Feb 12, 2016
Todd Burch For years, there has been speculation about when battery balancing occurs. Some speculated it occurred at 90% or higher. Others speculated that it only occurs with the battery fully charged.
I had my seatbelt inspection today, and while there the service tech was checking a few things in my diagnostic screens--the BMS tab happened to be up. This reminded me to ask about balancing, since everyone seems to have that question. What I learned is that the pack is *always* balancing. All the time. Which makes perfect sense and agrees with other things we've seen.
A while ago wk057 disassembled a pack and discovered bleed resistors. These bleed resistors are what balance the pack, by very slowly consuming power from the highest voltage cells to keep everything balanced. Since the bleed resistors are so tiny, the only way they can really be effective is if they are balancing all the time, and it looks like that is indeed what's happening.
This confirms a few things:
1. You don't need to charge to any charge level for the pack to balance. It always happens.
2. Running the pack down to near empty, then fully charging, appears to improve range, but of course this is only imaginary. Doing this doesn't help balance anything, it simply helps the range estimation algorithms get a better view of the voltage range of your pack. Don't do this to improve your range numbers--you're simply stressing the pack for no good reason. (This was generally accepted by many here to be the case).
3. Charging to 100%, and letting it sit there for a few hours (which in my recent experiments seemed to be helpful for indicated range), doesn't improve balancing at all. Again, it just seems to help the algorithms *maybe* improve the *indicated* range.
4. The long, long taper at the end of a 100% charge is not balancing. It's simply providing sips to the pack to top it off.
5. Apparently, an update is coming soon that will improve the algorithm. (This one isn't that newsworthy, but it's nice to hear).
So hopefully with this info owners will not go and abuse their pack to try to "initiate" a balancing session. As has been Tesla's suggestion all along, don't worry about the pack, and just let the BMS do it's thing.�
Feb 12, 2016
Ampster My only comment is that I has to be tricky for the BMS to turn on the bleed resisters because of the flat charge curve. Perhaps this chemistry has more slope than the LIFEPO4 cells I have observed.�
Feb 12, 2016
Todd Burch My best guess is that they try to get a good voltage estimate at certain times. For instance, if you fully charge, they can get a better idea of the voltage differences, then they record how much needs to be bled off and slowly do that. Maybe.�
Feb 12, 2016
AndY1 I know the system. It's similar to my RC Helicopter, when I charge the 6 cell pack. All the cells are being monitored and actively balanced when charging.
I believe, this in one of the causes of the 'vampire drain'. So many thousands of tiny cells, being balanced 24/7 consumes power. Some of those 50 watts of vampire drain is coming from this.�
Feb 12, 2016
David29 This makes eminent sense. It translates to another example of letting the battery management system worry about the battery (within limits).
Another way to say it is, don't go by folklore -- regardless of how well-intentioned the proponent or how well-versed he or she may seem to be -- if it isn't in the Tesla owner's manual, then most likely we do not need to know.�
Feb 12, 2016
Todd Burch Yes--certainly this contributes to vampire drain. I think wk057 listed it in his pack dismantling thread, but don't recall the value. I think it was pretty small though....maybe a few watts?�
Feb 12, 2016
MP3Mike Are you 100% sure about that?
I thought that wk057 found, during his hacking/exploration, that the balancing doesn't kick in until you hit the 93% charge level.�
Feb 12, 2016
Todd Burch Well nobody's information is 100% definitive I guess, but I don't see why they would impose a limit of 93% in order to initiate balancing. What would be the purpose of that, especially since there are people that almost never charge to that level? It makes more sense to just always be bleeding off higher cells. Those resistors are so small that it's a time-consuming process, and one in which you really wouldn't notice the consumption from day-to-day.�
Feb 12, 2016
CHG-ON I just do what Tesla tell me and I live in ignorant bliss:tongue:. It is Saint Elon, after all!
Seriously though, I simply do not worry about it. I have never had a problem with range. Except once, when it was completely my own fault. But I made it home. I learned now much headroom I need, based on my driving habits and terrain and just charge to that. Seems to be just fine. Even on trips. I used to have my battery display miles and worry about it. Now I use percent, just like the way I used to look at my gas tank.�
Feb 12, 2016
Todd Burch I should add that this tech has been with Tesla I believe since 2012, has 2 EVs of his own, and seemed to be very knowledgeable about everything I asked him--his knowledge was certainly above a typical tech who might be blowing some smoke.�
Feb 12, 2016
wk057 Well, a few notes based on my experience with the battery packs and the BMS.
I think the person you spoke with may be confused a little. Yes the bleed resistors on the BMS module boards can be active at any time at any charge level. However, unless they recently changed something, they never appear to get enabled until the pack reaches the constant-voltage phase of charging, at which point the BMS knows for sure which cells are higher than others and which ones to bleed. The resistors then stay enabled for a calculated period of time (based on the delta between the highest and lowest cells) and I guess you could then consider that "always" balancing... but it only starts once the pack hits CV, which is roughly 93% SoC (can be a little higher).
