Battery Management System - What I Learned At Tesla Service Center

[TimothyHW3]

“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.” Source.

Working on it haven’t gotten that low since my recent charge - I’m tracking it though and ready to report when it happens.

So this basically confirms the first part of the theory the 60-80% charge most people have been suffering from. Interesting thing is this is from 2014 and a lot of Model S/X drivers said they never experienced change in range even when doing 60-80. Maybe Tesla changed something and a new team worked on Model 3 and they didn't apply that fix? Or it was just placebo from the S/X drivers.

As for the second part, not sure, the BMS actively measures the voltage. But like I said, most people will never go below 5% to see this in effect. I have done a few 5% stunts so I would like the BMS to be working properly.

As for the 4v. Not sure if you understood me. The 4V should be around 65-75% or something. Haven't you gotten below that point in recent days?

 

[TimothyHW3]

As has been said the top and bottom 10% voltage actually changes so the BMS is measuring the state of the battery then.

What exactly do you mean by that? The voltage itself never changes - it only changes with degradation.

If you assume that the BMS doesn't recalculate itself unless <90% or less than 10% then this is wrong because with CAN Bus data we can read that the capacity is changing constantly regardless of SOC %

 

[SSedan]

I am no battery engineer but my understanding is that in vaguely 10-90% voltage is relatively flat compared to the variation it experiences outside this range.

Simple googling for lithium ion battery voltage curve has graphs that show this.

 

[drtimhill]

So how often do you think this should be done? Every few months? Twice a year?

My feeling would be twice a year, based on (a) LOTS of reading and cross-checking of facts/rumors/stuff, (b) gut feel after investigating what the BMS actually is doing (the posts in this thread correlate with what I discovered) and (c) not wanting to have deep cycles all the time as this DOES degrade battery life.

As for when the BMS does its job, both posts here are right. BMS is on all the time, and works all the time. However, the nature of the BMS system and the batteries means that it needs the occasional "deep cycle" (below 20% then back up to 100% then back down to 20%) to give it a chance to measure/balance the batteries fully.

So I added a reminder to my phone to deep cycle twice a year, and the rest of the time I charge every night at home to 70% (or 80-90% if I expect to go on a longer trip). And I leave the car showing percentage and not range as I dont want to join the "OMG I LOST 3 MILES OF RANGE!" crowd <grins>.

 

[Familyfirstj]

Greetings!
This was very very helpful, thank you!

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!

 

[TimothyHW3]

I am no battery engineer but my understanding is that in vaguely 10-90% voltage is relatively flat compared to the variation it experiences outside this range.

Simple googling for lithium ion battery voltage curve has graphs that show this.

Oh, so that is what you meant. Well yes, of course, depending on SOC % the voltage is different. It starts at 4.2V 100% and goes down to 2.7V or something. And once 100% is 4.19V you have degradation and then the curve changes.

But that still doesn't explain how the BMS works, which we most likely can vaguely guess.

 

[SSedan]

As others have said it measures input and output constantly.
The times it spends at the extremes just give it a chance to correct any cumulative rounding errors by measuring what voltage is doing in response to inputs and outputs. I suspect this is where it can see degradation and begins lowing 100% miles rather than always trying to push the cells to original max voltage.

Again I am no battery engineer, all I really know is from reading this forum, there is a lot of good info and a lot of bad. I just have enough internet experience to have gotten reasonably good at filtering info.

 

[hugh_jassol]

As for the 4v. Not sure if you understood me. The 4V should be around 65-75% or something. Haven't you gotten below that point in recent days?

That’s correct. From the time you asked me to to check it out...until now I was past that point when you asked.... I then depleted to ~10% and charged up to 90% and I’ve had a few days off work, so haven’t yet gotten down to 4V on the pack. I’m about 4.08 now ~77%

 

[pdx_m3s]

I use 10-15% per day in my SR+ and charge to 70%. I think I’ve only been down around 20% once or twice, and I’ve never charged past 90%. The BMS is probably having a hell of a time trying to estimate range.

 

[Jerry2]

(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

 

[Sunnfun]

Quick question that might be related to this topic: We've been charging our car using a NEMA 14-50 outlet at 40A to 80% every two or three nights, usually plugging the car in when the battery goes below 50%. We've had a visit by a mobile technician recently and he recommended to only charge at 30A to increase the battery life. Does that ring true?

