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

[orangezero]

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]

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]

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

 

[Candleflame]

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.

 

[rrolsbe]

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

Update: Found out my OBDLink LX was powered while my M3 was sleeping. Not sure why that changed or when. I unplugged the OBDII adapter and the car now sleeps around 28 hrs before topping off the battery; however, the bad news is it takes about three hours to perform the top off instead of two. So not much phantom drain power savings but the contactors and 12V battery get cycled less often.

 

[voltaholic]

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.

I would like to know WHAT issues a lithium replacement battery causes with the 12V system, and, only for " some cars"..? I am seriously looking to buy one.

 

[dfwatt]

I would like to know WHAT issues a lithium replacement battery causes with the 12V system, and, only for " some cars"..? I am seriously looking to buy one.

This is a subject for endless speculation, guesswork, trolling, and all kinds of other fun stuff on the forum.

The problem is that even though lithium iron phosphate batteries have virtually identical float voltages and charging voltages, which has allowed them for years to be swapped out with lead acid in lots and lots of ICE vehicles typically without issues, that kind of a relatively seamless and painless process was disrupted in the fall of 2021, when Tesla began roughly a year of experimenting and tweaking of the 12-volt subsystem including insertion of, we suspect, a so-called desulfation routine.

This is a routine that basically does a short bit of extra charging, once the battery has reached maximum charge. Of course this has to be fairly brief or otherwise it damages the battery but it can be done with Pb-sulfuric acid chemistries . These can help with the desulfation of lead plates (a primary correlate of mostly non-reversible battery aging) and the speculation is that Tesla was tired of hearing about poor longevity of their lead acid batteries and threw this in there, in the (vain?) hope that it was going to extend the lifespan of the standard 12 lead acid. This perhaps combined with other changes that BMS systems in various aftermarket lithium iron phosphate battery read as dangerous overcharging appears to cause the handshaking protocols between the battery and the 12 volt system to fail and then for the operating system to reject the battery as defective.

it is worth emphasizing that none of these issues existed for the better part of three years, and that it's only been since last fall that Model 3/Ys where this swap out has been very popular and have experienced troubles with their aftermarket batteries. if you read the real experts on this lithium ferric phosphate chemistry (not the self-styled experts that sometimes post on this forum), they will warn you about desulfation routines. They are not compatible with this chemistry. It would be one thing if all this tweaking and tuning meant that the lead acid systems on Tesla's that the lead acid systems on Tesla's lasted forever. Unfortunately it's just the opposite. There's no evidence that the frequent failure rate of stock batteries has materially improved. We also had failure of our 12 volt systems in our Florida powerwalls which meant that the power walls were basically bricked if drained. Not permanently damaged but not usable either. All of this goes to the evidence that Tesla does not know what they are doing with lead acid chemistries. That comment will probably open the door to multiple trolling responses but that's a fact.

Long story short - right now this is a "Tweakers Paradise" - if you are okay with tweaking your system and you're okay with doing occasional resets, and you like the idea of saving weight and also some energy (because after all these batteries don't have the internal losses or the heavy charging losses of lead acid), it might still be worth it, but it's not a set and forget deal anymore. Ohmmu has done a lot to try to get their internal BMS to accommodate to all this and have it conform with Tesla's handshaking but it's a moving Target, and even their latest version 4+ may get rejected by the 12 V operating system. Whether all of this settles down and goes back to the previous seamless and painless deal that it once was is anyone's guess. Caveat emptor.

 

[AlanSubie4Life]

It would be one thing if all this tweaking and tuning meant that the lead acid systems on Tesla's that the lead acid systems on Tesla's lasted forever. Unfortunately it's just the opposite. There's no evidence that the frequent failure rate of stock batteries has materially improved.

Probably too early to say, and only Tesla would know, in any case. I guess if 5 years down the road we have lots of people with batteries replaced this year (after 3 years) with batteries still going strong, then we’ll know it worked (maybe)?

 

[Flybyglass]

This is a subject for endless speculation, guesswork, trolling, and all kinds of other fun stuff on the forum.

The problem is that even though lithium iron phosphate batteries have virtually identical float voltages and charging voltages, which has allowed them for years to be swapped out with lead acid in lots and lots of ICE vehicles typically without issues, that kind of a relatively seamless and painless process was disrupted in the fall of 2021, when Tesla began roughly a year of experimenting and tweaking of the 12-volt subsystem including insertion of, we suspect, a so-called desulfation routine.

