LiFePo4 3.2V 280AH observer request for feedback

[DonPhillipe]

No offense but I'm looking only for someone who has spent a lot of time observing their cell voltages and hopefully during a period of 3 days or so in a 12V configuration. (In other words tapping into practice over theory.) I have two of these 12V systems set up separately, each on a 200a JBD BMS and the two banks are built from cells from 2 different vendors. One bank seems well matched, with voltage that slowly drops and uniformly from a hard-charge of 14.2V (cells normally gobble up a constant 50-60a flow until the demand just falls off to zero) and then once the JBD coulohm meter shows full, I disconnect them. These cells generally settle around 3.45V and all stay around this value while taking several days to slowly tick down lower, but the cells stay close in voltage.

On the second bank, I am interpreting this one as my "problem bank". But being inexperienced I think what I am now seeing is that contrary to what seems logical it's Ok for the cells to lose their original charge voltage much faster than the first bank and if so, the questions I have from this are:
a) is it OK that this group of cells deplete much faster than the other bank
b) is it OK that they deplete in voltage like shown in my chart below

I decided to just monitor my more rapid-discharging bank over a few days, then poll anyone here to see if this seems to be normal behavioir and thus not every bank is supposed to retain their "charge to" voltage once the initial charge voltage is cut??? (Guessing maybe this is why they call them 3.2V cells because once you charge them in the 3.5V range where my BMS shuts down passing any more charging current. I first thought this was severe internal resistance leaking but a chart I have since found indicates it may be normal???

I began with 4 "problem" cells that arrived as supposedly already top-balanced but unpacking them, they all read exactly 3.23 to 3.25V. I decided to put them on the 200aH BMS charging at 14.2V and they also absorbed around 50-60A of charge for nearly 2 hours but the BMS started tripping off on the second hour when the highest cell showed 3.64 and the lower cells were still in the area of 3.37V. From this I assumed they were not really top-balanced so rather than rewire them for a static voltage balance, I instead added a 5A active BMS and finally got them up to 3.54V and called it a day. At this point I removed the active BMS, plus removed the positive lead and continued simply to monitor cell voltages. From the equalized voltage of 3.54V (via the heavy hand of the 5A active BMS), the uniform charge appeared first to be stable but then within a half hour each cell began to show a surprisingly decreasing voltage level at mostly different change-rates.. I decided to just monitor the voltages over a 48 hr period and here is what I observed:

Cells connected in series, charged to 100% on the BMS coulomb counter, then the positive cables were removed from the battery bank. At full charge the active balancer was also removed:

(Hours after original 3.54 peak) (Cell 1 - 3 - 2 - 4) (BMS battery voltage display)
0 - 3.54, 3.54, 3.54, 3.54 - 14.1
3 - 3.50, 3.52, 3.51, 3.47 - 14.0
6 - 3.48, 3.50, 3.49, 3.45 - 13.9
8 - 3.47, 3.49, 3.48, 3.43 - 13.9
10 - 3.45, 3.48, 3.47, 3.42 - 13.8
13 - 3.45, 3.48, 3.47, 3.42 - 13.8
21 - 3.44, 3.47, 3.45, 3.40 - 13.8
24 - 3.44, 3.46, 3.45, 3.40 - 13.8
30 - 3.44, 3.46, 3.45, 3.40 - 13.8
35 - 3.42, 3.44, 3.43, 3.38 - 13.7
38 - 3.42, 3.44, 3.43, 3.38 - 13.7
49- 3.40, 3.41, 3.41, 3.37 - 13.6
55- 3.37, 3.36, 3.36, 3.36 - 13.5
58- 3.36, 3.35, 3.36, 3.35 - 13.4 (33F)
62- 3.39, 3.40, 3.40, 3.36 - 13.6 *1 (49F)
74- 3.38, 3.39, 3.39, 3.36 - 13.5 (36F)
78- 3.37, 3.38, 3.38, 3.35 - 13.5 (40F)

Normal???

*1 - No external power sources connected, no solar connected, no drain connected, positive disconnected with a battery switch and balane is turned off on the BMS; how the battery gained voltge during the past 8 hours I have not a clue (The temperature is at 39F when the reading 8 hours ago was at 49F)

 

[pellicle]

the BMS coulomb counter,

your BMS does coulomb counting? Not just measuring voltage?

 

[DonPhillipe]

your BMS does coulomb counting? Not just measuring voltage?

That could always be a wrong assumption, because the model is a JBD-200a BMS.
JBD-SP04S034-L4S-200A-200A-BU

The reason I guess this to be a feature is because the percentage remains on 100% regardless of what the added voltage levels deplete to.

