Battery Management System - What I Learned At Tesla Service Center

[Twiglett]

"should"? Well, I would say never if it's just for the sake of doing it. I'm seconding @gaswalla 's point that the best and healthiest thing for the battery is not going into those high and low ends of the state of charge. If you're doing it on a trip, then fine, you need to do it, but don't for no reason.

That number on the screen will drift down a bit from the algorithm kind of losing visibility in its estimation of the amount of energy, but you are really just trying to give yourself a warm fuzzy feeling by trying to go through these kinds of procedures to manipulate that number higher. It's not actually creating any extra energy or driving range in the battery, so there's not a practical reason to do it.

after starting to get really concerned about a sudden drop in rated range on my car, I've come to the same conclusion as you.
Will be setting the display to percentage instead of distance as soon as I get the car back from the SC - getting the HW3 upgrade

 

[rincewind]

I've always had it showing %age - the indicated range in miles is not real world and just causes too much thought about range. Out of curiosity I just had a look at my M3P (3.5k miles on it) - at 76% charge it showed 233 miles remaining (eqautes to 306.5 @ 100%). Not real world at all. Just to add, I top mine up pretty much every night to 90%, have never run it down to less than 25% and then only on relatively rare road trips.

 

[hugh_jassol]

I now realize that my "routine" charging that I've practiced since I got the car is basically sound for long term longevity. If the BMS cycling can add range when I need it, that's great.

Keep in mind that you are only changing the displayed range... you're not changing the actual capacity of the battery. So it really is only making you feel better. It's sort of like if you tried to keep track of how many empty spots there were in a parking garage with a sensor that just detects cars going in/out. Every now and then the sensor will miss or double-count and thus the displayed number of spaces available will be incorrect. If you empty the garage and reset the counter, you'd re-gain the accurate count (which would degrade over time again). In all of this, the actual number of cars the garage can hold didn't change, it's just the system trying to monitor it lost track a bit.

I guess my question is will the car limit itself to what it is estimating my range is, even if the number is not accurate? My mid range currently shows an estimated range of 229 miles at 100% according to teslafi. I know that part of that is that my daily commute is very short and it is winter so temps are really low, but I haven't really cycled my battery in a long time (i'm usually between 60 and 90 in the winter and I don't get that low often in the summer).

I also have a mid range. I was down to about an estimated 243 @ 100%. I also have Scan My Tesla... so I decided to supercharge (only the second time I ever did) up to 100% and record the data to see what would happen. The capacity hit 100% and the car kept charging for about another half hour... you can read more in my post here, but in summary, the BMS adjusted my "Nominal Full Pack" on the fly. I am now up to 250 estimated range at 100%.

 

[CCIE]

Every Tesla forum should have a post pinned to the top of every sub-forum that says:

"Relax, your battery is fine! Your range meter is just inaccurate because you don't drive enough everyday."

As someone else said, change the display to show percentage and forget about it. If the battery no longer has enough capacity to cover your daily needs, then it's time to worry.

 

[Milissalach]

Greetings!

Like many others I've noticed a drop in range on my 2018 LR RWD Model 3. The loss is about 8-10% over the last few months. I assumed that it was due to the new Version updates, vampire loss or a combination of both. When I brought my car to the Tesla service center to rotate my tires I mentioned it to the tech. He offered to check my battery, which he did, and his comments surprised me.

He said that Telsa has a Battery Management System (BMS) in all of its cars, and that the purpose of that system is to balance the battery load. It does this by various readings, but the key point is that if you don't discharge your battery below 20% capacity the BMS system is dormant. That means that it begins to sense that your capacity is being limited so it gradually decreases your range (I may be explaining this incorrectly). The point is that his instructions to restore full range were very different from what I thought was proper battery management.

I typically keep the battery charged from 30-80%, rarely going below 20% or above 90%. He said that will maximize battery life, but not maximize range. To do that you have to "cycle" the battery to use most of its range. He suggested that I NOT plug in the charger whenever the car is in the garage. Instead run the battery down to 10% or less then charge it up to 90% or more. Repeat this process for several cycles. The BMS will sense the changes in the battery usage and gradually restore the full range.