Correct, with the exception of the above about getting the bleeders enabled if it's been a while. Otherwise the rest is basically an illusion.
Same as above. The sitting at 100% is the bad part.
Correct, although the balancing is enabled at this time if required. The taper is the constant-voltage phase of the charge where the cells have reached peak voltage, but not peak capacity.
This would definitely be welcomed and hope it's true.
In any case, 99% of the time the BMS does a great job with the pack. Almost no one's pack is *actually* out of balance enough to lose any range. That's just a myth pretty much. Almost all range loss is due to the algorithm used to calculate capacity being miscalculated due to charging habits. If you never charge to 90%+ and/or never discharge to <10%, then eventually the error in the algorithm will start to add up in the form of false capacity loss. Basically Tesla just needs a way to let the BMS reset this and re-calibrate when possible, potentially by monitoring the potential sum of the calculation error and encouraging whatever type of charge or discharge is needed to help to the user when it's needed to re-calibration. Probably too complicated, so my guess is most likely they're going to refine the algorithm based on real world data from 10s of thousands of vehicles in the field in order to refine it a bit, although it still won't be perfect.
Long story short... just don't worry about it. If you've lost a substantial amount of range, just call Tesla and let them deal with it. There isn't much that you can do to "fix" it.�
Feb 12, 2016
JenniferQ I just switched to percent from miles on battery a couple of weeks ago and it felt like I'd finally "arrived" to full EV mindset at that moment. A very nice feeling. And after 6000 miles and 4 months with the car, I really don't worry about the battery at all anymore either.�
Feb 12, 2016
wk057 Well, I used percent for a while myself. Unfortunately, with only 1% resolution it's almost 3x less precise than rated miles. So, I switched back to rated miles so I had more granularity... not that the "miles" mean anything to me other than a capacity available measure. If Tesla exposed 0.1% resolution (it's there, they just round down to the nearest whole number it looks like on the UI when using percent) then I'd never look at a "miles" rating ever again.�
Feb 13, 2016
Todd Burch I don't think he was confused as much as just not going into details. I'll admit he didn't necessarily say what triggers balancing to start. I suppose I could interpret what he said to mean that the pack could be balancing while at any SOC--you don't have to be at 100%, for instance.
Regardless, I think what you state and what he stated can be pretty much in agreement.
Agree, and that makes sense too--it makes sense that the pack might have to be closer to full to perhaps get a better view of the voltage state of the cells and identify which should be bled off, and by how much.
Makes perfect sense.
I'm hoping it's a more substantial improvement in the algorithm, versus the tweaks we usually see. (Disclaimer: I understand that getting a true picture of the charge state of a pack is really hard, so I'm not dinging Tesla on this one).
I'm in complete agreement with you.
I think that based on your findings from disassembling the pack, and other corroborating evidence such as this, that we have a pretty good idea of what's going on now. And now we understand why Tesla never put any information in the manual about pack maintenance. You just don't have to worry about it.�
Feb 13, 2016
apacheguy Consensus amongst the technical crowd of this forum says that Tesla top balances. However, once the off balanced bricks light up during CV charging, the bleed resistors will continue to do their thing regardless of charge state. Perhaps this is what the tech meant when he said the pack is always balancing. Here's why:
�
Feb 13, 2016
Kipernicus Thanks for reporting your findings, Todd, and to wk057 for weighing in as well.
How do those bleeders work? Are they actively and individually switched on and off for those cells that need it?
Or is it through clever circuit design that they are bleeding off cells that are at higher voltage than other cells?
Otherwise it seems like they would be bleeding everyone at the same rate and won't help with balancing at all.
If the bleeders are only doing their thing when the cells are at peak voltage, then they shouldn't contribute to vampire loss when below 93% right?
Is this 93% peak voltage why when we first got the cars, before the battery slider was implemented, that the "standard" charge was at 93%?
The capacity miscalculation: is this why some people seem to be able to keep driving even after the range shows zero miles, while other cars shut down right at zero?�
Feb 13, 2016
apacheguy The bleed circuits each control a separate brick so only the bricks with excess Ah will trigger the bleed. There are 16 modules and 96 bricks in an 85 kWh pack.�
Feb 13, 2016
Todd Burch Actively switched on and off, yes. Someone else can correct me if I'm wrong, but this bleeding occurs at the module or brick level (not the individual cell level).
Well yes again, kinda. Again wk057 or someone else can correct me if I'm wrong (and wk057's battery disassembly thread has details on this), but I believe each module or brick has a circuit board that controls the initiation of bleeding for that module or brick.
