BMS Drift - does it really matter?

[gabaum10]

So I have seen numerous posts about range decreasing and the wonky BMS, but no one has actually addressed the problem with that. Of course, that's assuming you don't have a faulty battery. I feel like in those cases you'd be able to tell pretty easily.

Does having a BMS that's drifted cause any navigation issues or is it mostly just all your OCD? For example, would it ever reroute you to a charger thinking it wouldn't have enough range if it thinks say, 50mi are unavailable, in an otherwise healthy pack? Realistically, this is an edge case and most of the time a dozen miles of drift likely won't matter on long trips. I guess to rephrase, is there really any major benefit to correcting the drift?

My car is about a month old and I've only been charging every few days (generally like 50-80%). I'm curious if that will keep the BMS in alignment or if that's more of a hassle than it's worth, if there really isn't a major downside other than the number on the screen

 

[user212_nr]

or is it mostly just all your OCD?

OCD is the cause of 95% of all the range posts on here (including this one). That's why there is a sticky thread.

If your gas car's computer shows the wrong range, it doesn't affect the gas or the engine. Batteries charge based on their voltages and not on the number of miles they have.

Does having a BMS that's drifted cause any navigation issues?

I don't see why not, but it is kind of a stretch. They would need to be taking a long trip with no superchargers that maximizes their range and leaves little margin for error. These are the people who are charging to 80% and don't go far. Plus charging to 100% before the trip might fix some of their issues.

 

[gabaum10]

OCD is the cause of 95% of all the range posts on here (including this one)

I know, reading all these posts is making me crazy. I should have left well enough alone lmao. I'll just start charging to 80% daily and try and forget I ever delved into these threads.

So the assumption here is there is really not a benefit to forcing the correction aside from fixing the number on the screen?

 

[user212_nr]

So the assumption here is there is really not a benefit to forcing the correction aside from fixing the number on the screen?

There may be some benefit to charging to 100% every few months as there is some kind of "cell balancing" that supposedly happens then, but as far as the number being off there is no harm from that. I'm not sure if this cell balancing improves the battery, but it is probably splitting hairs.

Main point is that a user charging to 60% will have better battery health, but the "max range number on the screen" will be off. So just change it to percent.

 

[Matsayz]

OCD is the cause of 95% of all the range posts on here...

I'm SURE you meant to say "CDO"... phew! that was a close call

 

[Tam]

...there is really not a benefit to forcing the correction aside from fixing the number on the screen?

Correct.

It's not exact measurement so you can expect some drifting: 80% of yesterday may drift down to 79% today. The BMS will eventually correct that back to 80% someday.

Some people get anxious and rather than wait for the BMS to correct itself, they want to manually do it now.

 

[gabaum10]

Thanks! That's all really helpful info.

 

[Candleflame]

fact of the matter remains that people with i.e .5-6% degradation generally can only pull 69-70kw out of the battery when they should be able to pull 75kw on a brand new car. I dont think anyone is upset here if their BMS drifts by 10km or so but a lot of people do have i.e. 450km rather than 500km at 100%.

 

[Tam]

...I dont think anyone is upset here if their BMS drifts by 10km or so but a lot of people do have i.e. 450km rather than 500km at 100%.

That's 10% loss. To make sure customers don't get upset, Tesla guarantees the capacity to 70%, so there are still 20% more to go.

 

[Candleflame]

That's 10% loss. To make sure customers don't get upset, Tesla guarantees the capacity to 70%, so there are still 20% more to go.

so you drop to 80% after 2 years and then stay there. that is crap.

 

[Tam]

so you drop to 80% after 2 years and then stay there. that is crap.

Exactly. When planning for the future, many plan the best scenarios such as the car would still have 100% capacity and still go to the same destination without a charge in between.

However, I also plan for the worst scenarios what if the car depletes down to 71% capacity and will need a charge in between to reach the same destination.

That's why range is king: Plan the scenario of 70% and not just 100%. I would buy the longest range in case it would degrade.

And that's why there's a warranty in case of the worst scenario that the car needs towing to a Service Center.

 

[ElectricMozart]

For my day to day, my 11% range loss (reported by the BMS) doesn't really bother me. It just means my daily range is a little less. I drive 130+ miles commute so I'm charging 20 to 80% daily.

Where it does matter to me is when I'm taking long trips and my charging stops are increased slightly. Also, when traveling off of major interstates, I no longer have the estimated range to make it on some routes that have little charging infrastructure. When I visit family out of state for instance, there is a 250+ mile stretch between superchargers and I don't feel comfortable making it with only 275 miles of range. Especially considering its a hilly 250 mile stretch.