He noted several things:
1) My battery capacity hasn't been permanently lost. The BMS is curtailing range and following his procedure will help restore full range (he assured me that the Tesla battery test on my car shows that I have at least 8% more capacity than is being made available in normal use).
2) If the battery charger is plugged in the BMS doesn't work! You MUST keep the car disconnected from any charger to engage the BMS system.
3) Using the Supercharger after the BMS is reset does not decay the battery capacity or range on the M3 in any way. The car is designed for this type of use.

This is very different from what I thought was best practice, especially the part about not charging when I'm not driving the car. I'm not saying that he's right and everyone else is wrong, but I'm certainly going to try what he suggested and I'll report back after several cycles to let you know what I find. Frankly, it makes sense to a degree. There isn't any other way to explain my range loss.

It may also be that many of us who have blamed software or other losses on decreased range can regain the lost range by actively engaging the BMS program through the process I just described.

Hope it work for all of us!

 

[Milissalach]

Thanks for sharing this information. I'm nowhere near a service center and the few times I've had the mobile unit come out we were dealing with other issues. It is quite frustrating that there aren't more superchargers between Atlanta and NW Florida, since Atlanta is my closest service center.

 

[Kevy Baby]

Every Tesla forum should have a post pinned to the top of every sub-forum that says:

"Relax, your battery is fine! Your range meter is just inaccurate because you don't drive enough everyday."

As someone else said, change the display to show percentage and forget about it. If the battery no longer has enough capacity to cover your daily needs, then it's time to worry.

So very true. I made the switch to percentage a while ago and my life is better because of it. I try to avoid even coming into the Battery & Charging sub-forum as it is rare that there is anything new.

 

[schrodarkian]

I've got the type of personality where I'll obsess over this and spend waaaay too much time trying to optimize something that in the long run really doesn't matter all that much. I've watched countless YouTube videos on this, read a ton of threads, perused technical articles way above my paygrade... gah. I give.

Look below for a screenshot of what doesn't help: the BMS kinda freaks out when you're at a low state of charge. Long road trip coming home, down to about 10-15%, and the BMS (via the app slider) unhelpfully tells me I've trashed my battery. My AWD M3 is suddenly down from 300+ to 211 miles of potential range?! CLASS ACTION LAWSUIT!!! Ha. No. The BMS, or the app, or the interface between the 2, or something somewhere along the line is miscalculating the potential SOC of the battery. Nothing to see here.

The answer?

I put mine on % so don’t get concerned with stated range, but occasionally take trips that use 95% of battery.

That.

Also, this:

A Senior Tesla Executive's Comforting Answer to Concerns Re: "Loss of Range" | Tesla

The best part of this answer, from Jerome Guillen in 2014, is that the BMS only ESTIMATES the state of health of your battery. The actual state of health of your battery is a mystery. Kind of like, hey, we can't perceive the actual state of reality through the limited senses that human beings possess, but usually it's good enough to get us through the day.

A couple quotes from Guillen in that email:

"We have found that in cases where the battery is consistently charged to a lower state of charge- between 60-80% -capacity estimation becomes less accurate and tends to underestimate the true capacity of the battery. The result is an incorrect reduction in the displayed range estimate. This does not affect the true range of the vehicle, as the end-of-drive conditions are based on real-time battery measurements of reducing battery power rather than software estimates."

"You are correct that avoiding charging to high states of charge optimizes battery capacity retention. You should continue this behavior as best meets your daily range needs. We also recommend opportunistically charging, i.e, charge frequently, charge often. Avoiding deep discharges is another best practice for optimum capacity retention."

I added bold for emphasis. In the 5+ years since this was published, I don't think much has changed.

All the graphs I've looked at suggest that don't run the thing out completely, and don't maintain a high SOC especially in warm weather. If you have to charge to 100%, don't do it above 100 F, and drive the sucker immediately. It probably won't matter in the long run if you charge to 60%, 70%, 80%... and occasionally higher.

If you do in fact get below a real 70% (for the M3) during your warranty, Tesla will replace it with another battery, likely re-manufactured, that is > 70% of the original capacity.