They don't only do their thing at peak voltage. Wk057 is saying that bleeding is merely triggered at higher SOC. The bleeding can continue because the BMS can determine how much energy to bleed off of which modules or bricks--it can do that at virtually any SOC. And keep in mind these are very very small power resistors. We're talking maybe a couple watts here and there, so it won't be a significant contributor to vampire loss.
My guess is yes. My speculation is that the BMS calculates where it thinks zero ought to be, but if the algorithm is off and the actual voltage ends up being higher than expected, the BMS allows the car to continue to drive.�
Feb 13, 2016
Ampster Yes, the BMS is guessing where zero is in the aggregate and is letting the car use the pack until the pack Low Voltage Cutoff is reached or one of the modules is approaching its minimum voltage, whichever occurs first.�
Feb 13, 2016
garygid If the equilization is only activated when above 93% SoC, my car, and many others,
have never begun equalization. Also, the measurement of the 6 "cell" voltages
in each module is best done when neither charging nor driving (and not when heating
or recharging the 12v battery). Best to measure when there is essentially no current
going into, or coming out of the traction battery.
So, if making the equalization decision only while charging, and when only when over
93% SoC ... that would seem to be a substantially sub-optimal design decision.
IMO, it is more likely that the start and stop decisions are made on a "cell" basis,
in real time, and perhaps only when stopped or parked.�
Feb 13, 2016
kennybobby Here's a theory open to peer-review and open critique:
As Otmar has stated "The open circuit voltage at no load, ignoring internal resistance, can indicate SOC of a brick, but SOC is just indicating a percentage of available Ahrs from that brick."
My theory is that effective balancing can only occur when the main contactors are open, and this is also the condition when a meaningful OCV can be measured and SOC can be determined.
The balancing circuit is controlled by a TI chip, some aspects of which are noted in the Technical Forum here.
Once the desired balancing set-point voltage is loaded into the memory registers and the enable register flag is set, then balancing will be turned on whenever the brick hits that value regardless of the main contactor position, e.g. at 93% during charging. But to be effective the contactors must be open, since balance current is only ~ 100 mA for a whole brick.
There are 6 bricks per module, 16 modules per 85kWh pack. The balance resistance is 39.5 Ohms. It is left as an exercise for the reader to calculate the total power consumed if the entire pack was balancing for 24 hrs, and compare this to the vampire load.�
Feb 13, 2016
Quantum` Gary it couldn't be on a cell basis as they're paralleled in each brick with no individual MOSFETs. (that would be too expensive for such tiny cells)
Kenny, keep in mind that the balancing chip on each brick is slaved to the main BCM, it's not operating locally. This was Tesla's (wise) design choice. I was the first to identify the chip's function and to point this out.
In any case, all this 90% business is just a rumor, probably started by wk057. There is absolutely nothing about not charging the pack to full in the User's Manual nor the Service Manual, nor in any advice from the SC.
And I am sure that balancing does not occur below some given SoC as cars have sat for months and only depleted proportional to their Ri. It makes sense that balancing only occurs while charging, as a) there is no real load on the pack so true readings can be had, and b) it would reduce range if done while on the road.�
Feb 13, 2016
kennybobby My theory is that balancing is triggered by a set-point voltage, not a SOC or %. Once loaded into the memory register from the master BMS board, aka BCM, then it is free to do it's thing once the enable flag is set.
Seeing is believing--this shows local self-activation of balancing from wk057's thread, hope he doesn't mind, probably won't see it anyway due to ignoration policy...
�
Feb 13, 2016
apacheguy Yikes, this balancing stuff get complicated fast. So my best bet now is to ensure the main contactors have opened so there is no load on the pack? What do I have to do next? Do a song and dance and fan my S with Palm fronds?�
Feb 13, 2016
wk057 I don't start rumors.
That FLIR shot shows that the balancers stay on once commanded by the main BMS, and stay on until commanded otherwise. In this case it actually turned out that they had run for far too long and were in fact pulling the module OUT of balance. I had speculated that they may be acting on their own and making the decisions, but turns out they were just following their last orders. Something to keep in mind if breaking down a pack and removing the BMS.
I've confirmed that the balancers only ever enable while trying to reverse engineer the protocol used to talk to the BMBs (the boards on each module with the balancers). I could throw cell groups way out of balance, and the BMS would still not enable any balance resistors until the pack was in a CV charge mode (constant voltage, taper portion, close to full), at which point immediately the expected balance resistors came online for the groups I had not partly discharged.
This could have changed at some point, but I don't know why it would. The cells are in fact top balanced.�
Feb 13, 2016
Cottonwood It's really pretty simple. Reading from earlier posts by @wk057 and @Todd Burch, as well as insight from many other threads, here is my view of getting a pack balanced and calibrated. Comments are welcome.
