Cell health in a 16s pack

[Alexplose]

Here is the measured voltage this morning :

1. 3.583
2. 3.577
3. 3.584
4. 3.584
5. 3.588
6. 3.585
7. 3.576
8. 3.575
9. 3.574 (former #13)
10. 3.575
11. 3.586
12. 3.581
13. 3.586 (former #09)
14. 3.588
15. 3.581
16. 3.585

It seems that the cells that were the farthest from the charger legs, have the lowest voltage. (former #13 is the lowest, maybe the weakest)

Do I need to individually charge the lowest ?

I'll rebuild the pack when I have some time, I soldered the bms wires into the crimps to make sure connection is good.

 

[solardad]

Here is the measured voltage this morning :

1. 3.583
2. 3.577
3. 3.584
4. 3.584
5. 3.588
6. 3.585
7. 3.576
8. 3.575
9. 3.574 (former #13)
10. 3.575
11. 3.586
12. 3.581
13. 3.586 (former #09)
14. 3.588
15. 3.581
16. 3.585

It seems that the cells that were the farthest from the charger legs, have the lowest voltage. (former #13 is the lowest, maybe the weakest)

Do I need to individually charge the lowest ?

I'll rebuild the pack when I have some time, I soldered the bms wires into the crimps to make sure connection is good.

@Alexplose so these numbers look really good! Better than my pack when I put it together. evidence I believe that this points to a bms or settings issue vs. a cell issue.

 

[TomC4306]

Time to reassemble in series. Those numbers look great.

Carefully clean with isopropyl alcohol every single metal piece. Every battery terminal, every ring lug, every post (switch, breaker, bus bar)

Do this as you go. Less chance you'll miss one.

And also torque as you go.
I use this one.

https://www.amazon.com/TEKTON-24320-4-Inch-Torque-2-26-22-6/dp/B00C5ZL2EG/ref=asc_df_B00C5ZL2EG

Use a sharpie to make a mark (known to you) on the connection after you're done with each connection.

Clean, assemble, torque, mark every single connection.

 

[sunshine_eggo]

+1 to @solardad and @TomC4306 on them looking great.

I recommend you hit each one individually and surprise yourself how each cell runs to 3.65 in short order.

 

[Alexplose]

Hi everyone,

I did put my pack back online a few days ago, here are the new curves I have.



Really impressive, I'll continue checking what it looks like on the high end and low end, to see if I have any issues, It seems that #3 has a connection issue, maybe a bit dirty. But so far I like what I see.

Thanks for your help

 

[Steve_S]

The little gremlins that pop up can really lead you on a Wild Goose Chase !
Our favourite Aussie just had yet another battle with busbars getting hot and the reason, far too well known but again popped up to zot his project.

Sometimes it is really the dumb things that many miss that cause issues.... With copper/plated busbars the ridges & burrs are culprits in many cases and often the absolute LAST thing looked at...

 

[solardad]

Hi everyone,

I did put my pack back online a few days ago, here are the new curves I have.

View attachment 87389

Really impressive, I'll continue checking what it looks like on the high end and low end, to see if I have any issues, It seems that #3 has a connection issue, maybe a bit dirty. But so far I like what I see.

Thanks for your help

@Alexplose great news. It has been my experience that at the high and low end of the charge / discharge curves the cells start to deviate. Since I keep my overall pack to a rough 20-80% range I avoid this scenario and do not have to rely on the BMS to do any balancing. For my cells that usually means cells above 3.45v and below 3v.

 

[HRTKD]

@Alexplose , that graph looks MUCH better. I'm sure you're breathing a huge sigh of relief.

 

[pellicle]

If you "top balanced" your cells to 3.55 you didn't top balance you cells. Fully Top balance to 3.65 you won't hurt the cells if you're doing this properly.

actually having spent my time on other chemistries (lithium, but no fepo4) I wonder how you can actually "balance" if you don't take them to 3.65.

Basicallyy, how I understand things such as how a BMS works is that if you stop charging at 3.55 you won't get balance occurring because (as I understand it) balance to lagging cells is handed out when they get to 3.65V (and thus significantly drop their ability to absorb current). I understood that it is on measuring this current that "charge completion" was calculated inside the BMS.

For instance, while this diagram is for lithium ion (cobalt) I understood that aside from the actual voltages involved the rest of the curves are the same.



Can someone provide an explanation of how actual charge balance occurs if charge is shut off at 3.55 and why this will not result in cells drifting slowly out of balance with each discharge curve?

