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New 12v DIY Pack, Bad Cell?

Zeravnos

New Member
Joined
Aug 17, 2020
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Hey all, first post here and first LiFePo4 pack was just made! I already reached out to the shop I bought these from on Amazon but I figured I'd ask you guys while I wait for a response.
So I bought a set of matched Sinopoly cells from Amazon, and when I got them in I checked that they were all matched and that their voltages were within 50mv, which they were. However, and here is where I may have messed up, one of the cell terminals had some strange blue powdery residue on it, almost like a dab of white paint got on it. It didnt smell and it wiped off easily, so I didnt think anything of it. Anyway, I dont have a DC power supply, so I didnt top balance them (perhaps second mistake), but I figured they seemed well enough that I'd try playing around with them. They discharged down to ~15% great and when I got my solar delivered, I decided to then charge the battery. They seemed to charge great, nothing heated up, and they stayed within .07V of each other the whole time, up until I got up to around 3.4v per cell. Suddenly one of the cells started shooting up in voltage until the BMS tripped the overvoltage protection, going from 3.45ish to around 3.6+V in probably less than a minute. Turns out this was the cell that had the residue on the terminal.

Did I get a bad cell? Or is it more likely because I didnt top balance it? Sorry if I'm a bit rambly!
 
.05V deviation in "matched" cells suggests that you didn't get matched cells.

Was BMS balancing active or not?
 
Yes, cell balancing was active. I suppose I have incorrect understanding of how close cell voltages should be?

I assume the implication is that these cells aren't matched. Like I said, I've contacted the seller on Amazon to see if they'll help me, but if they dont, what is my recourse? Setting the charge limit to something like 13.4V or 13.5V? Can I just try to get a single replacement cell and deal with them not being matched?

ETA, I added a screenshot of the cell voltages as reported by the BMS right before I disconnected the solar.
 

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IMHO, they're not matched, so a replacement may be better or worse. Matched cells are pulled from the same manufacturing batch and have established performance characteristics.

Based on your testing, did the cells meet spec?

Did the other 3 stay very near one another, but the 4th was an outlier?

IMHO, you should run the cells through their entire range with balancing active, charge and discharge, 2.5-3.65V and assess if the cells perform as advertised. A low current charge with balancing active might help bring these into step with one another and top balance them.
 
I'm not going to act like I did any hard data testing, but my testing did involve running an 800W load for a bit over an hour and landing at around 20%SOC as reported by BMS. I didnt write anything down, but at the time I felt satisfied in that performance...

And what seems to be the trend is that 2 of the batteries stay relatively close in voltage, one is a bit low, and one is rather high when at high SOC. Though when at 13.3V pack voltage and below, all 4 cells stay rather close.

I suppose I will run a few more charge cycles with that load I mentioned before. What should I set my cell overvoltage to? Or should I keep it at 3.65? I just fear the voltage running away on that one cell, perhaps unfoundedly.
 
Some details on the cells/BMS would help.

You probably should do some documented testing. It's useful as proof to you that you bought the correct product for yourself, or proof that you didn't for the seller.

Given your description, it sounds like it's been bottom balanced, which is the least ideal situation for unbalanced cells, and IMHO, never really desirable when one wants to capture as much capacity as possible.

Fully charge as you've been doing.
Set your cell undervoltage to 2.5V/cell and repeat the discharge. Note the capacity extracted. Compare to rated.
Leave your overvoltage to 3.65V/cell and charge at a medium rate (just enough to get it charged in a day) with balancing enabled. See if it improves.
 
Alright, I'll try and run it through a few cycles over the next couple days when I have time to spare and I'll document my findings. Hopefully the seller gets back to me as well and perhaps they can offer me another solution.

Thanks for your help, snoobler. I'll report back with findings.
 
