When does cell balancing happen?

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MikeinPA said:
xa, what a great idea. I am finally replacing some modules in a 2013 pack, and have been pondering how to get all the old cells as close as possible without taking them all up to 4.10 volts. Your technique is perfect--I will take all the cells down to 3.70, then let it balance for a while. The new modules will have greater capacity, so I will need to raise the voltage on those after using your technique.
I have done this (replaced modules in a pack), so a couple of comments.
  • It's not so much what you charge the pack to, but what you (manually) charge the replacement module(s) to. It's much easier to charge everything to "full"; after all, you aren't going to leave it there long and (trust me) it's difficult to get all the cells within the replacement module even/equal.
  • Worst case, charge the replacement module(s) a bit above the pack, because it's better to have your charge stop early (in my case I have one really good cell that I didn't balance well) than get a surprise at the bottom of your pack (which is what happens with "weak" modules).
I don't know if I explained that last part well, but top-balancing is definitely the way to go when replacing cells/modules, and the easiest way to do that is > 4.1V. I still don't have a well-balanced pack, but I probably have ~90% of my potential capacity, and the good/strong module is (very) slowly coming back in line with the rest of the pack over multiple charge/discharge cycles.
 
Stanton, thanks for the comments on preparing the replacement modules. I am placing the three new modules into the lowest voltage stack under the passenger seat, and moving good modules from there into the rear stack and elsewhere in the passenger seat stack. Hopefully this is the first and last time I will have to work on the rear stack. But how the BMS will handle the new modules is still unknown to me.

Call the existing good modules (45 of them) group "A", and give them 50 ahr. Call the three new modules group "B" and give them 55 ahr. I am thinking that at end of a 100% charge, you want group A at 4.10 volts, and group B at around 4.05 volts. At low battery warning, the group B modules will now be at a higher voltage than group A. So group A is available to fully charge and discharge, and group B is making a smaller voltage cycle within group A's larger voltage cycle. The issue now is ( and still somewhat on topic for this thread) is how the BMS is going to handle the voltage difference between the third group B module and the first group A module.
 
The BMS will deal with the lower capacity cells in a way that will limit the pack's capacity to match that of the weakest remaining cells. It will use the voltage of the weakest cell as the trigger point for Turtle mode.
 
Leftie, I understand that part. What I am wondering is how the BMS will atttempt to balance between module three and four. Turbo3 has the schematic here: https://www.mynissanleaf.com/viewtopic.php?t=17470 (third schematic). Imagine that B91 through B96 on that drawing have 55 ahr, and B90 and lower have 50 ahr. At higher SOC, B90 on up will have a slightly higher voltage than B91, and at lower SOC, B90 will be slightly lower than B91. It is comforting to think that the BMS will just try and make B6 and B7 closer--but I think it will be more complex than that.

Edit. If the function of the tiny resistors on the BMS is just to bleed off voltage to bring higher cells down (one per cell), then the behavior should be amenable to modeling in a spreadsheet. I will look online to see if that is indeed the case, then try a spreadsheet.
 
MikeinPA said:
Call the existing good modules (45 of them) group "A", and give them 50 ahr. Call the three new modules group "B" and give them 55 ahr. I am thinking that at end of a 100% charge, you want group A at 4.10 volts, and group B at around 4.05 volts. At low battery warning, the group B modules will now be at a higher voltage than group A.
I don't know how you arrive at that last conclusion, but no matter: you are "over thinking" things.
You have two different sets/age of modules (at least in your example)...and the lower capacity set will never "catch up" to the higher capacity set. Also, in my experience (at least so far), the BMS (or more specifically the circuitry you referenced) doesn't have the ability to "pull" the voltages that far either way at any given time; in other words, they tend to end up (after charging) with the same spread/delta as when they were installed.
To sum it up, the best way is to top balance as closely as you can--especially since you seem to be replacing many more modules than I did (I simply had one weak/bad module). Which brings me to another point: if you are just trying to "fix" a weak/old pack, it's much easier to replace the whole pack with a better/newer pack. Swapping modules around is like re-arranging the deck chairs on the Titanic (and a lot of work).
 
I don't know how you arrive at that last conclusion, but no matter: you are "over thinking" things.
You have two different sets/age of modules (at least in your example)...and the lower capacity set will never "catch up" to the higher capacity set. Also, in my experience (at least so far), the BMS (or more specifically the circuitry you referenced) doesn't have the ability to "pull" the voltages that far either way at any given time; in other words, they tend to end up (after charging) with the same spread/delta as when they were installed.

Simple. Both groups are having the same ahrs removed during discharge, but since group A has less total ahr, its voltage will drop faster. Group A may start higher, but end lower. I am trying to figure how the BMS is going to respond to this behavior over the long run. If the BMS operates by dissipating watts from the higher voltage cells, then at higher SOC it will be dissipating group A and at lower SOC it will be dissipating group B.

Maybe simpler for you to locate and install a 40 kwh pack (was it >12k?) and then replace a module. My situation is different and I am perfectly happy learning how to repair these packs as well as the whole car.
 
The batteries are connected in a 2P96S arrangement. The BMS can't control where the power is coming from since there are 96 individual cells (in 48 modules) connected in series. The current is coming from ALL of them simultaneously.

The power will drain from all cells more or less equally so the cells with a lower capacity will reach their minimum voltage before the cells with a higher capacity do. Bottom line is the car will be limited by the lower capacity cells and when the first individual cell reaches the cut-off voltage the car will go into Turtle mode.

The balancing circuitry will try to even out the voltages of the cells - but as you know - that current is miniscule compared to what is needed to move the car so any balancing effect will be insignificant.
 
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