Battery Management System - What I Learned At Tesla Service Center

[Svandevo]

(Responding to Post #41)

Thanks AlanSubie4Life - as usual, more complicated than I hoped.

You brought up a different subject by saying "... if you have an energy measuring charge meter ...". I do NOT, but I would LOVE to have one! (Otherwise, how can I know how much it's costing me to charge at home???) For 120V circuits, I use a Kill-A-Watt meter, which is great. But I have not found an equivalent (nor even close) meter/tool for use on my NEMA 14-50 charging circuit. Is there such a thing? I am not an electrician nor EE guy, so rolling my own is probably not an option. I eventually ran across TeslaFi, which I now use. After 43 charges (mostly once per day), TeslaFi shows an average Charge Efficiency of 97% (range of 92.8% to 99.6%), which sounds extraordinarily high to a layman like me. And it is much higher than the 88% range shown in your spreadsheet column labeled Charge Efficiency. Any suggestions? (And is there already a separate thread for this somewhere?)

(FWIW, I used Kill-A-Watt once when I charged from a 120V circuit, and TeslaFi said the efficiency was 82.1%, while Kill-A-Watt said (indirectly) that it was 84.3%.)

Thanks

 

[navguy12]

(Responding to Post #41)

Thanks AlanSubie4Life - as usual, more complicated than I hoped.

You brought up a different subject by saying "... if you have an energy measuring charge meter ...". I do NOT, but I would LOVE to have one! (Otherwise, how can I know how much it's costing me to charge at home???) For 120V circuits, I use a Kill-A-Watt meter, which is great. But I have not found an equivalent (nor even close) meter/tool for use on my NEMA 14-50 charging circuit. Is there such a thing? I am not an electrician nor EE guy, so rolling my own is probably not an option. I eventually ran across TeslaFi, which I now use. After 43 charges (mostly once per day), TeslaFi shows an average Charge Efficiency of 97% (range of 92.8% to 99.6%), which sounds extraordinarily high to a layman like me. And it is much higher than the 88% range shown in your spreadsheet column labeled Charge Efficiency. Any suggestions? (And is there already a separate thread for this somewhere?)

(FWIW, I used Kill-A-Watt once when I charged from a 120V circuit, and TeslaFi said the efficiency was 82.1%, while Kill-A-Watt said (indirectly) that it was 84.3%.)

Thanks

Not sure if someone has offered any techniques, but these two links will explain my setup and my results from same:

The first is a sensor attached to the circuit itself:

Eyedro monitoring system for a wall connector circuit - Tesla Owners Online

The second is what 19 months of data looks like:

Mike's monthly Model 3 efficiency report - Tesla Owners Online

As always, YMMV.

 

[bhzmark]

Charge to 100%! There is no reason not to, at all, for a road trip like this. It will have no impact on your battery and Tesla specifically says to do so. Just set it to finish charging roughly just before you leave.

+1

 

[RickW]

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!

Truly interesting post. I can confirm that the whole BMS, Battery issue is very confusing. I've read some very "heady" articles on this subject and this weekend I was able to try out your method.

A little background. I took delivery of my car in October, 2019. I would charge to 90% daily during the first 2 weeks, but after discussing with a few individuals on this board I changed to 80% nightly charging with no charging over the weekend. I have used the Stats App since October and at 80% charge, the estimated range for 100% was 308-310. That was until an update in December. Then the craziness began.

I watched as the estimated range fell to 298, then a week later 294. By January it was down to 284. I watched many people post here and determined that there was no way in hell that everyone was decaying at the same time and that this had to be a software issue. My car was still showing the same percentage drop of 5-7 on my regular trips. Weirdness !!!!

I had never let the car go below 45% without charging. Except this weekend. I charged 5 days ago and did not recharge until this morning. However scary, I ran the car down to 10%. An interesting thing happened. I charged the car to 100% this morning before leaving and the estimated and real range came to 297 miles. Although it's lower than the 310, it was a lot better than the 284 that I was getting.

I may take your advice and let the car run down a few more times to 10% to see if it finally breaks 300 miles in range on charging. It will also be interesting to see if some other things that have been suggested will work, ie. A change in the weather with less use of the heater which decreases the percentage battery use.

Great post by the way, thank you.

R.