This is a routine that basically does a short bit of extra charging, once the battery has reached maximum charge. Of course this has to be fairly brief or otherwise it damages the battery but it can be done with Pb-sulfuric acid chemistries . These can help with the desulfation of lead plates (a primary correlate of mostly non-reversible battery aging) and the speculation is that Tesla was tired of hearing about poor longevity of their lead acid batteries and threw this in there, in the (vain?) hope that it was going to extend the lifespan of the standard 12 lead acid. This perhaps combined with other changes that BMS systems in various aftermarket lithium iron phosphate battery read as dangerous overcharging appears to cause the handshaking protocols between the battery and the 12 volt system to fail and then for the operating system to reject the battery as defective.

it is worth emphasizing that none of these issues existed for the better part of three years, and that it's only been since last fall that Model 3/Ys where this swap out has been very popular and have experienced troubles with their aftermarket batteries. if you read the real experts on this lithium ferric phosphate chemistry (not the self-styled experts that sometimes post on this forum), they will warn you about desulfation routines. They are not compatible with this chemistry. It would be one thing if all this tweaking and tuning meant that the lead acid systems on Tesla's that the lead acid systems on Tesla's lasted forever. Unfortunately it's just the opposite. There's no evidence that the frequent failure rate of stock batteries has materially improved. We also had failure of our 12 volt systems in our Florida powerwalls which meant that the power walls were basically bricked if drained. Not permanently damaged but not usable either. All of this goes to the evidence that Tesla does not know what they are doing with lead acid chemistries. That comment will probably open the door to multiple trolling responses but that's a fact.

Long story short - right now this is a "Tweakers Paradise" - if you are okay with tweaking your system and you're okay with doing occasional resets, and you like the idea of saving weight and also some energy (because after all these batteries don't have the internal losses or the heavy charging losses of lead acid), it might still be worth it, but it's not a set and forget deal anymore. Ohmmu has done a lot to try to get their internal BMS to accommodate to all this and have it conform with Tesla's handshaking but it's a moving Target, and even their latest version 4+ may get rejected by the 12 V operating system. Whether all of this settles down and goes back to the previous seamless and painless deal that it once was is anyone's guess. Caveat emptor.

So far with the V4+ December build it has been running for 3 weeks perfectly with no issues/warnings or messages. Software change occurred in early December (minor adjustments but could have made the difference) and the app is expected to have some functionality improvements in February.

 

[dfwatt]

I've had to do two resets in the last month. Both once again associated with long periods of sitting where the car is not doing anything for days. That seems to be an area where some kind of mismatch or failure of the handshaking protocol between the battery operating system and the vehicle 12 volt sub system manifests itself. The other thing I've noticed that seems to be associated with some kind of handshaking failure is that it appears after the float voltage on the lithium ion gets above 14.6 or 14.5. What's puzzling is that if you try to manually charge the ohmmu battery past 14, the battery's internal operating system disconnects it. At least that's what I found when I got the new version 4 plus.

So I don't even understand how it can get to 14.5 or 14.6 as a float voltage. Anyway, I'm okay with doing an occasional reset and since I've done so many it takes me literally a minute. In exchange for that bit of hassle once in awhile I get a much more efficient 12 volt subsystem with less parasitic loss due to the differences between lead acid and lithium iron phosphate chemistries in terms of how efficiently they charge. Plus I get significant weight reduction and of course the ability to see what's going on with the battery through the app. People forget that the 15% efficiency difference between those chemistries means that 15% of every kilowatt hour that goes in and out of that battery system is something you're going to save and that's ultimately going to contribute to your vehicle range. That's not a lot each charge cycle but it adds up.

 

[dfwatt]

I've had to do two resets in the last month. Both once again associated with long periods of sitting where the car is not doing anything for days. That seems to be an area where some kind of mismatch or failure of the handshaking protocol between the battery operating system and the vehicle 12 volt sub system manifests itself. The other thing I've noticed that seems to be associated with some kind of handshaking failure is that it appears after the float voltage on the lithium ion gets above 14.6 or 14.5. What's puzzling is that if you try to manually charge the ohmmu battery past 14, the battery's internal operating system disconnects it. At least that's what I found when I got the new version 4 plus.

So I don't even understand how it can get to 14.5 or 14.6 as a float voltage. Anyway, I'm okay with doing an occasional reset and since I've done so many it takes me literally a minute. In exchange for that bit of hassle once in awhile I get a much more efficient 12 volt subsystem with less parasitic loss due to the differences between lead acid and lithium iron phosphate chemistries in terms of how efficiently they charge. Plus I get significant weight reduction and of course the ability to see what's going on with the battery through the app. People forget that the 15% efficiency difference between those chemistries means that 15% of every kilowatt hour that goes in and out of that battery system is something you're going to save and that's ultimately going to contribute to your vehicle range. That's not a lot each charge cycle but it adds up.

It seems pretty clear that the 12 volt sub system rejecting the lithium iron phosphate battery is significantly related to cold weather. I've only had this an issue when the night time temperatures dip into the 50s or lower. Don't know if that squares up with what other people have experienced. Risk of having to do a reset is heightened if the car is sitting for multiple nights or days in cooler weather.

 

[Flybyglass]

It seems pretty clear that the 12 volt sub system rejecting the lithium iron phosphate battery is significantly related to cold weather. I've only had this an issue when the night time temperatures dip into the 50s or lower. Don't know if that squares up with what other people have experienced. Risk of having to do a reset is heightened if the car is sitting for multiple nights or days in cooler weather.

I found under 40f, charging issues. The car trickle charges below 1 amp (sometimes .01-.05 amps) along with momentary discharging at the same rates. This was observed in service mode LV battery. I suffered a discharge and no charging at 31F. I put the Ohmmu on the bench with a 1 amp trickle charger overnight and the battery discharged!. I swapped for a 5 amp charger and the battery then charged to 100% normally. So temperature affects low amperage charging. The self heating technology added to the Ohmmu's is supposed to remedy and allow low amperage low temperature charging.