 

[DonPhillipe]

Reading further from the BMS user manual:

4.7. Capacity calculation
The battery pack can be accurately measured by integrating the current and time. SOC Calculation. The full capacity and cycle capacity of the battery pack can be set by the
host computer, and the capacity can be automatically updated after a complete charge and discharge cycle. It has the function of calculating the number of charge and discharge
cycles. When the cumulative discharge capacity of the battery pack reaches the set cycle capacity, the number of cycles increases by one.
Note: For newly installed batteries, please set the nominal capacity and cycle capacity according to the battery capacity, and perform a capacity study, otherwise capacity inaccuracy
may occur. Capacity learning operation: first fully charge to overvoltage protection, then discharge to undervoltage protection, and then charge again.

 

[acdoctor]

No offense but I'm looking only for someone who has spent a lot of time observing their cell voltages and hopefully during a period of 3 days or so in a 12V configuration. (In other words tapping into practice over theory.) I have two of these 12V systems set up separately, each on a 200a JBD BMS and the two banks are built from cells from 2 different vendors. One bank seems well matched, with voltage that slowly drops and uniformly from a hard-charge of 14.2V (cells normally gobble up a constant 50-60a flow until the demand just falls off to zero) and then once the JBD coulohm meter shows full, I disconnect them. These cells generally settle around 3.45V and all stay around this value while taking several days to slowly tick down lower, but the cells stay close in voltage.

On the second bank, I am interpreting this one as my "problem bank". But being inexperienced I think what I am now seeing is that contrary to what seems logical it's Ok for the cells to lose their original charge voltage much faster than the first bank and if so, the questions I have from this are:
a) is it OK that this group of cells deplete much faster than the other bank
b) is it OK that they deplete in voltage like shown in my chart below

I decided to just monitor my more rapid-discharging bank over a few days, then poll anyone here to see if this seems to be normal behavioir and thus not every bank is supposed to retain their "charge to" voltage once the initial charge voltage is cut??? (Guessing maybe this is why they call them 3.2V cells because once you charge them in the 3.5V range where my BMS shuts down passing any more charging current. I first thought this was severe internal resistance leaking but a chart I have since found indicates it may be normal???

I began with 4 "problem" cells that arrived as supposedly already top-balanced but unpacking them, they all read exactly 3.23 to 3.25V. I decided to put them on the 200aH BMS charging at 14.2V and they also absorbed around 50-60A of charge for nearly 2 hours but the BMS started tripping off on the second hour when the highest cell showed 3.64 and the lower cells were still in the area of 3.37V. From this I assumed they were not really top-balanced so rather than rewire them for a static voltage balance, I instead added a 5A active BMS and finally got them up to 3.54V and called it a day. At this point I removed the active BMS, plus removed the positive lead and continued simply to monitor cell voltages. From the equalized voltage of 3.54V (via the heavy hand of the 5A active BMS), the uniform charge appeared first to be stable but then within a half hour each cell began to show a surprisingly decreasing voltage level at mostly different change-rates.. I decided to just monitor the voltages over a 48 hr period and here is what I observed:

Cells connected in series, charged to 100% on the BMS coulomb counter, then the positive cables were removed from the battery bank. At full charge the active balancer was also removed:

(Hours after original 3.54 peak) (Cell 1 - 3 - 2 - 4) (BMS battery voltage display)
0 - 3.54, 3.54, 3.54, 3.54 - 14.1
3 - 3.50, 3.52, 3.51, 3.47 - 14.0
6 - 3.48, 3.50, 3.49, 3.45 - 13.9
8 - 3.47, 3.49, 3.48, 3.43 - 13.9
10 - 3.45, 3.48, 3.47, 3.42 - 13.8
13 - 3.45, 3.48, 3.47, 3.42 - 13.8
21 - 3.44, 3.47, 3.45, 3.40 - 13.8
24 - 3.44, 3.46, 3.45, 3.40 - 13.8
30 - 3.44, 3.46, 3.45, 3.40 - 13.8
35 - 3.42, 3.44, 3.43, 3.38 - 13.7
38 - 3.42, 3.44, 3.43, 3.38 - 13.7
49- 3.40, 3.41, 3.41, 3.37 - 13.6
55- 3.37, 3.36, 3.36, 3.36 - 13.5

Normal???

I don’t leave mine just sitting idle but charge daily to 3.55 with a small load of inverter they drop back to 3.4 in a couple of hours. If the fridge is running at the end of the charge cycle they drop off to 3.4 per n a half hour. When I first built the packs and top balanced they dropped from 3.65 to 3.5 in 12 hours and I think to 3.4 in 12 more hours.
Running different brands with possibly different internal resistance could result in one changing and discharging faster than the other especially in parallel application.


I am not sure what you are saying here. Are you saying cell get out of balance? Packs get out of balance?
“the uniform charge appeared first to be stable but then within a half hour each cell began to show a surprisingly decreasing voltage level at mostly different change-rates.”