I've resorted to displaying my battery percentage. I'm sure I'll peak at the BMS' guesstimated range during long road trips to check how things are going. Otherwise? You're looking at the state of the battery through pretty smudged lenses.

 

[navguy12]

So very true. I made the switch to percentage a while ago and my life is better because of it. I try to avoid even coming into the Battery & Charging sub-forum as it is rare that there is anything new.

View attachment 511169

I've been using % since day one, 20 months ago.

Since none of my regular "critical length with no stopping" trips have suddenly become problematic, I must assume any real battery degradation is still lost in background noise.

 

[Kevy Baby]

I've got the type of personality where I'll obsess over this and spend waaaay too much time trying to optimize something that in the long run really doesn't matter all that much. I've watched countless YouTube videos on this, read a ton of threads, perused technical articles way above my paygrade... gah. I give.

This was all that was necessary in your post as you have realized the futility of it, changed your car to percentage and moved on

(At least I hope that's what you did.)

 

[schrodarkian]

This was all that was necessary in your post as you have realized the futility of it, changed your car to percentage and moved on

(At least I hope that's what you did.)

Nope, still obsessing! Of course, anyone reading this thread is likely in the same boat!

In all seriousness, these days I try to confine my obsessions to Powerwall and solar related issues.

 

[Tam]

...I guess my question is will the car limit itself to what it is estimating my range is, even if the number is not accurate?...

Contrary to what you were told verbally, the BMS works constantly whether you are plugged in or unplugged, parking or driving...

As explained in writing by Tesla staff and Senior Executive on the above 2 links, to get an accurate measurement, BMS needs to know the top and the bottom but doing so would reduce your battery longevity. They said they would implement a new algorithm to guess the range soon. And it looks like from the Service Center's receipt, that algorithm would reference 90% every night.

Since it is a guess, the range estimate will gradually get inaccurate.

However, it still continuously calculates the correct range in real-time so sometimes you can see the range shrinks when the weather suddenly gets colder or expand when cold weather becomes hotter.

That's the same when you go uphill then downhill. The BMS works all the time to give you their best guess of what the range is.

Thus, it's possible that when you see battery gauge saying there are 10 miles left in one second but then your car's dead on the very next second and you look at the battery gauge again and it no longer says 10, it says 0!

That's your BMS at work at all times!

The opposite is true when your battery gauge says there are 10 miles left and the number hardly goes down after you drive another 3 miles. So actually, there were more than 13 miles left, not 10. For some, battery gauge says 0 miles and they can still drive a little bit longer than that.

For others, 0 means 0, and it's dead!

It's a lottery when your battery is low: It's hard to know which above scenario will be.

 

[Jerry2]

(Dual Motor owner since Oct 1st, now with 4800 miles.)

This thread is another inconclusive list of various people's divergent opinions. Oh, well ... welcome to the Tesla community, apparently.

I haven't a clue about this issue (though diminished advertised range began affecting my car almost exactly a month ago), except for this tesla.com webpage: Range, which says: "Whenever possible, don’t let the battery go above 90% or below 20%.” That's about as clear as words can be ... and it's from Tesla. But I suppose it could be wrong ... ?

One (long) observation and question. I have stuck with range (rather than percent) from the beginning, for 2 reasons: range has greater meaning to me, and range has 3 times the precision compared to percent -- ie, 310 parts vs 100 parts, since both are reported only as whole numbers. However, that first reason has fallen to essentially useless now that I understand how stupidly erroneous the "EPA rating" of 310 miles of range is. I get about 60% range "efficiency" in my daily use -- ie, on average, I only get 60 miles on the odometer for 100 miles of range used up, which means my real range is about 186 miles instead of 310. (This is CRAZY!!! But it's a different issue.) But the precision difference remains. However, precision is not as important as accuracy. SO, does anyone know FOR SURE whether range and percent have the same accuracy? In my mind, they SHOULD be the same accuracy, but my mind didn't design the hardware/firmware/software that leads to the 2 numbers. (Presumably one is a simple calculation from the other ... but who knows for sure?) Thanks.