- To ensure balance, charge to over 93% (I like 95% or higher to make sure), then wait a few days for balancing to complete. Going over 93% SoC triggers a good balance measurement at the constant Voltage part of the charge cycle. Once the balance measurement has been done, go ahead and use that extra SoC and discharge back to 90% or less. The balance measurement is used to bleed the charge off of stronger bricks[SUP]*[/SUP] to bring the bricks into balance over the next few days. This bleed cycle can bleed off about 1% per day of charge, so giving the process a few days, allows for a worst brick correction of a few percent. The bleed/balance operation will continue at any SoC.
- To get a good calibration of the battery pack capacity, charge once to over 90% and then discharge down to 10-20% (slightly lower is better for low end calibration). If you have time, do this after the above balance cycle; the calibration will still be pretty good if you overlap balance and calibration.
- If at all possible, don't leave your pack for too long a time at over 90% or under 20% SoC. No need to be parinoid, but try to correct back into the mid-range in less than a day or so; correcting within hours is slightly better. High SoC for long periods at high temps should be avoided.
- Charging to 100% does not help balancing, but don't worry about charging to 100% SoC before a long trip, or long segment. Just don't leave the pack at a high SoC for too long.
[SUP]*[/SUP]The battery pack in the 85 consists of 7,104 individual cells. These are arranged into 96 "bricks" of 74 cells in parallel (96*74=7,104). The 96 bricks are packaged into 16 modules of 6 bricks each. In each module, the 6 bricks are in series. The 16 modules are in series, putting all 96 bricks in series. Each of the 96 bricks has it's own bleed resistor set for balancing, i.e. there are 6 bleed resistor sets in each module.�
Feb 13, 2016
Todd Burch I'm sure the determination of how much bleeding is necessary is made after charging has stopped or slowed down significantly.
But there are over 7000 cells in the Model S. There aren't that many bleed resistors, and there isn't that much silicon to switch at such a granular level. There isn't a reason to have that many anyway, since the cells in parallel will (by nature of being in parallel) be at the same voltage.�
Feb 13, 2016
OddB
Are you saying ; Charge to 95%+ and leave the car at that SOC, without driving for a couple of days ?�
Feb 13, 2016
mspohr I think the point of what the tech told the OP is that you don't have to worry about it. The car will take care of balancing by itself and you don't have to do anything. Just follow Tesla's recommendation for daily charge up to 90% and leave it plugged in overnight when possible or when you are gone for extended periods. If some special procedure was necessary, it would be in the manual. Just relax and enjoy your car.�
Feb 13, 2016
LargeHamCollider Yes, NCA has way more slope than LFP.�
Feb 13, 2016
kennybobby fyi there's lots of brick voltage data found in Lolachampcar's CAN bus logging thread in the driving dynamics section, e.g.
�
Feb 13, 2016
Cottonwood Sorry, no; poor wording. I updated my post to try to make it more clear.
Take the SoC to 95% then discharge to 90% or less within hours. Wait for days for balancing to complete.�
Feb 13, 2016
Quantum` Or, charge it to 100% and leave it plugged in. It will only balance while plugged in.
100% is not 'radioactive'. Ask your SC.
It's clear here that balancing is only taking place at the top.�
Feb 13, 2016
cynix What do I do if I have no home charging facilities and rely on superchargers/public charging, and therefore cannot leave it plugged in for an extended period of time?�
Feb 13, 2016
Todd Burch Whoa, whoa, whoa. This information is contradictory. What do you base it on?�
Feb 13, 2016
zwede I don't see any reason it would only balance while plugged in. Seems it needs to be charged to the CV phase (>~93%) at which point the BMS figures out how many watt-hours it needs to bleed off from which bricks. It will then enable the bleed resistors and leave them on for a specific time. It doesn't matter if you drive the car before balancing is complete as the BMS already knows how much it needs to bleed off. The balancing can take days.
That's what I seem to glean out of several threads on the subject, at least.�
Feb 13, 2016
CygnusX1 Any idea where this obvious periodicity within the 96 brick voltages comes from? Different voltage drops due to different brick positions within the pack repeating in a regular order? This could explain the "balancing" at the top: When charging current is reduced, voltage drop is reduced as well... in this case, the differences in voltage would not be real but a current artefact. Just a theory...�
Feb 13, 2016
wk057 This is false. The car does NOT have to be plugged in for balancing to continue once it's started.�
Feb 13, 2016
ecarfan Ah, a voice of reason. Thank you.�
Feb 14, 2016
mkjayakumar That is exactly how I understood from reading all the posts in this thread. You summarized it nicely.�
Feb 14, 2016
Todd Burch I was just thinking that it would be nice to feed that bleed power to the 12V battery instead of burning it off as heat. I know it's not a lot of power, but would make things more efficient. Tesla probably considered something like this, but the additional complexity or cost is probably not worth it.�
Feb 14, 2016
ZBB Fwiw, your edited post is still not clear. This response is much better wording.�
Feb 14, 2016
jpet I'm trying to imagine how this works, given that each of the 96 bricks report their own voltage. Is it the brick that gets to 4.2V first that determines CV charging for all? Fascinating stuff and lots of ways to implement optimization/balancing if you look at it at this detail.�
Feb 14, 2016
mspohr Interesting speculation. We are all like the blind men and the elephant here. Only Tesla really knows the full story of the specs,test data and the BMS algorithm so best to take all of the advice here with a large dose of skepticism.�
Feb 14, 2016
kennybobby Some of Lola's data to show brick voltages at the 'CV' knee in the charge curve and an overnight settling (balancing?):
The SC session (thanks to Jan aka jpet)
Bricks at the knee
Overnight
�
Feb 14, 2016
deonb But nobody inside Tesla with any real knowledge is speaking, so then you're left with the SC's... where the employees are kept just in the dark as us. The main difference is 95% of those employees don't even drive a Tesla.�
Feb 14, 2016
Cottonwood OK, let's try one more time...