 

[DonPhillipe]

I don’t leave mine just sitting idle but charge daily to 3.55 with a small load of inverter they drop back to 3.4 in a couple of hours. If the fridge is running at the end of the charge cycle they drop off to 3.4 per n a half hour. When I first built the packs and top balanced they dropped from 3.65 to 3.5 in 12 hours and I think to 3.4 in 12 more hours.
Running different brands with possibly different internal resistance could result in one changing and discharging faster than the other especially in parallel application.


I am not sure what you are saying here. Are you saying cell get out of balance? Packs get out of balance?
“the uniform charge appeared first to be stable but then within a half hour each cell began to show a surprisingly decreasing voltage level at mostly different change-rates.”

I think I am saying that charging up to 3.54 and disconnecting, cell number 4 appears to take a more rapid dive in voltage but only if I should be worried about this - since after 50 hours they all have finally seeked out the same level of 3.36V.

 

[acdoctor]

I am no expert but I believe so long as they settle to the same voltage. If it continued to drop past the others that would be considered self discharge and that is bad. Have you capacity tested then?

 

[DonPhillipe]

Not as of yet.

 

[acdoctor]

So long as they check out capacity wise. I believe they will be fine.

 

[DonPhillipe]

Trying to get my China cells up to 3.65 is like herding cats and I don't know how long that would take with 3.65V set as the high trip voltage when attempting to charge all to this level; in this case the BMS keeps tripping, and with 3.45 set as the BMS "try it again" voltage, that's actually why I finally gave up at 3.54V because the number one cell (not seen in the chart because we are idling and not charging). This notable "hot" cell would shoot way past 3.65V while the slow-mover would lag always around 3.37 or so. So what happens with the disproportionate charging rates is the 3.65 of the "hot" cell trips the BMS at 3.65 and it may then take 4 hours for the 3.65 to dip down to 3.45 to wake the BMS up again. That's why I added the active balancer that seems to do a great job of keeping them nearer-level as long as they are under idle or at least resting at the house voltage of the 13.6V of my stock camper converter. But trying to get them up to the levels you are talking about at the end of the charge cycle, the high leaning cell will always trip the BMS even with the balencer active and with the low leaning cell lagging back around 3.42.

After reading a lot of these sailor's blogs, they don't spend too much time sweating the top voltage it seems. Many of them also say that 3.45 is good enough for them. Still trying to describe all this is confusing as hell, e.g. is that the "going into the charge voltage" or the "going out of the charge voltage". And not to mention the further confusing questions like the fact that if we are wishing to see 13.8V on the BMS not much talk describes that this will actually require the alternator or solar outputting at a level of 14.2 and 14.4 respectively for them to be near successful when trying to drag what the BMS voltage rating up to a level of 13.6-13.8.

 

[acdoctor]

The 3.54 is good for first charge from there you need to top balance.
Reconfigure them in parallel. Charge at exactly 3.65 until amperage goes to near 0. If you are not familiar there are detailed instructions on the forum. Until you do that you really don’t know anything about the cells.

 

[RCinFLA]

Cell voltage dropping to 3.45v per cell is nearly nothing for capacity. It is surface capacitance charge that amounts to about 30 mAH of capacity (0.01% of 280 AH), or about 1.8 Amp-Minutes of capacity. You can dissipate that capacitance charge with 2 amps load for about 1 minute.

The cell will eventually leak that small surface capacitance charge off from cell self-leakage rate which is variable between cells. It can take less than a day or as much as several days to bleed off on its own. Just variations in BMS current consumption could cause one battery pack to drop to 3.45v per cell faster than another pack with a different BMS.

It is not normal for cell to continue to drop below about 3.45v if fully charged and left unloaded.

Pushing 50-60 amps bulk charge current is moderately substantial charge rate. Absorbing at 3.55v per cell with 50-60 amps bulk charge rate needs to ensure current drops during absorb phase to less than 2 amps before charging is stopped or cell will not be fully charged. This could explain cells continuing to drop to 3.36v indicating they are not quite fully charged. The fact they all dropped down to similar voltages of 3.36v enforces the premise they were not fully charged, and nothing is wrong with cells. If there was a cell problem, it would be unlikely they all dropped to similar cell voltage.

If cells are cold this could also be another factor as 50-60 amps bulk charging with 3.55v absorb voltage could take a lot longer time for current to taper off.

Other, somewhat long-shot, possibility is something is wrong with the BMS and it is drawing too much operating current of its own that is creating the extra discharge current bringing the SOC of cells down a percent or two in the observed timeframe. You could try swapping the BMS's and see if the issue follows the BMS.

Most likely reason is the cells were not quite fully charged.