Thanks

 

[sunshine_eggo]

actually having spent my time on other chemistries (lithium, but no fepo4) I wonder how you can actually "balance" if you don't take them to 3.65.

Balance can be achieved in the upper portion of the charge curve. 3.65V is ideal, but 3.55V can get you close enough. The goal is to get cell SoC imbalance at peak charge to a very very low %.

Basicallyy, how I understand things such as how a BMS works is that if you stop charging at 3.55 you won't get balance occurring because (as I understand it) balance to lagging cells is handed out when they get to 3.65V (and thus significantly drop their ability to absorb current). I understood that it is on measuring this current that "charge completion" was calculated inside the BMS.

Most BMS begin balancing well before 3.55-3.65. Once cells hit 3.40V under modest currents, they are close to full. Typically, a BMS begins pulling <100mA off the high voltage cells to balance as the cells fill.

For instance, while this diagram is for lithium ion (cobalt) I understood that aside from the actual voltages involved the rest of the curves are the same.

View attachment 87632

Not even close.

NCA, NCM, LMO are near constant slope linear between the legs. LFP is FLAT.

Can someone provide an explanation of how actual charge balance occurs if charge is shut off at 3.55 and why this will not result in cells drifting slowly out of balance with each discharge curve?

Thanks

Your experience with Cobalt chemistry is clouding your perception. LFP voltage in the upper/lower legs is "mushy" like lead-acid. When you terminate Cobalt at 4.20V, there is very little voltage settling. LFP behaves much more like lead-acid. Charge at 2.4V, settle at 2.15V in a day or two. LFP isn't that bad, but it is similar. LFP resting voltage will fall from 3.65V to 3.50V within a day or two. Smaller cells can drop dramatically faster. I have some CALB cells that will drop from 3.65V to 3.35V within just a few hours. After a year of sitting, they dropped to 3.30V, yet they retained 97% of their charge, i.e., were at 97% SoC even at 3.30V.

The combination of charge termination current and peak voltage matter. Most specs call for charge to 3.65V @ 0.5C terminating at 0.05C. If you terminate at even lower current, you can get fully charged at 3.55V. I have personally charged cells to >98% full at 3.45V with very long absorption periods and low charge termination current.

The practice of balancing only during charge and starting at or around 3.40V when cells are out by more then 20mV can maintain the balance of healthy cells even if they aren't charged all the way to 3.65V.

 

[Berseker]

Is this top balanced when the voltage accross the pack is 3.654V, and the individual cells are about 3.650-1V, charger is still pushing some amps, I disconnected, will let rest and see the resting voltage.

charger is still pushing some amps??. @ 3.65v?..if yes..the top balance isn't complete yet. amps to battery should be zero, before you can conclude that top balance is successfully over

 

[Steve_S]

I bulk/absorb charge LFP to 3.500 and Float to 3.475Vpc allow Amps to drop to 5A taken *fully saturated with 280AH cells
Within 45 Minutes after charge the cells will all settle to 3.400 +/- 0.10mv across all 5 Packs.
LFP will never charge to 3.650 and stay there, even if you have over saturated them down to where they only take <1A, they will still settle to below 3.500, more typically A+ Cells will drop to 3.450 or 3.425 within an hour Post Charge.

LFP delivers it's Amp Hours from the Working Voltage Range (3.000-3.400Vpc), NOT the Allowable Voltage Range (2.500-3.650).
Most LFP Makers recommend EndAmps be at 0.05XAH Rating (0.05x280AH=14A) see their docs.

 

[pellicle]

Your experience with Cobalt chemistry is clouding your perception. LFP voltage in the upper/lower legs is "mushy" like lead-acid. When you terminate Cobalt at 4.20V, there is very little voltage settling. LFP behaves much more like lead-acid. Charge at 2.4V, settle at 2.15V in a day or two. LFP isn't that bad, but it is similar. LFP resting voltage will fall from 3.65V to 3.50V within a day or two. Smaller cells can drop dramatically faster. I have some CALB cells that will drop from 3.65V to 3.35V within just a few hours. After a year of sitting, they dropped to 3.30V, yet they retained 97% of their charge, i.e., were at 97% SoC even at 3.30V.

interesting, thanks for that.

 

[pellicle]

NCA, NCM, LMO are near constant slope linear between the legs. LFP is FLAT.

PS: wouldn't this mean that measuring current (as I said) was the more reliable way of measuring the state of charge of the cell?