 

[gabaum10]

For my day to day, my 11% range loss (reported by the BMS) doesn't really bother me. It just means my daily range is a little less. I drive 130+ miles commute so I'm charging 20 to 80% daily.

Where it does matter to me is when I'm taking long trips and my charging stops are increased slightly. Also, when traveling off of major interstates, I no longer have the estimated range to make it on some routes that have little charging infrastructure. When I visit family out of state for instance, there is a 250+ mile stretch between superchargers and I don't feel comfortable making it with only 275 miles of range. Especially considering its a hilly 250 mile stretch.

So you're saying it DOES impact the computer calculations for trips? If so, that is kind of annoying.

You're saying you have an 11% degredation. Does that include taking some of the steps to restore the rated range?

 

[jerry33]

In the S I charged to 90% almost all the time, with 100% charge for trips. After seven years and 130K miles (50% trip miles) as best I can tell the degradation was around 8%. Mostly BMS drift is OCD. There's so little difference between charging at 90% and charging at 80% that it's not worth bothering about (despite what you might have read because there are many other variables that affect battery life). If you intend to leave the Tesla for many months without driving, then set it to 50%, but otherwise, just 90% and 100% charging is fine, so long as you don't leave it at 100%. (Set the stop time for when you intend to start driving and don't sweat it, also don't let the battery sit at below 20%.) This is kind of the opposite of stocks (time in the market is better than timing the market, time sitting in the extremes is worse than reaching the extremes.)

 

[ElectricMozart]

So you're saying it DOES impact the computer calculations for trips? If so, that is kind of annoying.

You're saying you have an 11% degredation. Does that include taking some of the steps to restore the rated range?

Yea as far as I can tell, the navigation calculates route and charging stops based on what range the car thinks it has. So if my BMS (battery icon) shows 275 miles at 100%, it seems to plan my routes based on that and not the 310 "rated" miles I'm supposed to have.

Yes I'm around 11%. Actually I just charged to 80% this morning and my estimated 100% is a lowly 272. I've tried all the tricks aside from driving it down to 0 miles to see if its 100% real. I've gone down as far as 5% and the estimated range seems on point (as long as I'm making the rated wh/mi). I don't think driving to 0 is healthy for the battery and at this rate, I don't want to push my luck. So it is what it is. Tesla says I'm "within spec". I was following the restored range sticky thread on here but after a few weeks, I had about a 5 mile uptick but then it went away.

 

[jerry33]

Yea as far as I can tell, the navigation calculates route and charging stops based on what range the car thinks it has. So if my BMS (battery icon) shows 275 miles at 100%, it seems to plan my routes based on that and not the 310 "rated" miles I'm supposed to have.

As far as I know, this is correct, but it also recalculates along the route, so it can change for lower or higher depending upon circumstances. It seems to be that some of these circumstances have to do with what the system thinks the battery is holding. I've started with a 15% remaining at the finish of a charge and gone up to as high as 30% remaining.

 

[AlanSubie4Life]

So you're saying it DOES impact the computer calculations for trips? If so, that is kind of annoying.

Of course it does. If your rated range is lower, you have less energy available as far as the car is concerned, so you can't go as far. This is the desired behavior! It would be much more annoying if it did not do this.

It's not like the BMS is going to say "well it looks like I only have 68kWh of capacity, but maybe I'm wrong, so I'm just going to plan this trip for my owner assuming I have 75kWh like I 'should'!"

That would end in tears.

In reality you can compensate for the loss of capacity a little by cutting the margins tighter, but obviously not everyone likes to do that. The loss of capacity is nearly always real (of course there can be some recovery if the estimates are off, but it's not like you're going to increase 10% over time). And thus it has real impact on the trip planning.

There seems to be a misconception that the BMS and the rated miles is "just an estimate" and will adjust upwards once it "gets it right." While there are a few cases where the rated miles recover through the various procedures covered elsewhere, they only recover partially, and it's a small percentage of overall cases. Generally battery capacity inexorably marches lower! That's how entropy works!

So the short simple answer is the rated miles nearly always represent your actual available energy. There are few exceptions.

 

[Tam]

...If your rated range is lower, you have less energy available as far as the car is concerned, so you can't go as far...

I would say that what the drifted BMS lower number does not affect your real battery capacity.

It's just like a stereo volume that can have only a maximum volume but the volume now maybe painted as "10" maximum and someone else would repaint it as "8" maximum.

Repainting the numbers does not change the maximum volume availability.

That's the same with your car. If your car requires 353 miles or 100% on the battery gauge to get to the destination without stopping for a charge in between before the BMS drift, now after the 10% BMS drift which displays as 317.7 miles at 100%, your battery capacity has not changed and your car can still make to the same destination just like before.