Reading from earlier posts by @wk057 and @Todd Burch, as well as insight from many other threads, here is my view of getting a pack balanced and calibrated. Comments are welcome.
- Take the SoC to 95% then discharge to 90% or less within hours. Wait for days for balancing to complete.
Details: To trigger a balance cycle, charge to over 93%, the constant Voltage part of the charge cycle (I like 95% or higher to make sure). After the balance cycle is triggered, you need to wait a few days for balancing to complete. Once the balance measurement has been done, go ahead and use that extra SoC and discharge back to 90% or less. The balancing is done by bleeding the extra charge off of stronger bricks[SUP]*[/SUP] to bring the bricks into balance over the next few days. This bleed cycle can selectively bleed off about 1% per day of charge from each brick, so giving the process a few days, allows for a worst brick difference of a few percent. The bleed/balance operation will continue at any SoC.- To get a good calibration of the battery pack capacity, charge once to over 90% and then discharge down to 10-20% (slightly lower is better for low end calibration). If you have time, do this after the above balance cycle; the calibration will still be pretty good if you overlap balance and calibration.
- If at all possible, don't leave your pack for too long a time at over 90% or under 20% SoC. No need to be paranoid, but try to correct back into the mid-range in less than a day or so; correcting within hours is slightly better. High SoC for long periods at high temps should be avoided.
- Charging to 100% does not help balancing, but don't worry about charging to 100% SoC before a long trip, or long segment. Just don't leave the pack at a high SoC for too long.
[SUP]*[/SUP]The battery pack in the 85 consists of 7,104 individual cells. These are arranged into 96 "bricks" of 74 cells in parallel (96*74=7,104). The 96 bricks are packaged into 16 modules of 6 bricks each. In each module, the 6 bricks are in series. The 16 modules are in series, putting all 96 bricks in series. Each of the 96 bricks has it's own bleed resistor set for balancing, i.e. there are 6 bleed resistor sets in each module.�
Feb 14, 2016
Andrew I'm still unclear on what you mean by "wait a few days for balancing to complete." Are you're saying that after charging to 95% (and then within a few hours driving to <90%), we should:
1. Continue to drive the car normally over the next few days, but do not charge during those few days? Or,
2. Let the car sit idle for a few days, without driving and/or charging? Or,
3. Just drive and/or charge as normal?�
Feb 14, 2016
jerry33 Drive and charge as normal.�
Feb 14, 2016
mspohr Great advice! Don't worry about it. Let the car take care of itself. If the car couldn't take care of itself and did need special attention, Tesla would tell you. You should just relax and enjoy the car.�
Feb 14, 2016
Cottonwood Exactly!
The car will do the balancing on its own from the measurement made by taking the SoC to over 93%. You don't have to do anything special, but the balancing process may take up to a few days to complete.�
Feb 14, 2016
apacheguy Problem with that is if you charge to 90% daily you're leaving the cells at 4.05 V which is not good long term. Oh, and your pack will never balance. Needs to see CV phase of charging.�
Feb 14, 2016
Quantum` Exactly.
And the BMS doesn't know exactly how much capacity it has even when balanced, unless you run it flat at least once. This means run it until it stops, not to 0 miles range. This would calibrate your system to be much more accurate, but how you do that is up to your imagination.�
Feb 14, 2016
Andrew Thanks for clarifying.
�
Feb 14, 2016
jpet So brick 80 was in the lead here with 4.198V.
Speculation: maybe the CV phase and balancing are triggered at around 4.197V?