 

[Jerry2]

I don't know what TeslaFi is doing but those numbers, I can assure you, are incorrect. The numbers from the spreadsheet are measured by Tesla, and validated by the charge rate at a given input power. In fact, this is another way to determine your battery capacity without actually having a meter - check your loaded voltage and current, and then very carefully monitor the length of your charging session, from a very low % to 90% (or whatever). The input energy is V*A*time. The output energy added to the battery, as we know, is your discharge constant for your vehicle (about 230-234Wh/rmi for an AWD, and about 223Wh/rmi for a LR RWD) times the number of miles added. (Do not use the charging constants, as that is not a useful metric for available energy added.) You'll find these have a ratio (for a 7.7kW charge rate) of about 0.89. If that ratio holds, everything is consistent and you can simply multiply the input power by 0.89 to tell you how much available energy you have and compare to the values in the spreadsheet for what you originally had (after accounting for the buffer of course).

There are lots of options for energy monitoring, I have no recommendations, but here is one from a quick search:

https://www.amazon.com/gp/product/B...search_detailpage?ie=UTF8&psc=1&tag=tmc064-20

Thanks again. Coincidentally (I hope), TeslaFi reported that my very next charge (after my post) had a 100.8% Charge Efficiency! Quite a feat! But ignoring that one, could the average report of 97% efficiency be realistic if it was calculated by ignoring all the energy-using support functions while charging, and just included the amount of energy "directed toward" the battery?

Re your link to the DROK product, I only see one CT, so how can it tell the whole story? Are the amps on the 2 hot wires so equally balanced in this application that one can just double what the meter says for energy?

 

[G26okie]

Mine has also gotten a little weird with the latest updates going out.

For the first couple weeks I charged to 90% nightly, at 279 miles (310 miles at 100%).

Then I started charging to 80% as there are only 2 days during the week I use my car significantly. Miles dropped by 1-2 to 247 at 80% or 277 at 90%.

After one of the latest updates the car started charging to 252 (80%) and 281 (90%), then a week later its down to 250 (80%) and 280 (90%).

This is on a stealth performance with 3.3k miles. I know they changed the range constant to be 322 on everything except the performance with 20 inch wheels, however I'm still only charging about 10-12 miles down from that, and my work commute is using the same exact percentage of battery it was using before.

Not super bothered by it as I use percentage all the time, but I wish Tesla would give us definitive knowledge on what exactly is going on with the BMS etc...

 

[AlanSubie4Life]

Then I started charging to 80% as there are only 2 days during the week I use my car significantly. Miles dropped by 1-2 to 247 at 80% or 277 at 90%.

After one of the latest updates the car started charging to 252 (80%) and 281 (90%), then a week later its down to 250 (80%) and 280 (90%).

Exactly what you would expect. Your battery/BMS has estimated about 75.4kWh at 100%, down from at least 76kWh.
Most likely you started with a battery below 77.6kWh (which is required to get 322 rated miles at 241Wh/rmi) but it is hard to say.

 

[G26okie]

Exactly what you would expect. Your battery/BMS has estimated about 75.4kWh at 100%, down from at least 76kWh.
Most likely you started with a battery below 77.6kWh (which is required to get 322 rated miles at 241Wh/rmi) but it is hard to say.

So...is that good or bad and I should keep doing what I'm doing?

 

[AlanSubie4Life]

So...is that good or bad and I should keep doing what I'm doing?

It has nothing to do with what you are doing, most likely.

Probably you just started with a battery which just had somewhat lower capacity for whatever reason. Which isn’t great, but also isn’t that consequential. I think I’d expect that over the course of the next year you’ll probably end up around 300 rated miles, but I am guessing and I really have no idea.

 

[G26okie]

Just to update, looks like I am at about 313 at 100% (extrapolated from 80% and 90%). Stealth P 2020 with 3.4k miles.

 

[G26okie]

Just to update, looks like I am at about 313 at 100% (extrapolated from 80% and 90%). Stealth P 2020 with 3.4k miles.

Can't seem to edit my original post, but my coworker with a 2020 Stealth with about 2k miles fully charges to 314, so within the margin of error of mine.

 

[G26okie]

It has nothing to do with what you are doing, most likely.

Probably you just started with a battery which just had somewhat lower capacity for whatever reason. Which isn’t great, but also isn’t that consequential. I think I’d expect that over the course of the next year you’ll probably end up around 300 rated miles, but I am guessing and I really have no idea.

You would think that just maybe they could test batteries at the factory to make sure they atleast go out with their max capacity. Kinda sucks that I'm starting the game 10ish miles down.

 

[N54TT]

You would think that just maybe they could test batteries at the factory to make sure they atleast go out with their max capacity. Kinda sucks that I'm starting the game 10ish miles down.

