balancing strategies as they relate to BMS

[ArthurEld]

I finished a couple capacity tests using 3.6V top and 2.8V bottom and no absorb.
I got 275Ah on one and 276Ah on the other. That seems about right since I didn't go all the way to the top or bottom and I didn't absorb.
2 more are being tested now.
I tested the internal resistance @ 3.35V, 3.6V and 2.8V. They were all .17 or .18
I can't find anything significantly different about these cells.
So far no clues as to why one cell took off at a little over 3.4V when I was charging in serial.
I'm starting to wonder if the resting voltage was fooling me when I did the original serial run.
Hopefully I won't run out of motivation before I find the answer.

 

[Gazoo]

I am very close to finishing my capacity test. I am noticing as the cells voltages get lower there is a larger difference of the voltages between the cells. Right now my pack is at 25.27 volts and the difference between the cells is 76mv. I will finish it later today and update.

 

[ArthurEld]

That is fantastic Gazoo. I'm sure you'll get good capacity you will be done with balancing.
I will finish capacity testing these 16 cells and if they all seem the same I will top balance and see how my cells behave.
No balloons! ?

 

[Colonel.lp]

I finished a couple capacity tests using 3.6V top and 2.8V bottom and no absorb.
I got 275Ah on one and 276Ah on the other. That seems about right since I didn't go all the way to the top or bottom and I didn't absorb.
2 more are being tested now.
I tested the internal resistance @ 3.35V, 3.6V and 2.8V. They were all .17 or .18
I can't find anything significantly different about these cells.
So far no clues as to why one cell took off at a little over 3.4V when I was charging in serial.
I'm starting to wonder if the resting voltage was fooling me when I did the original serial run.
Hopefully I won't run out of motivation before I find the answer.

Interesting. The video I got from xuba showed my cells IR at 3@0.12 and one at 0.14, voltage 3.32xxV, and that would be at 99% charged. Assuming my 265ah cell is the 0.14mohm one.

What I have seen is that the last run up to 3.65V is so quick, even a small amount of imbalance will case one to sprint ahead.

 

[John Frum]

@Craig
Finally got around to testing my balance strategy.

1. Assembled cells into 8s serial pack with bms.

Used 10AWG wires for balance leads so that I could use the leads to charge and discharge cells independently​

2. Configured BMS to trip on first cell to 3.55VDC
3. Charged 25A@28.6VDC until high cell tripped the BMS.

The spread was ~.15VDC​

4. Used a little buck converter to top each cell separately to 3.625VDC using the balance leads.
5. Let cells settle over night

Voltage spread was ~.05 volts​

6. Turned on BMS balancing

Waited 24 hours​

Spread was ~.005 volts​

7. Turned off BMS balancing
8. Discharged pack at ~40 amps until low cell hit 3.00VDC

Cells stayed with .01VDC all the way down to 25VDC and then diverged to .05VDC into the low knee​

10. Recharged 25A@28.6VDC

Cells re-converged as soon as they got out of the low knee​

Spread was ~.02VDC at the top​

Let settle overnight​

Spread was ~.01VDC​

I'm gonna call this pack balanced.

tl;dr classic parallel top balance does not appear to be stictly necessary.

 

[ArthurEld]

The balancer is just going to redo it anyway. I've come to the conclusion that there are lots of ways to get the states of charge in line.
Just the balancer will do it too. Just gradually keep moving the voltage up so it has time to balance before hitting the cell overcharge limit.
It takes time but little effort.

 

[John Frum]

The balancer is just going to redo it anyway.

I have since enabled the bms balancer for charging only and only above 3.375VDC.
So its not really going to redo things, just tweaks.

 

[ArthurEld]

I have since enabled the bms balancer for charging only and only above 3.375VDC.
So its not really going to redo things, just tweaks.

There's many ways to skin a cat.