When you look at the first SC chart, the overall voltage still increases during the CV phase as the lower voltage bricks get closer to 4.20V as well.�
Feb 14, 2016
Soolim Keeping balance is good. knowing how much capacity when balance is less important. Running flat just to know the capacity is not something Tesla recommends.�
Feb 14, 2016
Ampster Can you explain to us why it is not good long term to leave the cells at 90% (4.05v)? I understand why it is ocassionaly good to balance, but I have heard that the cells are often shipped at 50%. I have also heard that they should not be left at 100% for a long time.�
Feb 14, 2016
Soolim Charging to 93% gets the charging circuit to be at constant voltage charging. Upon reaching 93% and when the charging stop, BMS takes voltage measurement of each of the 16 modules and decides which modules need to bleed off some charge (the ones with the higher voltage) by turning on the bleeding resistor of those modules. These resistors are fairly low wattage, so as not to cause overheating and not to highly impact the remaining charge of the whole pack. Balancing remain on for a long time for those module until the next time you charge to 93% to have the CV charging again, at which time the BMS makes another determination on which module's bleeding resistor needs to be turned on.
So occasionally bringing the SoC to 93% is good practice to cause the BMS to identify the higher charge (voltage) module. I have not done any measurement to determine what the "occasional" time period should be. And looks like it is not a "Must", because it is not documented in the user manual. But I would say that balancing the battery pack is a good thing, so as to bring the nominal stored charge of the battery closer to its initial state of health. A balanced battery pack has more stored energy than a highly unbalance pack. Long term unbalance pack means one is not getting the optimal energy storage capacity of the pack.
An analogy: Balancing the pack is a like a virtuoso pianist practising to to get all his ten fingers to achieve equal dexterity. One can still play without perfect dexterity, but not as good if all the ten fingers are of equal dexterity.
I look to the day that Tesla would provide a maintenance note suggesting the owner to initiate pack balancing at the most convenient time. Or maybe when you go to your SC for service, they automatically return the car to you with the balancing initiated after they run some diagnostics on the health of your battery.�
Feb 14, 2016
Soolim Prolong storage of battery in high SoC results in coating of the anode inhibiting ion transfer and reduces the remaining life of the battery.
50% SoC is good for storage due to the battery chemistry.�
Feb 14, 2016
Ampster I am trying to understand @apacheguy's statement that leaving NCA cells at 4.05 is not good for them. I think that is 90% of Tesla stated capacity and probably less than that for a battery chemistry that can be charged to 4.2v.�
Feb 14, 2016
Soolim I edited my response and hopefully it answers your question.�
Feb 14, 2016
mkjayakumar So storage SoC as 50% and daily driving SoC as 90% is that good practice ?
Or are you suggesting that even daily driving SoC should be less than 90%, if you only need say around 70 miles of range per day ?�
Feb 14, 2016
Ampster If that is addressed to me, yes. There is already a cushion built into Tesla's number, but everything I have read says you will shorten life if you contantly keep your battery full. My question to @apacheguy was rhetorical, hoping that he would clarify his statement. The great thing about a Tesla is you can set a percentage and that is what I intend to do with mine. I will charge it to 100% before any journey that I will need the full range.�
Feb 14, 2016
zwede I guess you could do it the same way the service center does it: Open the windows and run the heater on max.�
Feb 14, 2016
Quantum` Please read my post more carefully.
The idea is not to run it flat so you know the capacity... when you hit zero miles it won't go any lower, but you still have capacity. The only way the BMS can know the actual capacity of the pack, is to calibrate it by running the pack flat and then charging to 100%.
The theory is that ostensibly at 93% balancing starts. Some modules may be at full charge then, and some will not. So the BMS starts bleeding down those which are higher, and charging up those which are lower. Eventually all the modules will be close to the same voltage and all will be practically fully-charged. That is full capacity.
And you can see that if balancing were done when the car is not charging, it would impair range. That would be senseless.
Some have suggested that, instead of using bleed resistors to drain off higher packs (just venting that energy as waste heat), that those higher modules should be diverted to a resistive element in the coolant loop when the weather is cold. I think it's a fantastic idea, but suspect that Tesla hasn't done it because that would drop the module voltage way too quickly, and also the voltage to the element would be quite variable depending on how many modules were diverted to it.
Good idea. Everyone should calibrate their system say, once a year (birthday?) after you're pretty sure you have the pack balanced. (This is where individual module voltage readings would be very handy) You might end up with a few more miles than you thought. Heck if most here have been charging only to 90%, you're surely out of balance, so just fixing that might gain you a few miles.
The car was designed to be charged to 100%. No theorizing is stronger than Tesla's silence on the issue.�
Feb 14, 2016
David99 Keeping the car plugged in doesn't do anything special. Many people people have monitored their power draw from the grid. The Model S does not draw power from the grid other than while charging or when pre-heating/cooling. There is no other time the car draws power from the grid to do anything else. So there is no indication that balancing or anything else only happens when the car is plugged in.