It does affect psychologically to plan for the trip as 100% and 317.7 miles won't make it as one charge but in reality, there's no change and you'll make it as if it was 353 miles at 100% as before.

Most likely, along the way, the BMS would actively recalculate and may slow down the miles or % display.

For example, I bought my Model new 3 months ago and it used to display 353 miles / 100% for my road trip. When I am at 3/4 of the way, it would display 88.25 miles / 25%.

Now, 3 months later, it displays 317.7 miles / 100% for my same road trip. When I am at the halfway, it should display the new drifted BMS number of 79.425 miles / 25% but the BMS might correct itself along the way and display it as the correct number 88.25 miles / 25%.

Slowing down the consumption is well known in cold weather. If I started at 317.7 miles / 100% and I drove and a few miles and % wouldn't go down just yet because the battery was warming up.

On the other hand, the BMS might wait to recalculate at the last minute, so after driving to 317.7 miles the display says 0 miles / 0% but the car still not shuts down, and it would continue to drive another 35.3 miles before shutting down (for a total of 353 miles) and it displays 0 miles / 0% for that negative 35.3 miles below 0 miles / 0%.

 

[AlanSubie4Life]

For example, I bought my Model new 3 months ago and it used to display 353 miles / 100% for my road trip. When I am at 3/4 of the way, it would display 88.25 miles / 25%.

Now, 3 months later, it displays 317.7 miles / 100% for my same road trip. When I am at the halfway, it should display the new drifted BMS number of 79.425 miles / 25% but the BMS might correct itself along the way and display it as the correct number 88.25 miles / 25%.

I would say that what the drifted BMS lower number does not affect your real battery capacity.

I don't know why you would say that. If you look at the trip meter consumption it will tell you that indeed you have less energy in the battery, for a single continuous drive without stopping. For example:

Relatively new 2020 Model 3 18", displaying 320 rated miles at 100%. Start a road trip, drive down to 16 rated miles (5%).

The trip meter will show that you consumed: 304rmi*0.955*0.99*241Wh/rmi = 69.3kWh.

If you now take another 2020 Model 3 18", "BMS drifted," displaying 300 rated miles at 100%. Do the same trip, drive down to 16 rated miles.

The trip meter will show that you consumed: 284rmi*0.955*0.99*241Wh/rmi = 64.7kWh.

Alternatively, you could take the FIRST 2020 Model 3 (with 320 miles @100%) and instead start at 300 rated miles, and drive down to 16 rated miles.

You'll see the same 64.7kWh.

These are approximate numbers, but they are within 1-2% of the result (way tighter than the 7% difference between the numbers).

If you don't believe me, try it yourself. It's easily verified for whatever vehicle you have.

In the rare case of a large BMS drift that is actually just an error (which I agree may be possible in rare cases, of course), the numbers of course may not align. But this is not the NORM.

I'm not sure, but I think you were also making the point that practically it doesn't make much difference, which is true in most situations. But it's not true in the remaining situations, which are typically the ones of interest.

 

[Candleflame]

I don't know why you would say that. If you look at the trip meter consumption it will tell you that indeed you have less energy in the battery, for a single continuous drive without stopping. For example:

Relatively new 2020 Model 3 18", displaying 320 rated miles at 100%. Start a road trip, drive down to 16 rated miles (5%).

The trip meter will show that you consumed: 304rmi*0.955*0.99*241Wh/rmi = 69.3kWh.

If you now take another 2020 Model 3 18", "BMS drifted," displaying 300 rated miles at 100%. Do the same trip, drive down to 16 rated miles.

The trip meter will show that you consumed: 284rmi*0.955*0.99*241Wh/rmi = 64.7kWh.

Alternatively, you could take the FIRST 2020 Model 3 (with 320 miles @100%) and instead start at 300 rated miles, and drive down to 16 rated miles.

You'll see the same 64.7kWh.

These are approximate numbers, but they are within 1-2% of the result (way tighter than the 7% difference between the numbers).

If you don't believe me, try it yourself. It's easily verified for whatever vehicle you have.

In the rare case of a large BMS drift that is actually just an error (which I agree may be possible in rare cases, of course), the numbers of course may not align. But this is not the NORM.

I'm not sure, but I think you were also making the point that practically it doesn't make much difference, which is true in most situations. But it's not true in the remaining situations, which are typically the ones of interest.

Imho the displayed typical range is very accurate and a direct reflection of what the BMS thinks the capacity is (in fact it aligns perfectly with the nominal full pack w/o the buffer). Yes, there might be some battery imbalance etc which isn't accounted for but it is very rare for a Model 3 battery to become unbalanced.