 

[Tam]

...SO, does anyone know FOR SURE whether range and percent have the same accuracy?

I think it's a matter of experience. If I know from the past that 90% would get me there 200 miles away or a battery gauge displaying 280 miles or an extra buffer of 80 miles will get me there in summer, then that's what I'll do the next time and replicate the driving style (posted speed limit, A/C at 77F...).

 

[milleron]

If the BMS cycling can add range when I need it, that's great. I also have another simple choice - slow down 5MPH on the highway and extend range as well. So slowing down & cycling BMS may provide the difference between making an additional stop at an inconvenient time.

Resetting the BMS canNOT, in fact, "extend range;" it merely allows the indicated range to more accurately reflect the actual range. The "range" number on your display may well be higher after the reset, but the distance your car could actually go on a given charge has not changed.

 

[hugh_jassol]

SO, does anyone know FOR SURE whether range and percent have the same accuracy? In my mind, they SHOULD be the same accuracy, but my mind didn't design the hardware/firmware/software that leads to the 2 numbers

The mile range number is the Wh the BMS thinks is remains in the pack divided by a constant. That "charge constant" is set by Tesla and is different for each variant of the Model 3. For the LR AWD it's 248 Wh/mi.

The percent is (Wh the BMS thinks is remaining - Buffer)/(Full Pack kWh - Buffer).

These have been confirmed by many using CANBus readers.

So if you read that carefully, you will see there is a little white lie going on. They calculate the percentage without the buffer - that is when the percent in your car reads "0%" you still have "Buffer" kWh left. This is ~4.5% of the full pack capacity. They also tick miles off the range at a slightly faster rate than 248 Wh/mi (I think it's 237 Wh/mi for LRAWD) so it hits 0mi when the percent hits 0. This is all to protect the battery by really making you want to charge before you run the pack ALL the way down (which is very bad). Because legally,I believe, they HAVE to make all of the energy in the pack available to you so they can't stop you from running the pack all the way until the car wont move - so they chose to make the gauge read "0" when there was still 4.5% of the pack remaining.

Here's a somewhat simplified example: So let's say you bought a 310 mi rated Tesla and charged to 100% (and the display now reads 310mi). If you then drive it at exactly the "constant" energy per mi (equivalent to driving an ICE car such that you were getting exactly the window-sticker mpg), you would get 310mi before the car wouldn't move any more. However, the "miles" display would have reach zero at approximately 296 mi and you would have driven the last 14 mi or so with the gauge reading 0.

This is somewhat equivalent to the gas gauge reaching "E" when there is still a gallon or so left in the tank as a way to keep people from running out of gas. Although in the case of an EV, you can do damage to the pack, so it's a bit more important.

I know this is all very confusing -it was for me too- but you should read many excellent posts by @AlanSubie4Life like this one. He has dug deeeeeep on this stuff.

 

[N54TT]

I've got the type of personality where I'll obsess over this and spend waaaay too much time trying to optimize something that in the long run really doesn't matter all that much. I've watched countless YouTube videos on this, read a ton of threads, perused technical articles way above my paygrade... gah. I give.

Look below for a screenshot of what doesn't help: the BMS kinda freaks out when you're at a low state of charge. Long road trip coming home, down to about 10-15%, and the BMS (via the app slider) unhelpfully tells me I've trashed my battery. My AWD M3 is suddenly down from 300+ to 211 miles of potential range?! CLASS ACTION LAWSUIT!!! Ha. No View attachment 511156

I thought the battery only turned red when below 10% not above 10%? Was your car showing 10-15% then suddenly dropped below 10% at the time you took that screen shot? Has your range increased since or still at 211?

 

[hugh_jassol]

ong road trip coming home, down to about 10-15%, and the BMS (via the app slider) unhelpfully tells me I've trashed my battery. My AWD M3 is suddenly down from 300+ to 211 miles of potential range?! CLASS ACTION LAWSUIT!!! Ha. No. The BMS, or the app, or the interface between the 2, or something somewhere along the line is miscalculating the potential SOC of the battery. Nothing to see here.