The reason Tesla recommends to keep it plugged in whenever possible is for other reasons.�
Feb 14, 2016
Canuck The car says "Trip" after 90% and Tesla recommends not to charge in the "Trip" range unless going on a trip and that extra range is needed. In fact, on the app you even get a pop up warning when going above 90%. I don't see this as silence on Tesla's part. I see it as Tesla actively telling us not to go above 90% unless needed.�
Feb 15, 2016
Ampster Actually that quote was by @quantum. I agree with your statement that Tesla is implying not to go above 90% unless needed.�
Feb 15, 2016
Quantum` "Tesla recommends"? Reference please.
If you never charge above 90% your pack will always be out of balance. Take it or leave it.�
Feb 15, 2016
wk057 This is false. The car has a max delta it will allow the pack to slide down from the user's set point before it will bump the pack back up to that point automatically. For example, in the extreme cold, the pack power will be used to keep the pack warm. Once the pack drains by a set amount (which has changed in various firmwares, so I don't know what exactly that number is today) the car will correct this by charging the pack back to the set point (at the scheduled time, if applicable). So if you leave the car sitting for an indefinitely period of time it will certainly always use grid power to keep the pack topped off.
Balancing only INITIATES when the car is plugged in and charging. It will continue to balance, however, long after this even if the car is not plugged in.
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Also false. While never charging past 90% will cause the pack to never trigger the active balancing circuits, it doesn't mean the pack will "always be out of balance." Again, and I think this is very very important for people to understand: In all likelihood your pack is NOT actually out of balance anyway! All this talk about balancing ends up being confusing because of the capacity estimate calibration errors that people keep referring to as "out of balance," which just is not true in the vast majority of cases.
Just drive your car, charge as much as you need to get where you're going, and let the car do its thing. Simple as that.�
Feb 15, 2016
Canuck Press the battery icon in your car. Note the "Trip" area that starts after 90%. The "Daily Driving" range is lower than 90%. Why does Tesla want you below 90% unless on a trip? Because lithium ion batteries degrade less when they are not fully charged:
http://powerelectronics.com/site-files/powerelectronics.com/files/archive/powerelectronics.com/portable_power_management/battery_charger_ics/804PET22li-ion-battery-life.pdf
Now, why doesn't Tesla tell spell it right out, like you want them to? That's because when a car has different charge levels the EPA averages the two levels. This was done on the 2011-2012 Leaf and the 2012-2014 RAV4 EV, to name a couple of examples. Nissan removed the 80% charge option in response, and sacrificed battery health for higher EPA numbers.
So Tesla is being extra careful not to fall into that trap. In fact, I just slid the slider on my phone to 100% to read Telsa warning, and I note it is now gone! My phone is Android so I don't know if it's gone on Apple too. I used to slide it 100% a lot just before leaving to my cabin (before they put a Supecharger in Hope)and I constantly got that warning, and for good reason too. However, Tesla probably removed it to avoid an EPA hit on range.�
Feb 15, 2016
wk057 Now it only gives you the warning if you leave the car set at > 90% for several charge sessions. It'll pop up with the warning message letting you know that you shouldn't do this regularly (don't know the exact warning). Probably works out so that the EPA didn't see it. lol.�
Feb 15, 2016
mspohr We don't know this is true. There is no official word from Tesla. They have only said they recommend daily charging up to 90%.
What we do have is speculation based on a lot of intensive detective work. We have some indication that looks like the pack balancing when charged to 93% but that could be an incorrect inference. The pack could be balancing at other times or it might be doing something different at 93%. In the absence of something official from Tesla, I would be cautious about regularly going over Tesla's recommendation of 90% max for regular charging (and also of draining the pack).�
Feb 15, 2016
jerry33 That's how my Model S has worked for the past three years. It's not new. (Unless I'm miss understanding)�
Feb 15, 2016
wk057 On the app or the car, when I first got my P85 (firmware 5.x) it would always pop a warning when setting above 90%. Now it only does if you leave it there a while.�
Feb 15, 2016
Quantum` Fine. You'll live with an unbalanced pack. Sorry but I really don't care.�
Feb 15, 2016
PaulusdB All this battery balancing stuff has a lot of similarities to hard disc (de)fragmentation to me.
Even down to the hypnotising graphic representation!�
Feb 15, 2016
jerry33 On the car. It's always popped up if I charge over 90% three times in a row. As far as I know this has been the case since 4.1 (the first version I ran).
I've never seen it on the App (iPhone), but I haven't range charged recently (last October was the last time).
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I can assure you that disk fragmentation is real, but it's only a problem if your disk is close to being full (over 85% used). I had several production servers that regularly needed defragmentation or they would practically stop working.�
Feb 15, 2016
qwk I can now confirm this to be false. My last max charge was a couple months ago, followed by partial charges to less than 90%. Last Friday the car was set to max charge, BUT unplugged at 232 Rated miles and driven right away. 232 rated is more than 90%, but less than 93% SOC. One more partial charge from 180 rated to 209 over the weekend, then it sat until today. Service screen confirmed the battery is perfectly balanced(like a new car would read). It's safe to say that the BMS takes care of the pack so you don't have to worry about it, and that there is a ton of estimation drift that goes on with rated range.�
Feb 15, 2016
garygid The initiation of brick (what I meant when I used "cell" to mean 74 cells in parallel, effectively
one "cell") equalization DURING charging might indeed wait until 93% SoC, but that does not
mean that equalization is not attempted at other times, like when parked.
