Battery Management System - What I Learned At Tesla Service Center

[lencap]

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!

 

[electrongeek]

Actually the BMS always works. What your SC guy is saying is that the calculations for rated range are aided by a discharge from 90% to 10%. In addition, because cells age at different rates and internal resistances of cells vary from one another, some cells in a parallel group may be totally charged at a given top voltage before other cells with higher internal resistance have reached that state. Charging occasionally above 90% allows the BMS to more fully charge those laggard cells and so give you a little more actual capacity/range.

 

[lencap]

Thanks for the clarification. Despite my misunderstanding it still seems as if it's beneficial to consider charging from 10-90% (+/-) instead of standard 30-80 or 20-80. Is that correct?

 

[apacheguy]

Thanks for the clarification. Despite my misunderstanding it still seems as if it's beneficial to consider charging from 10-90% (+/-) instead of standard 30-80 or 20-80. Is that correct?

The only reason to go 10-90 is to recalibrate the capacity algo. The BMS does an excellent job of keeping the battery balanced. On my S, I used to always charge to 70% and the cells were always within 4-6 mV of each other.

 

[Kevy Baby]

Thanks for the clarification. Despite my misunderstanding it still seems as if it's beneficial to consider charging from 10-90% (+/-) instead of standard 30-80 or 20-80. Is that correct?

I think the point of the 10 to 90 sweep is to do it ONCE IN A WHILE - not as a normal practice. And from what I am reading (including your post) is that this only affects range.

 

[SlyOne]

I think the point of the 10 to 90 sweep is to do it ONCE IN A WHILE - not as a normal practice. And from what I am reading (including your post) is that this only affects range.

I understand the once in awhile, but am wondering if this affect the actual range (number of miles that can be driven), or the “stated” range. In other words does this process just affect the interpretation or actually range vehicle can be driven?

 

[derotam]

I understand the once in awhile, but am wondering if this affect the actual range (number of miles that can be driven), or the “stated” range. In other words does this process just affect the interpretation or actually range vehicle can be driven?

So it shouldn't affect the ACTUAL range that you can travel...as long as you don't have ACTUAL battery degradation.

As far as what the service guy told the OP, the service guy doesn't know how the system works evidently. The BMS is always working and balancing the cells.

 

[SlyOne]

So it shouldn't affect the ACTUAL range that you can travel...as long as you don't have ACTUAL battery degradation.

As far as what the service guy told the OP, the service guy doesn't know how the system works evidently. The BMS is always working and balancing the cells.

Thanks, I put mine on % so don’t get concerned with stated range, but occasionally take trips that use 95% of battery (250 miles on LR AWD in winter)

 

[TydalForce]

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

 

[gaswalla]

The goal should be to maintain battery health. This means maintaining battery capacity. Thus avoiding deep discharges and full charges is beneficial. Many folks notice the calculated range increases after long road trips. This is because they are doing deep discharges and full charges. My thought is avoid the purposeful rebalancing acts and let them occur naturally with use

 

[TJKR]

The goal should be to maintain battery health. This means maintaining battery capacity. Thus avoiding deep discharges and full charges is beneficial. Many folks notice the calculated range increases after long road trips. This is because they are doing deep discharges and full charges. My thought is avoid the purposeful rebalancing acts and let them occur naturally with use

I agree with this and drdumont, cycling BMS is almost pointless. In the long run, you are not doing the battery any favors.

I think the problem with people obsessing about range is they have a “gas tank” mentality about electric cars. Electric car batteries do not function like a gas tank.

Most important thing about the car is know your daily driving distance. This applies to Model 3 LR... If you drive 50 miles or less a day, charge it to 60%, 100 miles or less per day, charging between 70%-80% is plenty. If you drive 200 miles, charge it to 90%. If you drive more than that routinely every day.. god help you.

For road trips, Superchargers are spaced about 150 miles apart – 2 hrs of driving/restroom break… charge to about 60% and go down to 10%. Most locations and times, your car will be ready to go after you use the bathroom.

