Info: When Balancing Occurs, and Pack Maintenance

My only comment is that I has to be tricky for the BMS to turn on the bleed resisters because of the flat charge curve. Perhaps this chemistry has more slope than the LIFEPO4 cells I have observed.

Todd Burch said:

What I learned from this is that the pack is actually *always* balancing. All the time.

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I know the system. It's similar to my RC Helicopter, when I charge the 6 cell pack. All the cells are being monitored and actively balanced when charging.

I believe, this in one of the causes of the 'vampire drain'. So many thousands of tiny cells, being balanced 24/7 consumes power. Some of those 50 watts of vampire drain is coming from this.

This makes eminent sense. It translates to another example of letting the battery management system worry about the battery (within limits).
Another way to say it is, don't go by folklore -- regardless of how well-intentioned the proponent or how well-versed he or she may seem to be -- if it isn't in the Tesla owner's manual, then most likely we do not need to know.

Todd Burch said:

For years, there has been speculation about when battery balancing occurs. Some speculated it occurred at 90% or higher. Others speculated that it only occurs with the battery fully charged.

I had my seatbelt inspection today, and while there the service tech was checking a few things in my diagnostic screens--the BMS tab happened to be up. This reminded me to ask about balancing, since everyone seems to have that question. What I learned is that the pack is *always* balancing. All the time. Which makes perfect sense and agrees with other things we've seen.

This confirms a few things:

1. You don't need to charge to any charge level for the pack to balance. It always happens.

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Are you 100% sure about that?

I thought that wk057 found, during his hacking/exploration, that the balancing doesn't kick in until you hit the 93% charge level.

I just do what Tesla tell me and I live in ignorant bliss:tongue:. It is Saint Elon, after all!

Seriously though, I simply do not worry about it. I have never had a problem with range. Except once, when it was completely my own fault. But I made it home. I learned now much headroom I need, based on my driving habits and terrain and just charge to that. Seems to be just fine. Even on trips. I used to have my battery display miles and worry about it. Now I use percent, just like the way I used to look at my gas tank.

Well, a few notes based on my experience with the battery packs and the BMS.

Todd Burch said:

1. You don't need to charge to any charge level for the pack to balance. It always happens.

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I think the person you spoke with may be confused a little. Yes the bleed resistors on the BMS module boards can be active at any time at any charge level. However, unless they recently changed something, they never appear to get enabled until the pack reaches the constant-voltage phase of charging, at which point the BMS knows for sure which cells are higher than others and which ones to bleed. The resistors then stay enabled for a calculated period of time (based on the delta between the highest and lowest cells) and I guess you could then consider that "always" balancing... but it only starts once the pack hits CV, which is roughly 93% SoC (can be a little higher).

Todd Burch said:

2. Running the pack down to near empty, then fully charging, appears to improve range, but of course this is only imaginary. Doing this doesn't help balance anything, it simply helps the range estimation algorithms get a better view of the voltage range of your pack. Don't do this to improve your range numbers--you're simply stressing the pack for no good reason. (This was generally accepted by many here to be the case).

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Correct, with the exception of the above about getting the bleeders enabled if it's been a while. Otherwise the rest is basically an illusion.

Todd Burch said:

3. Charging to 100%, and letting it sit there for a few hours (which in my recent experiments seemed to be helpful for indicated range), doesn't improve balancing at all. Again, it just seems to help the algorithms *maybe* improve the *indicated* range.

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Same as above. The sitting at 100% is the bad part.

Todd Burch said:

4. The long, long taper at the end of a 100% charge is not balancing. It's simply providing sips to the pack to top it off.

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Correct, although the balancing is enabled at this time if required. The taper is the constant-voltage phase of the charge where the cells have reached peak voltage, but not peak capacity.

Todd Burch said:

5. Apparently, an update is coming soon that will improve the algorithm. (This one isn't that newsworthy, but it's nice to hear).

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This would definitely be welcomed and hope it's true.


In any case, 99% of the time the BMS does a great job with the pack. Almost no one's pack is *actually* out of balance enough to lose any range. That's just a myth pretty much. Almost all range loss is due to the algorithm used to calculate capacity being miscalculated due to charging habits. If you never charge to 90%+ and/or never discharge to <10%, then eventually the error in the algorithm will start to add up in the form of false capacity loss. Basically Tesla just needs a way to let the BMS reset this and re-calibrate when possible, potentially by monitoring the potential sum of the calculation error and encouraging whatever type of charge or discharge is needed to help to the user when it's needed to re-calibration. Probably too complicated, so my guess is most likely they're going to refine the algorithm based on real world data from 10s of thousands of vehicles in the field in order to refine it a bit, although it still won't be perfect.

