up). Unfortunately, I haven't been able to find a single discussion of when the main battery is used to charge the 12v.. any idea? I hope to start getting some measurements for my own car with a multimeter soon.

It appears the lifepo4 battery options are $$$ and not exactly the identical charging requirements, but perhaps is within the tolerance of the cells in regards to voltage and so things generally work okay? I saw one guy report back numerous times about his ohmmu battery and seemed happy.

I'm not quite sure I've seen anyone comment on lifepo4 12v battery replacement in colder climates. As the temperature decreases, it eventually becomes a bad idea to use or charge a lifepo4. It seems rather vulnerable to heat loss being just below the windshield.

So, for example, lets say hypothetically I filled my frunk with a HUGE 12v battery. Would this be good since the tesla wouldn't have to charge the 12v nearly as much? It would just sense the voltage and see it still at a high voltage and not charge. Or would this cause some unknown problem I haven't considered. Or am I all wrong and tesla is actually monitoring watt hours used and not using voltage?

Thanks for anyone providing any details about voltage questions. Sorry, just trying to wrap my head around options.
 

Sophias_dad

Active Member
Supporting Member
Jul 29, 2018
1,867
2,066
Massachusetts
So, for example, lets say hypothetically I filled my frunk with a HUGE 12v battery. Would this be good since the tesla wouldn't have to charge the 12v nearly as much? It would just sense the voltage and see it still at a high voltage and not charge. Or would this cause some unknown problem I haven't considered. Or am I all wrong and tesla is actually monitoring watt hours used and not using voltage?
I'd wager that the huge 12V battery would self-discharge at a rate SIGNIFICANTLY larger than the small 12V battery, and the HV battery would be used even more than with the small 12V battery. If we assume its just monitoring voltage, it might take longer to get to the trigger voltage, but when it did, the HV battery would need to dump a lot more power toward recharging the huge battery in the trunk.

I'd assume its just monitoring voltage, since most types of battery care more about going below a particular voltage than other factors. I mean, lets suppose they are instead monitoring power delivery from the 12V. What if there's drift? What if the capacity of the battery actually decreases over time, and now they are oscillating between 10V and 12.5V instead of 11V and 13.5V. I'd imagine that eventually even the lead acid battery won't recharge well after getting down to the 9V arena repeatedly.
 

rrolsbe

Member
Feb 18, 2017
308
191
Albuquerque
The following is how the 12V charges on my 2018 M3 Long Range RWD. If the UMC/Wall connector is not plugged into the car, the car wakes up to top off the 12V battery approx every 17 hours and charges for approx 2 hours. When the voltage drops to around 12.5V the two hour charging window begins. It charges at around 14.55V during the two hour charge cycle and the float voltage after charging is complete/contactors reopen is around 13.1V to 13.2V. If you drive the car once or multiple times daily, the 17 hour sleep window between top-off obviously varies . This charging regime has changed significantly over the past few years. At the start of covid, anytime the car was awoken for ANY reason it would many times stay awake for six to eight hours and the charging voltage was close to 15V the entire time. This made the phantom drain quite significant and probably was not good for the 12V battery. When I measured the sleep power draw 18 months back, it came in around 7 to 8 watts. The power currently required to replenish the phantom drain (power used when sleeping) and the associated overhead while charging is around 0.5KWH/day. AFAIK, most or all of the above is fairly accurate but if not corrections are welcome! Hope this helps you and other forum members.

Side note: One improvement that might extend the life of the 12V battery would be to top off every seven to eight hours instead of 17 hours. Doing this could minimize the Depth of Discharge and extend the 12V battery life while still using approx the same 0.5KWH of power daily.

Regards, Ron
 
  • Informative
Reactions: AlanSubie4Life

Candleflame

Active Member
Mar 9, 2015
3,678
2,009
QLD, Australia
I've tried and failed to search in the various similar threads already, lots of good info. I've seen someone list 230w as a minimum the tesla model 3 uses when "awake" and may be more like 25-50w when sleeping.

What I'm after is more how the 12V battery is monitored and charged by the software. This has changed several times it appears. Some seem to mention their car charges every few hours on a regular schedule. At 230w of power draw, before powering anything else, I can see why this constant charging is needed. I can also see why tesla might find a lead acid as the most appropriate per cost of the options at the the model 3 was released. And also why they fail, as lead acid really are worked differently than ICE cars where they need a large burst to start the engine.

I imagine the computer recognizes the voltage drops below 11v (or whatever the trigger is) and then it charges until the voltage gets above 14.2v (again, random voltage I've just made up). Unfortunately, I haven't been able to find a single discussion of when the main battery is used to charge the 12v.. any idea? I hope to start getting some measurements for my own car with a multimeter soon.

It appears the lifepo4 battery options are $$$ and not exactly the identical charging requirements, but perhaps is within the tolerance of the cells in regards to voltage and so things generally work okay? I saw one guy report back numerous times about his ohmmu battery and seemed happy.

I'm not quite sure I've seen anyone comment on lifepo4 12v battery replacement in colder climates. As the temperature decreases, it eventually becomes a bad idea to use or charge a lifepo4. It seems rather vulnerable to heat loss being just below the windshield.

So, for example, lets say hypothetically I filled my frunk with a HUGE 12v battery. Would this be good since the tesla wouldn't have to charge the 12v nearly as much? It would just sense the voltage and see it still at a high voltage and not charge. Or would this cause some unknown problem I haven't considered. Or am I all wrong and tesla is actually monitoring watt hours used and not using voltage?

Thanks for anyone providing any details about voltage questions. Sorry, just trying to wrap my head around options.

the car uses like 5w when sleeping. And tbh more recently sleep seems to use even less as evident by the car being able to slumber for much longer.

the car doesnt use a normal lead acid battery. it uses a gel deep cycle battery. constantly recharging the battery is healthy for both those gel batteries as well as traditional lead acid and leads to increased longevity.

The car monitors the voltage of the battery as evident by shorter intervals between waking and recharging the older the battery gets - however, over the last 6 months or so i have noticed a drastic increase in sleeping intervals suggesting that sleep is even more efficient now. From my old calculations it waits til the 12v is at around 20% calculated capacity. i think they are 35-40 ah batteries so you get around 400w out of them before they have to be recharged - or 2.5 days until 20% and a car wakeup.

a lithium ion battery is a waste of money and causes issues with the 12v system for some cars.
 

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