 

[Tam]

...especially the part about not charging when I'm not driving the car...

Just a reminder that this advice is verbal and not written documentation. This is contrary to the documented Owner's manual.

Just like a salesperson who might say lots of things but if they are not written down on a receipt, it's questionable.

...There isn't any other way to explain my range loss...

Your loss can be explained by not following written documentation of plugging in when you park and charging to 90% at least every night:

Service appointment for 12 month old Model 3 for range loss /invoice for out of warranty inspection?

...run the battery down to 10% or less then charge it up to 90% or more. Repeat this process for several cycles...

If you need to use your range on a road trip, then use it, but to deep cycle your battery for the purpose of "gaining" range is not recommended:

Tesla Battery Range Degradation


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

In summary, talks are cheap but if are they willing to put those in a Tesla written documentation?

 

[electrongeek]

The only reason to go 10-90 is to recalibrate the capacity algo. The BMS does an excellent job of keeping the battery balanced. On my S, I used to always charge to 70% and the cells were always within 4-6 mV of each other.

You are not able to monitor all 4000+ cells individually, are your? A certain number of cells are paralleled in each package, so can't be individually monitored for voltage. It would be possible for individual cells in each "package" to become out of balance unless steps are taken to mitigate that.

 

[Az_Rael]

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.

I would file a lot of that under "stuff the SC says", which is often not correct information.


If you are really interested in learning about the BMS and tracking the battery for your car, I recommend buying or making a CANbus cable for the 3 and using ScanMyTesla. Lots of information can be extracted from the bus including how imbalanced your battery is, etc. The ScanMyTesla website has some info about what hardware you need.

 

[Knightshade]

Thanks, I put mine on % so don’t get concerned with stated range,

Here's the correct answer

 

[lencap]

Thanks for the replies/info. My OP was intended to better understand why my range was decreasing. I do take longer trips on occasion, and when I do having the range extend as far as possible gives me some additional options about where/when to charge. Often I'm arriving home after midnight. During those times having 30 or so extra miles of range may allow me to skip a charging stop - that's welcomed during bad weather, or late at night when I'd rather not be charging.

The information about how the BMS works (and my explanation and description could be inaccurate) was new to me. It seemed to explain why my range declined, and also gave me some insight into how to balance the long term health of the battery versus having greater range for when I may need it. Following the discussion with the service tech I decided that potentially for my use the preferred strategy may be to "cycle the BMS system" occasionally by following the method I described earlier, especially in anticipation of longer trips.

The other thing I decided was that, for me, since I intend to keep the car for a prolonged period being aware of long term battery health is important. 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. 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.

That's all I was trying to understand. If your charging methodology works better for you, that's great, but regardless having a better understanding of how BMS can affect my car for my needs is valuable. And that's why I posted - it may be helpful for others as well.

 

[Teedub21]

Most important thing about the car is know your daily driving distance. This applies to Model 3 LR... If you drive 50 miles or less a day, charge it to 60%, 100 miles or less per day, charging between 70%-80% is plenty. If you drive 200 miles, charge it to 90%. If you drive more than that routinely every day.. god help you.

Or just charge to 90% like Tesla recommends and forget about the rest.

 

[Rocky_H]

So how often do you think this should be done?

"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.

 

[sraatc17]

"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.

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).

We are planning a road trip to Philly soon and my question is i'm wondering if the car will find some of the missing range or re calibrate the battery estimates with this longer drive (getting the battery down to 10% before supercharging). Is my range actually 229 miles until that happens because the car will shut down at that point because it thinks the battery is drained?

 

[apacheguy]

You are not able to monitor all 4000+ cells individually, are your? A certain number of cells are paralleled in each package, so can't be individually monitored for voltage. It would be possible for individual cells in each "package" to become out of balance unless steps are taken to mitigate that.

Well I was reading directly from the BMS so if couldn’t read the individual voltages then neither could the BMS. But yes, you are correct that I was only reading the voltages at the “brick” level. The cells that are in parallel necessarily must have the same or very close to the same voltage.