I wonder what the QA is for the cells and battery packs. You have to keep in mind though. The model 3 is made up of over 4000 cells(2170). I got into collecting led flashlights and 18650 cells for over 7-8 years now. Even with a single 18650 cell there’s not only variation between different brands but also within the same brand.

 

[VT_EE]

So I followed the directions given during my service visit. I ran the battery below 10% and began charging at home to 90%. Still charging, about another hour to go. Based on the percentage charge and switching to distance it appears as if the calculation for a full charge on my LR RWD is 285 miles - still far below what EPA states and consistent with the loss of range I originally posted about. Tomorrow I have a trip to take, about 200 miles, and I after I arrive at my destination the SuperCharger is nearly 50 miles further from my destination. That suggests that I'll need at least a 250 mile range to make it to the SuperCharger. With a projected range of 285 miles at full charge, I'm only charging to 90%, about 255 miles of range. That doesn't give me a lot of confidence, especially since were expecting cooler temperatures and rain tomorrow.

And that's why I posted. I want the range I paid for, and I'm apparently very confused about what my true range is and how to maximize it. Despite everyone's posts, I still don't know if I'm being overly concerned and tomorrow's trip will be fine. But I do know that if I leave the house with 255 projected miles of range, and need 200 just to get to my destination, and another 50 to get to the SuperCharger, and the weather is cool, I'm NOT going to enjoy a stress-free, non range-anxiety ride. And that's what's frustrating me. Even with a 100% charge, the estimated range is only 285 miles - nearly 40 lower than the EPA range, and enough to cause range anxiety.

I paid for the longer range (322 miles) and if that is indeed my true range, I'm fine. If not, I STILL don't understand what I'm doing wrong, and why the range estimate is so low. What am I doing wrong, and what can I do to fix it? Do I have 285 miles of range, 322 or something else? That's really all I want to know, and I'm REALLY frustrated that I don't have an answer to such a basic question.

I don’t think one deep cycle is going to drastically increase the displayed range. You would need to do it many times and perhaps even change your driving style depending on what variables the vehicle uses to decide what your “battery capacity” is. The more posts I read, the more I’m convinced Tesla’s bottom battery buffer is variable to ensure a user doesn’t experience a shutdown prior to hitting zero. This is not the hard ~2% buffer (for battery safety), but rather a soft buffer (what I’m calling it) used for calculating approximate range. The more uncertainty the BMS has regarding battery charge, the more the soft buffer is increased. This would manifest itself as reduced range display even though the battery is fine.

Tesla’s advertised range is based on EPA test cycles and represents mileage achieved when running the vehicle until a shutdown occurs. Theoretically, this EPA mileage should be achievable regardless of what the “remaining miles” display states (minus legit battery degradation).

This theory is based on my personal observations and reading thousands of posts on TMC. Nobody outside of Tesla will know definitively what is used for determining remaining capacity. I agree with everyone here that setting the display to % is the way to go for sanity’s sake.

 

[AlanSubie4Life]

The more posts I read, the more I’m convinced Tesla’s bottom battery buffer is variable to ensure a user doesn’t experience a shutdown prior to hitting zero. This is not the hard ~2% buffer (for battery safety), but rather a soft buffer (what I’m calling it) used for calculating approximate range.

However, every single SMT readback I have seen so far shows the buffer (as it is called out) is 4.5% of the full pack.

Of course there are other provisions the BMS could make which would hide battery capacity from the car itself, too. But no way to know that.

The more uncertainty the BMS has regarding battery charge, the more the soft buffer is increased. This would manifest itself as reduced range display even though the battery is fine.

Perhaps, again the BMS could hide energy from the car. But I would say that effectively this is not “fine” because it means there is less energy available to the user! The effective result is reduced energy available and I think we can all agree that is what matters to the user.

See this awesome thread which proves via the charging event time that a reduced rated miles indeed means less energy available. We just need someone with a mostly un-degraded SR+ to repeat the experiment and add 201 rated miles and show that it takes the same time, to really prove that available energy is directly proportional to charging time (assuming a warm environment).

Calibrating BMS with a software locked battery (SR)

 

[G26okie]

I'd really like to see how performance degrades based on battery degradation.

 

[lencap]

Well, I'm still at it. Drove the car over 1000 miles in the last 2 weeks. Tried bringing the charge below 20% each time (a low of 6%) and then charging to between 90-95%. I've cycled it twice, on the third try now. The results are interesting.