It's actually more simple than that. When you get to low single-digit percentages the rounding errors become huge. For example, let's say you had 10 mi left on the display and the percent showed 3%. That 3% could be actually 2.5 rounded up or 3.4 rounded down. If you take the 10mi remaining and assume there's 2.5% left, then you would estimate your full pack at 10/0.025=400 miles. If you assume 3.4%, you would estimate 10/.034=294. The display (and the API) just doesn't report enough signifiant figures when the percentage low.

Because of this you should try to be as high as possible SOC to estimate the "full" range. essentially you need more significant figures in the percent number so the calculation is more accurate.

 

[SSedan]

So this has been said but maybe not this simply on this thread, or at least I missed it.

Voltage is flat from something like 10-90% occasionally going past those numbers let's the BMS measure an actual voltage and find its place again.

 

[AlanSubie4Life]

SO, does anyone know FOR SURE whether range and percent have the same accuracy? In my mind, they SHOULD be the same accuracy, but my mind didn't design the hardware/firmware/software that leads to the 2 numbers. (Presumably one is a simple calculation from the other ... but who knows for sure?) Thanks.

I can say for sure they are not equivalent. A % is just that - it is the formula given above by @hugh_jassol, so it's simply a measure of your usable energy remaining (that is your energy not including the buffer), relative to what the maximum energy you could have remaining (not including the buffer), according to the BMS. Meanwhile, the rated miles are equivalent to available energy (above the buffer) at any time, and not related to the maximum energy remaining. They are not equivalent.

Accuracy? Technically accuracy is the difference between the measure and the actual value...in that respect, once you know your maximum energy you can have, the two (% and miles) are basically the same accuracy (the rated miles have a bit better resolution though). However, your maximum energy available according to the BMS can and does change over time.

Summary: So 1% when your car is new will not necessarily represent the same amount of energy as 1% a year later will represent. However (unless the constant is changed, which is rare), one rated mile will always represent the same amount of energy for your specific car, no matter how old your car is.

Here is a later version of the linked post above, which may be helpful:

2020, 2019, 2018 Model 3 Battery Capacities & Charging Constants

I'd actually recommend sticking to the indicated "Trip Meter" constant in this spreadsheet, since that most directly represents how the trip meter behaves relative to displayed rated miles, and the energy content of each rated mile. You can really only directly relate the charging constant to energy remaining easily at SoC of 100% (Max Energy Including Buffer = Rated Miles@100% * Charging Constant), due to the buffer, which screws up the math and makes the formula non-intuitive when trying to use the charging constant at other SoCs...

Available Trip Meter Energy Above Buffer = Rated Miles Remaining * Trip Meter Constant

I'm not going to delve into existential questions about how accurate the BMS is or whether it really represents a good measure - mostly because it's fairly hard to measure against anything, and I really don't know if it gets out of whack over time. To some extent you have to draw these conclusions on your own with your own observations. All I can say is that I generally see that the trip meter seems to read fairly consistently, for a given journey under a given set of conditions. That is to say, the trip meter seems to measure energy used precisely. Furthermore, no matter how widely I cycle my battery (5% to 100%), my rated miles at 100% have always been amazingly consistent (and on a downward trend which has now leveled off).

As far as the rated miles/BMS goes: One way you can approach this, if you have an energy measuring charge meter, is to monitor your consumption during a single controlled large charging event, at a particular charging wattage, and log the number of miles added and the energy from the wall. It needs to be a well controlled charging event (no opening the door to get the heat going, etc.) to get consistent charging efficiency. And then track the behavior over time. My feeling is that if it takes 84kWh from the wall to charge your battery from 0->100% when new, and then when your rated range shows 300 miles rather than 322 rated miles at 100% (or whatever) a year later, and it then takes 78.5kWh from the wall to charge from 0 ->100%, that's an indication that the BMS is actually pretty good at measuring available energy and represents a real change in capability (capacity) if it shows that behavior (it of course could indicate a software change improving charging efficiency too...at that point you'd have to compare to someone else's vehicle with a different number of rated miles available)! I've got some historical data on this from Chargepoint & Stats, and efficiency is amazingly consistent for controlled charging events over a wide range of durations, where I did not use preheat.