 

[drtimhill]

Quick question that might be related to this topic: We've been charging our car using a NEMA 14-50 outlet at 40A to 80% every two or three nights, usually plugging the car in when the battery goes below 50%. We've had a visit by a mobile technician recently and he recommended to only charge at 30A to increase the battery life. Does that ring true?

I've certainly never heard that and the Tesla wall chargers almost always charge at 40A or a little more depending on the circuit. He might be technically correct in that lower current slower charging is better for battery life, but the difference between 30A and 40A I'm sure is slight (compared to the vastly greater difference between that and supercharging).

 

[SSedan]

Quick question that might be related to this topic: We've been charging our car using a NEMA 14-50 outlet at 40A to 80% every two or three nights, usually plugging the car in when the battery goes below 50%. We've had a visit by a mobile technician recently and he recommended to only charge at 30A to increase the battery life. Does that ring true?

Now ignore the amperage being delivered for a moment stop to think about duration to charge from empty to full.

3s are ballpark equal mile per amp hour right?
So a 3 with 240miles of range could recharge from 0-100 in 6 hours, now remember there is a complex cooling system and in actual practice the top 10% is slower while the BMS balances modules and such, does that sound abusively bad?
Nowl us take that a step further and if Tesla makes supercharging available at rates up to 600miles added per hour 40miles is 6.7% of that and granted that rate is not held long term but just use it as a frame of reference. Heck even just look at regen.

Auto technicians are efficient parts swappers, relatively few truly understand how things really work. I don't say that as some arrogant academic engineer or the like but rather as an automotive enthusiast who had tried to talk cars with auto techs and found that many enthusiasts since they are interested and engaged know more than the professionals.

 

[Zoomit]

Does that ring true?

No

 

[lencap]

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.

 

[AlanSubie4Life]

. What am I doing wrong,

My guess is nothing.

and what can I do to fix it?

My guess is nothing.

Do I have 285 miles of range, 322 or something else?

I suspect it is more likely you have 285 rated miles (about 67kWh for a Model 3 LR RWD) than ~78kWh. But I could be wrong.

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.

If you want to figure out what is up, I feel like the only way to convince yourself of the true state of affairs is to measure how much energy you can put into your car, and then multiply by 0.89. That is how much energy is in your battery based on all available data. (Obviously you need to scale that value appropriate by the 0.955 for the buffer and any partial charge you do to get your full battery capacity - but in any case this will give you an idea of usable capacity if you are charging from close to 0 to close to 100%.)

I don’t think Supercharging is a good place to do this, since the multiplication/efficiency factor is unknown (it is a lot closer to 1), and also how Tesla meters the energy is unclear.

So you would have to wait for some time in the future when you are close to empty and use a free ChargePoint with a meter to charge from close to 0 to 90% or something.

No matter what the BMS is doing, it can’t create energy out of thin air. And it can’t significantly change the charging efficiency (it could make it a bit worse but probably pretty small effect). So this is a solid way to measure things - and even allows comparison to other vehicles using the same charger if they are available to you (can compare to other reports here too of course but that introduces more variables).

 

[AlanSubie4Life]

As I mentioned earlier, the 0.89 factor is for 7.7kW charging. Might be as high as 0.9 or 0.91 for 11.5kW charging. But for the difference of 67kWh vs 76+kWh at issue here it does not really matter - just has to be ballpark and these numbers are within 1% or so.

 

[ord3r]

Came across this tidbit on battery balancing. Probably need to leave the car at 85%+ for a week or so if you have frequently only charged to 80%.

 

[AlanSubie4Life]

Good piece of info. Notably, the first paragraph discussion about the capacity of the pack being limited by the brick with the lowest capacity may be relevant here.

Seems like for partial discharges, or when charging to less than 100%, you could compensate for such issues by shifting charge from the higher capacity bricks to the low capacity one, once it has discharged a bit, when idle (though not sure if that operation would be net energy positive). But for a continuous discharge from 100% it seems like there is no way to get around such a limitation. I don't know anything about balancing, so just thinking out loud, but I'm sure this is just one of the most basic fundamental issues in the well-understood techniques of battery balancing, which was addressed optimally long ago.

 

[AlanSubie4Life]

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?)

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