 

[RCinFLA]

Here is the measured voltage this morning :

1. 3.583
2. 3.577
3. 3.584
4. 3.584
5. 3.588
6. 3.585
7. 3.576
8. 3.575
9. 3.574 (former #13)
10. 3.575
11. 3.586
12. 3.581
13. 3.586 (former #09)
14. 3.588
15. 3.581
16. 3.585

You are being way too picky. If the cell charge current has dropped off to low levels, the cells are fully charged with various amounts of surface charge capacitance causing cell voltage above 3.45v when cell is rested and unloaded.

LFP rested no-load cell voltage greater than 3.45v is fully charged cell. Rested no-load cell voltage beyond 3.45v is surface charge that has no effect on cell capacity or state of charge. It amounts to about 0.01% of cell AH capacity and can be discharged with a 1 to 3 ohm load resistor in less than a minute, after which the cell rested open circuit voltage will be about 3.43v to 3.45v.

The reason for charging voltage above 3.45v is to speed up the charging process. Cell current creates a cell overpotential voltage that must be overcome at given charge current to charge cell. The overpotential voltage level is dependent on cell current so charging at higher current requires higher absorb voltage and longer dwell time at absorb voltage before charge current tapers off. Current induced overpotential voltage is traded for surface capacitance charge voltage at absorb voltage as charge current tapers off.

 

[Sojourner1]

Balance can be achieved in the upper portion of the charge curve. 3.65V is ideal, but 3.55V can get you close enough. The goal is to get cell SoC imbalance at peak charge to a very very low %.



Most BMS begin balancing well before 3.55-3.65. Once cells hit 3.40V under modest currents, they are close to full. Typically, a BMS begins pulling <100mA off the high voltage cells to balance as the cells fill.



Not even close.

NCA, NCM, LMO are near constant slope linear between the legs. LFP is FLAT.

View attachment 87637



Your experience with Cobalt chemistry is clouding your perception. LFP voltage in the upper/lower legs is "mushy" like lead-acid. When you terminate Cobalt at 4.20V, there is very little voltage settling. LFP behaves much more like lead-acid. Charge at 2.4V, settle at 2.15V in a day or two. LFP isn't that bad, but it is similar. LFP resting voltage will fall from 3.65V to 3.50V within a day or two. Smaller cells can drop dramatically faster. I have some CALB cells that will drop from 3.65V to 3.35V within just a few hours. After a year of sitting, they dropped to 3.30V, yet they retained 97% of their charge, i.e., were at 97% SoC even at 3.30V.

The combination of charge termination current and peak voltage matter. Most specs call for charge to 3.65V @ 0.5C terminating at 0.05C. If you terminate at even lower current, you can get fully charged at 3.55V. I have personally charged cells to >98% full at 3.45V with very long absorption periods and low charge termination current.

The practice of balancing only during charge and starting at or around 3.40V when cells are out by more then 20mV can maintain the balance of healthy cells even if they aren't charged all the way to 3.65V.

Please elaborate on "mushy" meaning, the only thing that comes to my mind is a fruit or vegetable just before rotting.

 

[sunshine_eggo]

Please elaborate on "mushy" meaning, the only thing that comes to my mind is a fruit or vegetable just before rotting.

When current is removed, voltage settles or rises over time - not just instantaneously as a function of the current itself. One charges to voltages higher than resting to fully charge and discharges to voltages lower than resting to fully discharge.

With the Co based Lithium, voltage is very stable with very little settling over time that's not related directly to actual charge loss. Charge to full @ 4.20V, and you have a cell sitting at very near 4.20V for some time after current is removed. Resting voltage is a very reliable indication of state of charge.

 

[sunshine_eggo]

PS: wouldn't this mean that measuring current (as I said) was the more reliable way of measuring the state of charge of the cell?

If you mean counting current to compute SoC based on Ah consumed vs. Ah capacity, yes.

 

[pellicle]

If you mean counting current to compute SoC based on Ah consumed vs. Ah capacity, yes.

I was meaning just observing / measuring the Amp flow, and cutting off when it falls below a set level , but if you were meaning coulomb counting that's even better (as long as your method is accurate, some hall effect systems are not specifically accurate).

PS: it would seem aside from our various views and emphasis upon the flatness of the curve of charge (where and when) we are on the same page.

 

[Alexplose]

Hi all after some time, here is the high end & low end of the curve.




Everything between seems find, even the low end looks correct.

Still some strange curves at the high end don't know why this is behaving this way.