So my first discharge has left me confused. It ran for 21 hours on a 64W load (as reported by the BMS), which suggests that I got 1344Wh out of this pack, even though the cell voltages started at roughly 3.31V each under load, and bottomed out at roughly 2.6V-ish under load, and 2.77V-ish after removing the loads. 'ish' is the best estimate I have since I wasnt watching the battery when it disconnected.

Does that sound remotely correct? I determined that it was a 64W load from the BMS (Overkill Solar 4s). It is worth noting that the inverter said 40W though, and the only thing else that would be drawing from the battery would be the solar controller and its monitor. 840Wh sounds a bit more accurate, maybe.

I'm going to resume testing tomorrow, aiming to fully charge and discharge the battery again and noting the throughput.
 
Glad you have the overkill. It seems pretty accurate. The inverter is probably not reporting its own power usage. My 4kW Victron uses 30W.

21*64=1344Wh.

I don't see that you included the cell specifications, so I don't know how that compares to rated.

"ish" is fine. Were they all pretty close to each other when you checked?

Can you include a snapshot of your balance settings screen?
 
Sorry, I've attached the cell specs. They're rated for 320Wh each and I'm running 4s. So, pretty close, and to my understanding having extended capacity out of new cells isnt uncommon?

The cells were within 5mV every time I checked, except below 12.8V pack voltage and above around 13.5V pack voltage. From there, the voltage delta grew in proportion to DoD either way. I think the biggest difference I saw was at high SoC, with once cell at 3.853V and another at 3.432V. Not sure why the BMS waited so long to overvolt, but it did at that point. At low voltage, the delta I saw was around .067V

As for the balance settings screen, I dont know if the second picture I've uploaded is what you mean? If not, could you tell me specifically what you'd like to see?
 

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Yeah, new cells frequently deliver a little more than rated.

Hardware overvolt is set to 3.9V for 8 seconds, so 3.853V qualifies?

Now the question becomes is it the same cell that's limiting the upper and lower end of the range? It's worth taking screen snaps during charge/discharge to establish how each cell is behaving.
 
Oh, wait, thats right. In that case I'm confused between the difference between the "Overvoltage" parameter and the "Hardware Overvoltage Protection" parameter.

As for it being the same cell, I've included screenshots of the cells at full discharge and at full 'overvoltage' charge. It seems its just the one cell that likes to spike after it reaches 3.4Vish, and the voltage climbs dramatically very quickly after that.

That being said, the third screenshot is about where I started discharging during this test where I got 1344Wh.
 

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Yep. That was my thinking in the OP that this cell might be somehow damaged. Still no word back from the supplier, either.
 
#2 is reversing its position, but it's not as bad as #3 as the limiter on charging.

if there's any way, I'd recommend you top balance those things so you can truly tell what's going on. I suspect the BMS internal balancing won't be able to overcome the disparity without a lot of cycling.
 
I think I'll wait a day or three to see if the supplier wants to just give me a fresh cell/pack before I go out and get a power supply to top balance these, maybe I'll get lucky. Until then, I'll try to fully cycle the pack once or twice more just to see if anything else odd happens. Failing anything else, I'll get these top balanced.

Again, I appreciate the support!
 
Try to do less deep discharges and slower charges. That might help equalize them.

Might also adjust balance to 3.1V and 10mV
 
Alright, just went through another recharge cycle. The pack behaved all the way up to 3.4V cell voltage, where Cell 3 gets too excited and its voltage flies up until I reach cell overvolt disconnect or even pack overvoltage at the Solar Controller. At which point, the battery goes into overvolt protection, balances everything back down, and when the protection releases cell 3 boosts up again, ad nauseum.

I've disconnected the solar and will be running another discharge cycle overnight to get a capacity, but if I get ~1300Wh again I might just leave well enough alone for now and set my charge limit to 13.6V for now.

The screenshots I've attached are in groups of 2, showing midway charging (at 3mV cell delta, whereabouts it stayed), an overvolt about to happen, and then a few minutes later when the cells balanced down a bit.
 

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