Info: When Balancing Occurs, and Pack Maintenance

[Todd Burch]

I was just thinking that it would be nice to feed that bleed power to the 12V battery instead of burning it off as heat. I know it's not a lot of power, but would make things more efficient. Tesla probably considered something like this, but the additional complexity or cost is probably not worth it.

 

[ZBB]

Sorry, no; poor wording. I updated my post to try to make it more clear.

Take the SoC to 95% then discharge to 90% or less within hours. Wait for days for balancing to complete.

Fwiw, your edited post is still not clear. This response is much better wording.

 

[jpet]

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

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

I'm trying to imagine how this works, given that each of the 96 bricks report their own voltage. Is it the brick that gets to 4.2V first that determines CV charging for all? Fascinating stuff and lots of ways to implement optimization/balancing if you look at it at this detail.

 

[mspohr]

I'm trying to imagine how this works, given that each of the 96 bricks report their own voltage. Is it the brick that gets to 4.2V first that determines CV charging for all? Fascinating stuff and lots of ways to implement optimization/balancing if you look at it at this detail.

Interesting speculation. We are all like the blind men and the elephant here. Only Tesla really knows the full story of the specs,test data and the BMS algorithm so best to take all of the advice here with a large dose of skepticism.

 

[kennybobby]

Some of Lola's data to show brick voltages at the 'CV' knee in the charge curve and an overnight settling (balancing?):

The SC session (thanks to Jan aka jpet)

Bricks at the knee

Overnight

 

[deonb]

Interesting speculation. We are all like the blind men and the elephant here. Only Tesla really knows the full story of the specs,test data and the BMS algorithm so best to take all of the advice here with a large dose of skepticism.

But nobody inside Tesla with any real knowledge is speaking, so then you're left with the SC's... where the employees are kept just in the dark as us. The main difference is 95% of those employees don't even drive a Tesla.

 

[Cottonwood]

Fwiw, your edited post is still not clear. This response is much better wording.

OK, let's try one more time...



Reading from earlier posts by @wk057 and @Todd Burch, as well as insight from many other threads, here is my view of getting a pack balanced and calibrated. Comments are welcome.

1. Take the SoC to 95% then discharge to 90% or less within hours. Wait for days for balancing to complete.
   Details: To trigger a balance cycle, charge to over 93%, the constant Voltage part of the charge cycle (I like 95% or higher to make sure). After the balance cycle is triggered, you need to wait a few days for balancing to complete. Once the balance measurement has been done, go ahead and use that extra SoC and discharge back to 90% or less. The balancing is done by bleeding the extra charge off of stronger bricks[SUP]*[/SUP] to bring the bricks into balance over the next few days. This bleed cycle can selectively bleed off about 1% per day of charge from each brick, so giving the process a few days, allows for a worst brick difference of a few percent. The bleed/balance operation will continue at any SoC.
2. To get a good calibration of the battery pack capacity, charge once to over 90% and then discharge down to 10-20% (slightly lower is better for low end calibration). If you have time, do this after the above balance cycle; the calibration will still be pretty good if you overlap balance and calibration.
3. If at all possible, don't leave your pack for too long a time at over 90% or under 20% SoC. No need to be paranoid, but try to correct back into the mid-range in less than a day or so; correcting within hours is slightly better. High SoC for long periods at high temps should be avoided.
4. Charging to 100% does not help balancing, but don't worry about charging to 100% SoC before a long trip, or long segment. Just don't leave the pack at a high SoC for too long.

[SUP]*[/SUP]The battery pack in the 85 consists of 7,104 individual cells. These are arranged into 96 "bricks" of 74 cells in parallel (96*74=7,104). The 96 bricks are packaged into 16 modules of 6 bricks each. In each module, the 6 bricks are in series. The 16 modules are in series, putting all 96 bricks in series. Each of the 96 bricks has it's own bleed resistor set for balancing, i.e. there are 6 bleed resistor sets in each module.

 

[Andrew]

1. Take the SoC to 95% then discharge to 90% or less within hours. Wait for days for balancing to complete.

I'm still unclear on what you mean by "wait a few days for balancing to complete." Are you're saying that after charging to 95% (and then within a few hours driving to <90%), we should:

1. Continue to drive the car normally over the next few days, but do not charge during those few days? Or,
2. Let the car sit idle for a few days, without driving and/or charging? Or,
3. Just drive and/or charge as normal?

 

[jerry33]

I'm still unclear on what you mean by "wait a few days for balancing to complete." Are you're saying that after charging to 95% (and then within a few hours driving to <90%), we should:

1. Continue to drive the car normally over the next few days, but do not charge during those few days? Or,
2. Let the car sit idle for a few days, without driving and/or charging? Or,
3. Just drive and/or charge as normal?

Drive and charge as normal.

 

[mspohr]

Drive and charge as normal.