Long story short... just don't worry about it. If you've lost a substantial amount of range, just call Tesla and let them deal with it. There isn't much that you can do to "fix" it.

CHG-ON said:

I just do what Tesla tell me and I live in ignorant bliss:tongue:. It is Saint Elon, after all!

Seriously though, I simply do not worry about it. I have never had a problem with range. Except once, when it was completely my own fault. But I made it home. I learned now much headroom I need, based on my driving habits and terrain and just charge to that. Seems to be just fine. Even on trips. I used to have my battery display miles and worry about it. Now I use percent, just like the way I used to look at my gas tank.

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I just switched to percent from miles on battery a couple of weeks ago and it felt like I'd finally "arrived" to full EV mindset at that moment. A very nice feeling. And after 6000 miles and 4 months with the car, I really don't worry about the battery at all anymore either.

JenniferQ said:

I just switched to percent from miles on battery a couple of weeks ago and it felt like I'd finally "arrived" to full EV mindset at that moment. A very nice feeling. And after 6000 miles and 4 months with the car, I really don't worry about the battery at all anymore either.

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Well, I used percent for a while myself. Unfortunately, with only 1% resolution it's almost 3x less precise than rated miles. So, I switched back to rated miles so I had more granularity... not that the "miles" mean anything to me other than a capacity available measure. If Tesla exposed 0.1% resolution (it's there, they just round down to the nearest whole number it looks like on the UI when using percent) then I'd never look at a "miles" rating ever again.

Todd Burch said:

Well nobody's information is 100% definitive I guess, but I don't see why they would impose a limit of 93% in order to initiate balancing. What would be the purpose of that, especially since there are people that almost never charge to that level? It makes more sense to just always be bleeding off higher cells. Those resistors are so small that it's a time-consuming process, and one in which you really wouldn't notice the consumption from day-to-day.

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Consensus amongst the technical crowd of this forum says that Tesla top balances. However, once the off balanced bricks light up during CV charging, the bleed resistors will continue to do their thing regardless of charge state. Perhaps this is what the tech meant when he said the pack is always balancing. Here's why:

Otmar said:

You can balance at any one SOC, but with real life cells (varying capacity) it will be out of voltage balance at any other SOC.
I will attempt to explain further, since I know this can be hard to visualize.
We know that every brick has a slightly different capacity in amp hours.
The open circuit voltage at no load, ignoring internal resistance, can indicate SOC of a brick, but SOC is just indicating a percentage of available Ahrs from that brick.
For an extreme example: Assume two bricks, one with 100 Ahr and one with 200 Ahr capacity. Balance them at the top, say 4.2V. Now remove 100Ahr from both in series. At this point if you tried to balance the voltage the 200 would have to lose a lot of energy since it's at 50% SOC while the 100 is empty. But, if you don't balance and you put 100Ahr back into them both they will magically comeback into balance, just like in the supercharging animation. (Nice animation!) Balancing takes much longer than what you see when supercharging. The reason one only wants to balance at one voltage is so that you are not constantly chasing the balance.

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Thanks for reporting your findings, Todd, and to wk057 for weighing in as well.

How do those bleeders work? Are they actively and individually switched on and off for those cells that need it?
Or is it through clever circuit design that they are bleeding off cells that are at higher voltage than other cells?
Otherwise it seems like they would be bleeding everyone at the same rate and won't help with balancing at all.

If the bleeders are only doing their thing when the cells are at peak voltage, then they shouldn't contribute to vampire loss when below 93% right?

Is this 93% peak voltage why when we first got the cars, before the battery slider was implemented, that the "standard" charge was at 93%?

The capacity miscalculation: is this why some people seem to be able to keep driving even after the range shows zero miles, while other cars shut down right at zero?

The bleed circuits each control a separate brick so only the bricks with excess Ah will trigger the bleed. There are 16 modules and 96 bricks in an 85 kWh pack.

Todd Burch said:

.................
.... My speculation is that the BMS calculates where it thinks zero ought to be, but if the algorithm is off and the actual voltage ends up being higher than expected, the BMS allows the car to continue to drive.

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Yes, the BMS is guessing where zero is in the aggregate and is letting the car use the pack until the pack Low Voltage Cutoff is reached or one of the modules is approaching its minimum voltage, whichever occurs first.

If the equilization is only activated when above 93% SoC, my car, and many others,
have never begun equalization. Also, the measurement of the 6 "cell" voltages
in each module is best done when neither charging nor driving (and not when heating
or recharging the 12v battery). Best to measure when there is essentially no current
going into, or coming out of the traction battery.

So, if making the equalization decision only while charging, and when only when over
93% SoC ... that would seem to be a substantially sub-optimal design decision.

IMO, it is more likely that the start and stop decisions are made on a "cell" basis,
in real time, and perhaps only when stopped or parked.