If 93% is the ONLY time that equalization is initiated, then my S85 has never been equalized.
When I get a working copy of TM-Spy, I will be able to monitor "cell" (brick) voltages better.
Bleeding down the highest bricks while driving does not reduce your Range,
since it is the lowest voltage "cells" that will stop the car.
Likewise, it is the highest voltage bricks that will STOP a "full" charging,
so those bricks will not get overly damaged.�
Feb 15, 2016
Quantum` This doesn't mean that I am false. It just means that your modules are very similar to one another. There is no evidence that balancing takes place below 90% nor when unplugged. I personally know of several cars which have sat for months at between 20 and 60% SoC and only lost proportionate to the Ri.
I for one will wait to see Gary's evidence.�
Feb 16, 2016
Soolim @Quantum
How often do you balance, i.e. charge above 93 SoC?
How often do you run flat (0 RM) the battery?
What is your km range at 100% SoC?�
Feb 16, 2016
Ampster It is possible that balancing is triggered not so much by pack state rather than by the first group of parallel cells (bricks?) That hits the balancing algorithm point. Obviously this can only be triggered during charging. This voltage might be around 4.05-.10 for this chemistry. There probably is also a high voltage cutoff (as @garygid pointed out) some where around 4.20. This is how it works in two BMSs (EMUS & Orion) that I have used. It is likely that this point would be reached at the higher SOC (state of charge) As mentioned the balancing would continue even after charging stopped.
If the above is true, then it is possible that an out of balance pack might trigger balancing at a state of charge less than 90%. Following that logic it is also an explanation for why if you never charge above 90%, your pack could still be in balance or conversely not be unbalanced.�
Feb 16, 2016
Cottonwood Very good point.
If I were designing this algorithm, I would measure the difference in Amp-Hr that it takes for each brick of 74 paralleled cells to hit the start of the constant Voltage charge state. The difference in Amp-Hr is a very good approximation of the Amp-Hr that need to be bleed off.
The constant Voltage trigger point is probably the best Voltage point at which to do delta-Amp-Hr measurements, but lower, but upper end Voltage points could be used as well.�
Feb 16, 2016
AmpedRealtor The range estimate might be off due to not knowing the full top end voltage, but that is not a balancing issue. My 90% varies from 224 to 227 miles, has nothing to do with balancing.
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Tesla service told me the pack balances all the time. Good enough for me.�
Feb 16, 2016
brianman I've found such assertions suspicious for along time. In 2012 firmware, there was "Range" (100%) and "Standard" (92-93%). This was changed to "Trip" (91-100%) and "Standard" (50-90%). If 93% was a magic rebalancing point there are two problems:
1. Older firmware - A "low" standard charge (92%) would not rebalance. This suggests that some cars that rarely or never Range charge would get inconsistent rebalancing.
2. Newer firmware - Because 90% < 93%, all cars that rarely or never Trip charge would get inconsistent rebalancing. Some cars that use Trip charge would get inconsistent rebalancing when doing so.
Some might assert that "91-93% is the rebalancing threshold in older firmware" and "89-91% is the rebalancing threshold in newer firmware". This would be a more viable theory given the two noted problems.�
Feb 16, 2016
Ampster I don't know how that would be done on a serial pack without a separate current meter on each brick. That could create impedence that could create it's own balancing issues. Even if you could measure the difference, it would depend on what state each brick was when charging started. Most balancing algorithms I know use voltage near the top because that is the easiest to measure. My experience is confine to off the shelf BMSs and I have no specific working knowledge of the Tesla BMS.
Defining the contstant voltage charge state presents its challenges as well. While charging these non Tesla packs I can set the max pack voltage and when the charger hits this point, current begins tapering. However since the cells are in series, each cell is in a unique state of its own. A cell that is at capacity could spike it's voltage as it approaches the steep part (the knee) of its charge curve, while others are at a lower voltage. That would be when the BMS would turn on the shunt resistor(s).
This may be more thread drift that necessary to understand the specifics around the Tesla algorithm.�
Feb 16, 2016
Soolim For 85kWh pack I thought the 96 bricks are all in series, therefore the current for all bricks at the same, but voltage for each brick could be different depending on the SoC?�
Feb 16, 2016
Ampster Yes, I you are correct, I didn't clearly state the issue. The SOC of each brick could be estimated from the voltage and the Amphours going into the pack.�
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