Great advice! Don't worry about it. Let the car take care of itself. If the car couldn't take care of itself and did need special attention, Tesla would tell you. You should just relax and enjoy the car.

 

[Cottonwood]

I'm still unclear on what you mean by "wait a few days for balancing to complete." Are you're saying that after charging to 95% (and then within a few hours driving to <90%), we should:

1. Continue to drive the car normally over the next few days, but do not charge during those few days? Or,
2. Let the car sit idle for a few days, without driving and/or charging? Or,
3. Just drive and/or charge as normal?

Drive and charge as normal.

Exactly!

The car will do the balancing on its own from the measurement made by taking the SoC to over 93%. You don't have to do anything special, but the balancing process may take up to a few days to complete.

 

[apacheguy]

I think the point of what the tech told the OP is that you don't have to worry about it. The car will take care of balancing by itself and you don't have to do anything. Just follow Tesla's recommendation for daily charge up to 90% and leave it plugged in overnight when possible or when you are gone for extended periods. If some special procedure was necessary, it would be in the manual. Just relax and enjoy your car.

Problem with that is if you charge to 90% daily you're leaving the cells at 4.05 V which is not good long term. Oh, and your pack will never balance. Needs to see CV phase of charging.

 

[Quantum`]

Problem with that is if you charge to 90% daily you're leaving the cells at 4.05 V which is not good long term. Oh, and your pack will never balance. Needs to see CV phase of charging.

Exactly.

And the BMS doesn't know exactly how much capacity it has even when balanced, unless you run it flat at least once. This means run it until it stops, not to 0 miles range. This would calibrate your system to be much more accurate, but how you do that is up to your imagination.

 

[Andrew]

Drive and charge as normal.

Exactly!

The car will do the balancing on its own from the measurement made by taking the SoC to over 93%. You don't have to do anything special, but the balancing process may take up to a few days to complete.

Thanks for clarifying.

 

[jpet]

Bricks at the knee

View attachment 111282

So brick 80 was in the lead here with 4.198V.
Speculation: maybe the CV phase and balancing are triggered at around 4.197V?

When you look at the first SC chart, the overall voltage still increases during the CV phase as the lower voltage bricks get closer to 4.20V as well.

 

[Soolim]

And the BMS doesn't know exactly how much capacity it has even when balanced, unless you run it flat at least once.

Keeping balance is good. knowing how much capacity when balance is less important. Running flat just to know the capacity is not something Tesla recommends.

 

[Ampster]

Problem with that is if you charge to 90% daily you're leaving the cells at 4.05 V which is not good long term. Oh, and your pack will never balance. Needs to see CV phase of charging.

Can you explain to us why it is not good long term to leave the cells at 90% (4.05v)? I understand why it is ocassionaly good to balance, but I have heard that the cells are often shipped at 50%. I have also heard that they should not be left at 100% for a long time.

 

[Soolim]

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.

Charging to 93% gets the charging circuit to be at constant voltage charging. Upon reaching 93% and when the charging stop, BMS takes voltage measurement of each of the 16 modules and decides which modules need to bleed off some charge (the ones with the higher voltage) by turning on the bleeding resistor of those modules. These resistors are fairly low wattage, so as not to cause overheating and not to highly impact the remaining charge of the whole pack. Balancing remain on for a long time for those module until the next time you charge to 93% to have the CV charging again, at which time the BMS makes another determination on which module's bleeding resistor needs to be turned on.

So occasionally bringing the SoC to 93% is good practice to cause the BMS to identify the higher charge (voltage) module. I have not done any measurement to determine what the "occasional" time period should be. And looks like it is not a "Must", because it is not documented in the user manual. But I would say that balancing the battery pack is a good thing, so as to bring the nominal stored charge of the battery closer to its initial state of health. A balanced battery pack has more stored energy than a highly unbalance pack. Long term unbalance pack means one is not getting the optimal energy storage capacity of the pack.

An analogy: Balancing the pack is a like a virtuoso pianist practising to to get all his ten fingers to achieve equal dexterity. One can still play without perfect dexterity, but not as good if all the ten fingers are of equal dexterity.

I look to the day that Tesla would provide a maintenance note suggesting the owner to initiate pack balancing at the most convenient time. Or maybe when you go to your SC for service, they automatically return the car to you with the balancing initiated after they run some diagnostics on the health of your battery.

 

[Soolim]

Can you explain to us why it is not good long term to leave the cells at 90% (4.05v)? I understand why it is ocassionaly good to balance, but I have heard that the cells are often shipped at 50%. I have also heard that they should not be left at 100% for a long time.

Prolong storage of battery in high SoC results in coating of the anode inhibiting ion transfer and reduces the remaining life of the battery.
50% SoC is good for storage due to the battery chemistry.

 

[Ampster]

I am trying to understand @apacheguy's statement that leaving NCA cells at 4.05 is not good for them. I think that is 90% of Tesla stated capacity and probably less than that for a battery chemistry that can be charged to 4.2v.