My range prior to this exercise was typically 286 miles at full charge. It's now edged up a bit to 296. Not a dramatic change, but about 3.5% greater range. This isn't enough to change my driving patterns, but it is interesting. Today when I started my nearly 200 mile round trip the temperature was below 35F. It peaked during the day at 56F. I briefly charged at a Supercharger during the higher temperature - only added about 50 miles (prior to the charge the projected range when I returned home was 8% - that's too small a margin for me to be comfortable, especially as the temperature was projected to fall by 15+ degrees during the time of my trip).

I arrived home with about an 18% range level - nearly 3% more projected range than when I left the Supercharger location. The weather was a bit warmer than forecasted, but I also was driving at 70-75MPH, and had the heater set at 69 to wear the battery down a bit more. But the basic question is still unanswered. Why did I arrive home with 3% more range than forecast? The temperature was a bit warmer than expected, but not significantly so (3 degrees warmer); was it the BMS adjusting to my new charging style? I don't know, and because I don't know I won't know whether that 3% range will repeat or is an aberration - and that makes me even more frustrated.

No conclusions yet, but there seems to be some changes in my range based upon following the service tech's suggestions. That is welcome, but also VERY frustrating. Why is the operation of the BMS such a secret? If we all knew how it works it would do several things: (1) give us an idea of how to maximize efficiency - which may be different for long versus short trips. When I take a long trip of 500+ miles I may "precondition" my battery to accept more frequent SuperCharging visits in brief succession, following the service tech's suggestion; (2) help us better understand the difference between maximizing range versus maximizing battery life - how are they related, and what is "best practice" if you intend to keep your car a long time (100K or more miles) or trade it more frequently; (3) clarify the impact on the battery pack from frequent SuperCharging, or charging to high levels (95-100%), or discharging to low levels (under 20%, under 10%, under 5%). I've read several posts covering this topic on the Forum, but even then there seems to be a wide range of advice about "best practice" and there doesn't seem to be universal agreement on any specific procedure.

Knowing how my car operates doesn't seem like such a difficult ask of Tesla. Why all the secrecy? On this thread alone posts made by very well intentioned and knowledgable people seem to conflict with each other. Which is "best practice", which is based on incomplete information? Who knows, and that's exactly why I started this post. Frankly, I'm still not much closer to knowing the right answers, and I bet I'm not alone. I'd like to know the answer, but I'm not tech savvy enough to buy specialized equipment to find out something that shouldn't be hidden from me as an owner. (Besides, I'm old, I'm expected to be grumpy!)

It would seem to be in Tesla's best interest to help us become better educated. Pretty soon a new battery pack will be developed, and it's likely that ranges will extend to nearly 500 miles. Who will want a SR battery pack that may be degrading more than anyone expected? Who wants to buy a car with a "322 mile range" only to find out that's not at all likely. Early adaptors, like most of us, were willing to accept these limitations, but with new EVs coming out at an increasing rate, an eventual increase on third party charging stations, and changes in technology, Tesla has more to lose than competitors if expectations aren't met.

Rant over - thanks, I feel a bit better.... Actually, no I don't..... Frustrating. I'd make a general statement that most Tesla owners are pretty educated about their cars, and at some point they'll want the same information I do. Tesla, please, reveal the wizard behind the curtain - trust us to understand what you're trying to do. We'll appreciate it, and continue to educate others so that we all benefit, and Tesla continues to expand and grow - a goal that we all share.

Elon, if you read these posts please understand that ALL of us greatly appreciate Tesla and want to help you succeed even more. To that end, I'll gladly be a Beta tester for an early delivery CyberTruck (I have a reservation already), or if you like I'll "stretch" and be a Roadster Beta tester -- all you would need to do is supply the car for me (;->).

There, now I really do feel better. Thanks to everyone for the informative, friendly, and detailed posts.

 

[Dr. J]

It would seem to be in Tesla's best interest to help us become better educated.

I don't know this, of course, but I suspect Tesla acts in the interest of keeping their warranty, service and support costs low. They also have an interest in making the user experience simple and not at all intimidating. They may have an interest in not letting their competitors know the ingredients of the secret sauce. All of those things indicate to me that their interests and mine may not be perfectly aligned when it comes to battery use, and I should act in what I believe to be my own best interests (mostly: to maintain battery longevity).

 

[diamond.g]

I think having more data is nice, but it may be more useful to have the EPA design a separate test for range of EVs instead of just running the highway test until energy is depleted. Maybe also having Tesla switch to using your last 30 to 15 miles energy usage instead of the fixed EPA estimate in the display would be useful as well.

 

[VT_EE]

Tesla needs to provide a simple metric for true battery degradation similar to what Apple does on the iPhone. Having people fret over a metric (miles remaining) that obviously is not a good indicator of degradation, is not helping the company or their customers.