Diagnosing possible LiFePO4 pack issues

[DW SD]

Would very much appreciate some feedback as cell and pack voltages seem to drop very quickly, compared to what I'm used to.

Battery / BMS
I have a 560Ah LiFePO4 pack with 8 x (280Ah) cells, pairs of cells wired in series. Yes, I'm monitoring pairs of cells with Chargery BMS vs. individual. I realize not idea. Still, I'm charging / discharging with low C rates. Max is about 150A either direction. Mostly (99% of the time), 40A max from solar or <10A draws from lights, water pump, etc.
I installed this pack in my camper about 18 months ago and have no more than 50 charge / discharge cycles, only 1 hitting LVD (more info below). I have the BMS set conservatively (HVD 3.55V, LVD, 3.0V) and use about 1/4 the capacity in a given day.
My charging settings have been conservative too. I charge to 13.9V w/ 1 hour absorb. I assumed this explained being a bit out of balance. The BMS never triggers.
With 900W solar, I'm usually charged to full end of the day. Evening consumption might hit 100Ah.

Anyhow, last night I top balanced using BMS. See first photo, holding voltage @ 14.1V for 10 hours or so. After cell differential - 11mV.

Then, 20 minutes later, cell and pack voltage with very minimal consumption (-2Ah) substantially down. I didn't unscrew the lid and pull out my fluke, yet, to measure individual cells.
Should be very little voltage sag @ 4A draw. When I first built my pack, after top balancing, I never had wide voltage differentials, especially at the top. (20mV or so max).

A few other data / info points:
The cells are mounted with terminals facing sideways, vs up. I was told by Amy (EVE cells sales) that the orientation was OK. Some on this board said no. I made a judgment call, given low C rates.
A friend recently used it as overflow sleeping at his house and I know triggered the BMS LVD, when the breaker tripped and he ran for several days just on the battery (without solar turned on). I have LVD set for 3.0 / 12.0V.

I had assumed catastrophic failure is the really only failure (IE. completely dead cells). It is possible the BMS draw took cell voltage below or near extreme levels, as I didn't inspect things myself. He charged the batteries back up and put in storage.
Store batteries between 40% and 70% SOC.







I will do a capacity test and see what that yields. And take some more photos today and post as the loads draw the battery down.
Should I prepare for the worst? If this battery has $hit the bed, I don't think I'd do another DIY (this is my third. I built two similar for friends). Just buy server rack and call it a day.

 

[DW SD]

Dang. Pack voltage dropping like a rock. w/ very minimal consumption -7Ah.

 

[DW SD]

I pulled the battery cover and the cells tell a different story than BMS. I measure 3.321V or 3.322V on all pairs with my meters.
The terminals are all tight, including voltage taps.
I guess I will reach out to Chargery technical support.

 

[MisterSandals]

Anyhow, last night I top balanced using BMS.

That's not top balancing, especially with 2P4S configuration.

There is a known issue with Chargery voltages being inconsistent/fluctuating when BMS is NOT powered fro and external source. Its ok to use the battery as the power source but use the Chargery power input port.

Also, for 2P4S to be effective, the cells need to be capacity matched for the pairs. In other words, each pair should be as equal in capacity as the other pairs. I tested by cells thru using them as 4S batteries and ranked them in capacity (far from scientific). I paired the strongest cell with weakest cell, second strongest with second weakest...

If you've labeled your cells and kept some data on how they've been performing, maybe you have an idea which are the strong and which are the weak.

 

[DW SD]

Hi MisterSandals-
Appreciate your feedback!
Ah ha on the power from external source. hmmm. I wasn't aware of that issue. I wonder if I have the plug to power it externally?

I did not capacity test all of the cells - when I first deployed, I charged them all to 14.1 and let the chargery balance, which started @ 180 to 200mV differential and slowly dropped to 10mV (ISH). They were 8 new, A-grade from Amy. Given my use case and low C-rates, I figured this was good enough. I rarely use more than 25% of capacity in a given day (by morning). And camp in sunny weather, so the pack gets recharged daily when in use.

I have watched the performance (my wife thinks obsessively) over the last 18 months closely. I typically had no more than 20mV voltage differential reported by Chargery BMS. I do only use the camper 30 days / year, though. So some longer trips and shorter. Data isn't daily.

Then when I took out of storage after 3 months, now, things were very wonky.

When I store, I'm careful not to store near top or bottom of SOC.

Thank you,
Doug

 

[MisterSandals]

I wonder if I have the plug to power it externally?

It is extremely common plug. You probably have a few wall plug converters in your house that have a scavangeable wire...
There is a firmware release that supposedly "helps" this issue. But external power is a good easy fix/test.

I figured this was good enough. I rarely use more than 25% of capacity in a given day (by morning).

Which may be true but this went out the window with an LVD event. It may have undone any pseudo (top-like) balancing you did.

Do you have notes on your cell history that you can use to better arrange your pairs?

 

[DW SD]

MisterSandals (MS)-
Note, my LVD event was @ 3.0V / cell - very conservative setting. Is it reasonable to assume the balance wasn't completely fouled?
I do also have photos of prior top balance, which look very similar to the data I captured last night. But that is relying on the BMS output.
I just measured the individual cell pairs again. And all four measure with DVM / Fluke @ 3.321 or 3.322.