 

[dfwatt]

I found under 40f, charging issues. The car trickle charges below 1 amp (sometimes .01-.05 amps) along with momentary discharging at the same rates. This was observed in service mode LV battery. I suffered a discharge and no charging at 31F. I put the Ohmmu on the bench with a 1 amp trickle charger overnight and the battery discharged!. I swapped for a 5 amp charger and the battery then charged to 100% normally. So temperature affects low amperage charging. The self heating technology added to the Ohmmu's is supposed to remedy and allow low amperage low temperature charging.

Interesting yet another curveball that Tesla appears to throw at aftermarket BMS. One question is is this all 4+ editions of the battery that have heating Tech? Also does the version 2.0 of the app allow you to tell what firmware you have in the battery?

 

[Flybyglass]

Interesting yet another curveball that Tesla appears to throw at aftermarket BMS. One question is is this all 4+ editions of the battery that have heating Tech? Also does the version 2.0 of the app allow you to tell what firmware you have in the battery?

I think it is a heating element added to the most recent models. On the site it shows which ones have heating and the number is growing. I added an external module to record charging cycles and behavior to see what the car is doing. I found after one day of no driving the car would wake every 4-6 hours at 12.8 volts, charge a few minutes up to 14.3 and shut down.

 

[ATPMSD]

This is my understanding

When the car is awake, assuming the 12v battery is charged, it does not go through a drain and charge cycle per se. When the car is awake the battery acts to stabilize the 12v circuit. Some devices draw a high load and will cause a voltage drop, but battery keeps the voltage up to prevent these drops. These small drains are then topped off by the nominal line voltage provided by the DC-DC converter.

And as to the 12v lead-acid battery discharge / charge cycle when the car is asleep - at one time the 50% rule was followed. That is, when the car is asleep the battery supplies the system until it reaches 50% SOC at which point recharging begins.

 

[Flybyglass]

This is my understanding

When the car is awake, assuming the 12v battery is charged, it does not go through a drain and charge cycle per se. When the car is awake the battery acts to stabilize the 12v circuit. Some devices draw a high load and will cause a voltage drop, but battery keeps the voltage up to prevent these drops. These small drains are then topped off by the nominal line voltage provided by the DC-DC converter.

And as to the 12v lead-acid battery discharge / charge cycle when the car is asleep - at one time the 50% rule was followed. That is, when the car is asleep the battery supplies the system until it reaches 50% SOC at which point recharging begins.

This is what I charted a few days ago .

 

[ATPMSD]

This is what I charted a few days ago .

Makes sense as it looks like you have a 16V lithium battery. The same article I read stated these batteries have a much smaller capacity and the recharge point is 80% SOC.

 

[dfwatt]

Makes sense as it looks like you have a 16V lithium battery. The same article I read stated these batteries have a much smaller capacity and the recharge point is 80% SOC.

Did you actually read the thread? You are out in left field.

 

[AlanSubie4Life]

yet another curveball that Tesla appears to throw at aftermarket BMS

Not following details, but it seems reasonable for Tesla to flag a 12V battery as bad if it does not accept charge in cold temps. That’s a bad lead-acid battery, or at least something is wrong with it!

Does not seem like a curveball; it seems like normal sanity checking.

If the Ohmmu is to properly operate, it needs to look like a 12V battery regardless of temperature, at all times (there's no reason for Tesla to support a lithium-ion 12V with software; they have a hard enough time with the regular 12V!). Otherwise the car will probably flag it as bad and do something which is incompatible with the Ohmmu.

In addition, it sounds like the Ohmmu does not behave correctly when charged at low amperage (see 1A story above) and somehow paradoxically discharges when it's supposed to be charging (not at all sure how this happens but maybe there is a shunt in the Ohmmu BMS (this seems weird to me so I really have no good way to explain the observation)?)

When the car is awake the battery acts to stabilize the 12v circuit. Some devices draw a high load and will cause a voltage drop, but battery keeps the voltage up to prevent these drops. These small drains are then topped off by the nominal line voltage provided by the DC-DC converter.

My understanding is that the 12V is not needed (the car will allegedly run just fine without a 12V - though I've never tried it - I think @rrolsbe has though I'm not sure). Most likely there is enough decoupling capacitance on the DC-DC output and the 12V main rail to take care of any startup demands (high load wouldn't be an issue as long as it is steady since the DC-DC will have no problem with that - it's the rapid changes in demand that are important and need to be properly decoupled or otherwise managed, via a ramp-up in current). Again, I don't really know, but if it's possible to operate without a 12V (of course the car would not sleep!), that would have to be the case. It's not clear to me exactly how effective a 12V is for decoupling, anyway. Even they need decoupling for certain types of demands (they have inductance as well).

 

[Flybyglass]

Makes sense as it looks like you have a 16V lithium battery. The same article I read stated these batteries have a much smaller capacity and the recharge point is 80% SOC.

I have a 12 volt system.