Power Queen LiFePO4, BMS cuts off charge at low voltage

[rruff]

EDIT: I changed the title to better reflect the actual issue. 1/23/2024

This is the Power Queen trolling battery (12v 100ah), which I purchased 3 weeks ago on Amazon. I figured it was very similar if not identical to the Li Time trolling battery that Will reviewed.

I started cycling it about a week ago to see if it was functioning ok (3 full cycles so far). I'm using a little Imax B6 to charge it, which will only maintain 4.5a without over heating. I noticed that the rate of voltage increase on the battery would stall at around the 13.5V area. The voltage would also fluctuate +-0.1V which was not true in any other part of the charge cycle. It took 32.5ah to go from 13.47V to 13.54V for instance, and ~8ah more until the BMS tripped. It doesn't trip at 14.6V though; 14.1-14.2V based on the two times I've been watching when it happened. According to the Imax, the full charge takes 116-118ah.

Unfortunately I don't have a way to measure capacity by discharging atm. I've been using a cheap inverter looked up to a 250W heat lamp, with a Kill-A-Watt attached. The big unknown is the inverter efficiency, but the Kill-A-Watt measures 1,140-1,160Wh which is in the ballpark of what I'd expect. I have a battery coulometer on order so I can get an accurate discharge capacity and a 2nd opinion on the charging.

I've heard that there is very little loss in the charge-discharge cycle for LiFePO4 at reasonable rates... and I very much doubt this battery has appreciably more capacity than spec. The stalling at 13.5V makes me think it might have a bad cell and that the BMS is burning a lot of energy to try to balance them.

Does that makes sense? Has anyone seen similar behavior before?

Thanks!

 

[740GLE]

I’m guessing over cell voltage disconnect.

You may only be missing out on a couple Ah.

I’d say keep the battery at a highish voltage14.0 but low current <100mA see if the BMS can balance a low cell.

This won’t solve a bad cell but it’d try and balance the straggler.

 

[LakeHouse]

I noticed that the rate of voltage increase on the battery would stall at around the 13.5V area.

This is pretty much what should happen. The charge curve is very flat, so you're storing energy without the voltage changing much.

14.1-14.2V based on the two times I've been watching when it happened.

This is 3.525V per cell, and really not that bad. At this point, your battery is charged.

According to the Imax, the full charge takes 116-118ah.

This is a good thing. If it's accurate, it means you have more capacity than you paid for. You're right about there being very little loss on the charge-discharge cycle, if it took 116Ah to charge, you'll get them all back.

The big unknown is the inverter efficiency, but the Kill-A-Watt measures 1,140-1,160Wh which is in the ballpark of what I'd expect.

That's what came out of the inverter, what went in was certainly more than that.

The stalling at 13.5V makes me think it might have a bad cell and that the BMS is burning a lot of energy to try to balance them.

Not really how that works. With passive balancing (which you likely have) there is some energy 'burned' but it's very small.

You have good cells, a good battery, and more capacity than you paid for!

 

[740GLE]

I didn’t read that the BMS tripped at 14.6. That’s 100% full. You have nothing to worry about.

I thought you were implying it tripped at lower voltages.

 

[rruff]

I didn’t read that the BMS tripped at 14.6. That’s 100% full. You have nothing to worry about.

It does trip at lower voltages, it hasn't made it it as high as 14.2 before the BMS tripped.

Just checked the manual for the battery. Under the "Controller Settings" section, it says to use these values:

14.4V (charge/boost/bulk/absorption).
Over voltage disconnect, 15.0V. Over voltage reconnect, 14.2V.

The BMS trips before any of these values are reached.

I’d say keep the battery at a highish voltage 14.0 but low current <100mA see if the BMS can balance a low cell.

I'm trying! I can't hold it there, but I've been running it in the high 13s anyway at 0.1A. For some reason the charger is now saying the battery is full when it gets to 14.0V and it stops, even though the charger is set for 14.4, and its voltage reading is correct.

This is a good thing. If it's accurate, it means you have more capacity than you paid for. You're right about there being very little loss on the charge-discharge cycle, if it took 116Ah to charge, you'll get them all back.

Ha, that would be a first I think! The last charge was 118Ah, so a cheap Chinese battery giving 18% extra... ?

That's what came out of the inverter, what went in was certainly more than that.

Yes, that's why I said it's about what I expected. I'd expect ~1300Wh out of the battery, and got 1150 from the meter, which would mean 88% efficiency for the inverter.

I'll know more when I can measure the charge and discharge energy with a semi-accurate meter.

Thanks for the help!

 

[rruff]

I did the 100mA charge at near-full several times, and discharged it using a monitor with a shunt. Unfortunately I set it to 100ah at full charge, and when the monitor got to 0.000 it didn't record negative values, it just stayed there. But if I had to guess it was 104-105Ah and that was still over 11V. So no capacity issue. Charging now, and will discharge again. I set the capacity to 120Ah this time...

I found this page where the author describes what I thought was a similar issue. https://marinehowto.com/critical-lifepo4-data/

He couldn't get the battery to charge past 13.81V... due to severe imbalance. I have a different battery, the "trolling version", while his is the "mini". The BMSs are different also, as mine has the low temp cutoff, and higher surge current before cutoff... so the BMS settings might be different. But... since the manual recommends charging to 14.4V, it probably is set the same.

The relevant info he was able to obtain from PQ:

Balancing begins at 3.525V (would be 14.10V for a pack).
The balancing action is that the high voltage cell dissipates 35mA of current.

Though the author claims cell balancing never started, I think this is incorrect... rather the high voltage cell(s) hit the balance voltage for a time, but it was not long before they reached the HVD setting. 35mA isn't nearly enough to correct for the degree of imbalance before the BMS trips.

So if the cells are merely out of balance, rather than one or more being in poor condition, repeated full charges would eventually bring them in line... I think? I now understand why you were suggesting full charges at low current @740GLE.

It seems odd to me that cells would arrive so far out of sync. Also that the balance function only occurs at such a high voltage.

 

[rruff]

I got an actual coulomb meter a few weeks ago and measured capacity, and it's fine... 106Ah both in charge and discharge. The Imax charger is just very inaccurate.

I also took a 2 week camping trip where I used the battery in parallel with another similar one. I have an Epever Tracer and set it to a max charge of 13.9V and 180 minutes of boost at 13.8V, trying to get the cells to balance without tripping the BMS. It was sunny and I had plenty of solar, so most days I got 3 hrs at 13.8V. The balance seems to be getting worse rather than better though, with the BMS now tripping at only 13.9V.

Intepretation? Either the cell balancing function isn't working, or there is a cell that is "bad" enough that the balancing function can't fix it. Any other ideas? Not sure what else I can do...