I'll figure out which plug it is and wire that up.

Doug

 

[DW SD]

MS - I found the other threads on external power source. Thanks for the heads up.
It does seem most of them are using 8S or 16S batteries. But maybe the same issue applies.

Doug

 

[NVS]

I'm a big believer in active balancers I would install one on your battery. They take all the guess work out of balancing cells.

 

[DW SD]

I'm a big believer in active balancers I would install one on your battery. They take all the guess work out of balancing cells.

which active balancer would you suggest?
Does the inaccuracy in the Chargery BMS put my battery at risk?
Per my DVM, the cells are within 1/1000V of one another.

 

[NVS]

I'm not familiar with chargery but a quick search shows they have issues. I would say they might be ok for smaller applications but for solar use I would change to a different bms. That has more meat to it.

As far as active balancers people have been using heltec.
Heltec 4S 5S 6S 5A Capacitor Active Equalizer Balancer Lifepo4 Lithium Lipo/Titanate LTO for Battery Car Audio Group Balancer https://smile.amazon.com/dp/B08JCL4G2X/ref=cm_sw_r_apan_i_0J4B1P1X89PGWNV58PPR

 

[MisterSandals]

Is it reasonable to assume the balance wasn't completely fouled?

I dunno, not sure i am an expert on reasonable.

It was never parallel top balanced. The cells were never matched. Now its not charging and discharging in a balanced manner.
I do not have a look of surprise on my face right now. People take these steps for a reason.

 

[NVS]

Have you seen this thread?

Chargery BMS News / UPDATE (Nov.25.2020) Ver 4.02 firmware

I am Cross Posting Jasons update info which is spread around to this thread. Any items that pop up related to this Firmware Update etc should be posted or linked to in this thread to make it easier to locate, track and address. It will help solve issues and answer questions. Thanks in Advance...

diysolarforum.com

 

[MisterSandals]

I'm not familiar with chargery but a quick search shows they have issues. I would say they might be ok for smaller applications but for solar use I would change to a different bms. That has more meat to it.

Um, Chargery is one of the premium BMSs that, at over $300 for a setup, is usually only installed on large battery systems. With shunts of 100A, 300A or 500A as standard equipment its got plenty of "meat to it".

 

[NVS]

Um, Chargery is one of the premium BMSs that, at over $300 for a setup, is usually only installed on large battery systems. With shunts of 100A, 300A or 500A as standard equipment its got plenty of "meat to it".

Well the few videos I've seen and the post on here about it. I don't know if I'd use it on RC batteries let alone my expensive solar batteries. Price doesn't make it good.

 

[DW SD]

I dunno, not sure i am an expert on reasonable.

It was never parallel top balanced. The cells were never matched. Now its not charging and discharging in a balanced manner.
I do not have a look of surprise on my face right now. People take these steps for a reason.

I definitely appreciate the feedback! I assume the Chargery balancing feature is capable of balancing the cell pack, perhaps in not the most efficient manner as an initial parallel balance, but also agree my A-grade 280Ah cells might not be perfectly matched. All the cells did start the battery process at the same voltage per my fluke. I realize that doesn't mean equal SOC. I hope I don't sound argumentative. Given I'm not using 100% of available energy, I think it will work well.

Note, when measuring voltages a few hours later with my fluke, the pairs of cells again reported within .001 (1/1000 th) differential. The data from Chargery had significantly changed again on the monitor screen. See attached, after 4A draw for a few hours, the cell level voltages went up and differential went down (my balancing is only activated >3.36V and that obviously won't bring up voltages.

I emailed Jason with the same question and he suggested mailing the unit back for repair, which he would cover under warranty.
I appreciate the great customer support!

I now believe my diagnosis is bad BMS. Not bad cells ... PHEW! I'm most grateful for all the input!


Doug

 

[MisterSandals]

I now believe my diagnosis is bad BMS. Not bad cells

Yea, if you have a quality meter that shows such a difference maybe it is the BMS.

Have you checked the voltage at the connector that goes into the Chargery? Pin 1 is negative so voltage to pin 2 should be ~3.2, pin 1 to 3 ~6.4V, pin 1 to pin 4 ~ 9.6V, pin 1 to pin 5 ~12.8V. This would be a sanity check that your balance wires are sound and making good contact at the battery and connectors. I'd do this before shipping it back to Chargery.

 

[DW SD]

In looking back, the sum of my DVM individual cells and Victron pack voltage are within 1/1000V. Tomorrow, I'll measure the voltage taps @ the connector. A great idea!
Thanks again.

On email, I asked Jason if I could leave a deposit and have him send a replacement unit while my current unit transits back to China for repair. He then told me the voltage discrepancy wasn't a big deal and I should just turn up the voltage differential.
UMMM. What is the point of the BMS if it is off by 60mV on a cell level??? Hopefully, just a misunderstanding. This seems like having your fuel tank misreading by 60%.

I have a trip coming up and prefer not to go w/o BMS. Still, I've had no BMS events in 18 months, except when trying to top balance.

Is there an interchangeable BMS (controlling a contactor) out there I might consider? I know the FET - based units could still control a contactor.