 

[DonPhillipe]

Thanks, I am going to give them 12 more hours of rest and hit the project again. I "think" they had fallen to 0 amps but then again I could have mistaken that from my high likihood of simply giving up when the charge cycle was so crazy with the one cell that kept tripping at 3.65. Since I don't own a bench power supply capable of the 3.65v at notable amps, I think I'll try again forcing them all to near 3.65 and I'll also change the reset voltage up from 3.45 to around 3.63 and finally use a dual stacked active balancer combing to 10A to see if I can get them "full" that way. I'm already about 300% over budget of this so-called cost-cutting project and since I quit bench projects along with retiring,, I am going to try to order anything else and instead dummy up the BMS to get the cells up by "cheating" if I can. Plus they are mounted and don't want to create a potentially shorting laberynth of parallel charging .... bla, bla, bla. Live and learn but I'll be back to work on them manana. Thanks!

 

[pellicle]

is because the percentage remains on 100% regardless of what the added voltage levels deplete to

I'm not sure what this means, but as I'm not really helping with your actual question there is little benefit to either of us to answer it.

Best Wishes

 

[Bud Martin]

"This notable "hot" cell would shoot way past 3.65V while the slow-mover would lag always around 3.37 or so." So just one cell reaches 3.65V before others? Did you recheck the connections and swap the cell location to see if the problem follows?

 

[DonPhillipe]

"because the percentage remains 100%"..... I am speaking in terms of the reading from the JBD BMS bluetooth display. And the answer was directed to the question regarding if the JDB BMS provided a SOC percentage indicator that was a coulomb meter or simply was it showing SOC based on battery voltages. I was saying that the indication of SOC did not seem to be related to voltage changes because during the 3.54V to 3.34V "cool down" of the cells, the SOC on the BMS had not changed SOC value down from 100%. The fact I stated it was likely a coulomb meter was based on first, an absence of any notable documentation on the JDB BMS and secondly that it had not moved since the batteries had hit the meter's 100% level shown once the cells reached the 3.54V level and the SOC hadn't changed while the unloaded static voltage petered down to 3.34. With the JBD BMS, there is a bluetooth panel and the display that includes amps in/out, SOC in %, temp averge of battery taps, voltage range delta of cells and time charging (cell voltage indicators turn red when outside the accepted range and grey when below). I did discharge initially discharge the bank down to near 34% (reading BMS bluetooth meter) and once I charged it back up to 100% (about an hour at 60A delivered directly from 150A alternator with no B2B and no terminal or junction overheating issues existed). So I'm only going from my existing instruments which include this relatively new technology "smart" BMS and periodically using a digital volt meter which does confirm what the "smart" BMS is reading from the cell voltage perspective.

"hot cell would shoot past" ....Yes, the "hot" cell follows the cell and was in a different position and showed the same type of activity under a different BMS indicated cell number and I also wrote it off then to it being the fact that I only thumb tightened the intercell bridge of the 3 nickle plated copper strips These are all very tight now and the hot cell persists. (Note before testing the charge cycle, I initially tested with a 160A microwave load for 3 minutes and no connection or the terminals of the BMS got even warm so that says I likely have solid enough connections. Still that's a good idea and I have to say it screwed me up on the very first dry-run of cell testing on the table when they were only hand tight. You have to keep the bridge bars tight for sure.

 

[HRTKD]

It sounds like you didn't top balance the cells. Not much more any of us should be doing to resolve the situation unless that was done.

"Top balancing" with a BMS is a very iterative process that could take weeks. A proper top balance, as defined in the Resources section of this forum, takes days at most.

Cells are commonly shipped at a lower state of charge. There may be a regulation that Lithium based cells/batteries cannot be shipped at full charge. For straight Lithium-ion batteries they cannot exceed a 30% state of charge to be air shipped.

Voltage is an unreliable gauge of state of charge.

 

[RCinFLA]

Cells look fairly well balanced.

BMS shows 100% state of charge whenever charger brings pack up to the set trigger voltage in BMS to reset AH counter. Normally this reset voltage is setup to be just a little below charger absorb voltage.

This full charge reference voltage level resets AH cumulation count that has been used since last full reference reset so any cumulation errors since last full reset are cleared. If BMS is never allowed to reestablish the full charge reference voltage, then it continues to work with input/output current time increments (like dead reckoning navigation) and will continue to accumulate errors in total Columb count for what it reports for battery state of charge.

With cell current flow, cell voltage is a poor indication of state of charge.

Cell voltage is a good indicator of state of charge if you have better than +/- 5 mV accuracy voltmeter, and cells have had no current flowing through them for at least 3 to 5 minutes. Temp has little effect on open circuit, no load, rested cell voltage

With load current on cells there will be overpotential voltage rise for charging and voltage drop for discharging, from rested no-load cell voltage. The amount of voltage rise or drop for given amount of current, charge or discharge respectively, depends on current rate, condition (aging) of cell, and temperature of cell. The overpotential rise/drop is proportional to logarithm of cell current and gets greater as cell gets older and gets greater as cell temperature drops.