TomC4306
Solar Obsessive
Raise your balance turn on voltage to 3.4. 3.3 is too low
Cell health in a 16s pack
- Thread starter Alexplose
- Start date
pellicle
retired ex-professional dingbat
to @Alexplose I'm going to say (having read the whole thing at least passingly) what I think you need to do is individually charge each cell and monitor current and take each cell to the 3.65V level and measure the input current (I use a tool like a 150A in line which reports to me the voltage and the amps and other details in summary). Do this for each cell. Then do a discharge test at something like C or half C and again document that. This will (imo)
- clarify cell capacity
- provide useful data for a claim from the seller
Just my view.
NOTE: this is my 150A
Lacking a good measurement system I just put my GoPro on a gorilla pod and set it to take a still every 5 minutes ... then I just go through from the last image and can plot data from that ... of course the summary of Wh and Ah in (and of course turn it around and "out") is immediately helpful without referring to the "history data".
also, if not using a BMS make sure you don't go into over discharge or over charge!
Best Wishes
pellicle
retired ex-professional dingbat
this is a good theory and different BMS's have different capacity for a delta V between cells (especially passive type BMS).
pellicle
retired ex-professional dingbat
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Have you tried turning off the balance function or at least raise the trigger voltage to 3.55 or 3.6? It would be interesting to see if that is causing the issue (BMS). Having that many cells go haywire like that makes me think this is not a single or two cell issue but a BMS issue.
FYI -
Here is my graph for a 16 280ah cells. Notice at the low range (AM) the cells have a delta of .05 or so (bottom of the cell knees) I do not balance since this is 'normal' for my cells, once they start to charge they have a delta of 0 or .01, again no balance function is enabled. They will stay this way until the upper knee but by then I have a shut off charge on the mppt to stop charging the battery, I stick to a 20-80% capacity range for longevity. The green line is the overall pack voltage.
Sojourner1
Itinerant
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.
The very first time you got your cells and you said you top balanced the was is 3.65 or 3.55?
The very first time you got your cells and you said you top balanced the was is 3.65 or 3.55?
@Alexplose Just be very careful going the last .1 volts. there is very little AH left and you'll reach 3.65 a lot faster than you expect... I know i overcharged a few cells when i first put my pack together.
sunshine_eggo
Victron's little biatch
"I disconnected" - assuming you mean disconnected from the charger.
Once you terminate charging, you need to disconnect them from being in parallel. If you leave them in parallel, they will all stay at the same voltage. Disconnecting them will allow you to see any outlier behavior.
Yes that's it, so now I need to take them apart and see which one is mis behaving.
I measured 3.642, 3.643 on all cells how many time before I can see something ?
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sunshine_eggo
Victron's little biatch
Longer the better. At least 24 hours.
+1 day but i am not sold it is a cell that is the problem from reviewing your graphs & given a large number of cells are showing wide swings. my bet is a bad bms or bad bms setting or wire / connection issue.
sunshine_eggo
Victron's little biatch
That's pretty decent.
Here is the measured voltage this morning :
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.
- 3.583
- 3.577
- 3.584
- 3.584
- 3.588
- 3.585
- 3.576
- 3.575
- 3.574 (former #13)
- 3.575
- 3.586
- 3.581
- 3.586 (former #09)
- 3.588
- 3.581
- 3.585
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.
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@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
Solar Obsessive
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.
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.
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.
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
Victron's little biatch
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
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
Emperor Of Solar
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...
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.
@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.
@Alexplose , that graph looks MUCH better. I'm sure you're breathing a huge sigh of relief.
pellicle
retired ex-professional dingbat
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
Victron's little biatch
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.
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.
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