Battery unbalancing

[trig2]

I have a rack of six 48 volt EG4 LLV1 batteries that I've just started using even though I purchased them almost a year ago. I just got my solar actually set up and charge them fully for the first time. I'm using an EG4 6500 inverter.

4/27 charged with USE settings to 55.5V with about 16 amps per battery. (Max my solar could do). One battery had over voltage protection kick in from an overvolted cell. (3.9V)

4/28 Try again but when it gets close to full I lower the amperage to 20 amps total which is 3.33 amps per battery. Several batteries overvolt this time.

4/29 this time I figure maybe the battery can do a better job if I set it up to use EG4 mode where the BMS controls the charge rate. I'm away when the charging is done but I can see from solar assistant that it took the battery voltage to 56 and charged for like 5 hours at only 1 amp across the entire rack of batteries so I figure that had to get it fairly balanced right.

4/30 charge again with EG4 mode two batteries over volt and all but one of them have one or two wildly out of balance cells.

Swapped it back to use mode and I can charge it at 54.7 without overvolting cells but if I go higher than that they will probably overvolt. I also took a picture of the worst offender the first day, and if I compare that to today the battery appears to be even more out of balance which is boggling my mind.

In the pictures you can see cell 4 which is the worst offender in the entire rack for overvolting the first day was at 3.87 and today is at 3.864 so roughly the same but cell 9 for example is at a lower voltage today then it was the first day. They should be getting closer together not farther apart yes?

 

[42OhmsPA]

Try 54.7V for a few days, or something just above balancing turn on voltage.
Contact SS.
I'm shocked the BMS is even letting the cells get that high...

 

[sunshine_eggo]

Batteries in storage routinely lose top balance. If they are not kept at elevated voltage, they will not balance.

Balancing works solely by burning off excess charge from the high cells. They do not transfer charge from high to low.

You need to HOLD the batteries at elevated voltage. 54.7 is a good start. Increment it 0.1V at a time until OVP triggers. Fall back and hold for 24 hours, rinse and repeat.

 

[sunshine_eggo]

Try 54.7V for a few days, or something just above balancing turn on voltage.
Contact SS.
I'm shocked the BMS is even letting the cells get that high...

EG4 datasheet indicates 3.9V cut-off.

 

[trig2]

EG4 datasheet indicates 3.9V cut-off.

Yes it cuts off at 3.9

 

[42OhmsPA]

EG4 datasheet indicates 3.9V cut-off.

What's all the fuss over 3.65 then?
Does it come down to cell manufacturer?

 

[Zapper77]

Try 54.7V for a few days, or something just above balancing turn on voltage.
Contact SS.
I'm shocked the BMS is even letting the cells get that high...

If I recall correctly the LL V1 had different cells.

 

[sunshine_eggo]

What's all the fuss over 3.65 then?
Does it come down to cell manufacturer?

Battery builder.

LFP can technically take up to 4.2V before electrolyte breakdown occurs, but it definitely reduces cycle life for a very small % gain in capacity.

I would guess that In their estimation, a 3.9V cut off that rarely / never occurs on an active system, will not adversely impact cycle life.

EG4/SOK, etc., that communicate with the inverters also don't do float. While not exactly correct, it's basically like this:

"Charge to 3.60V/cell and hold there until you can't"

No determination that the battery is full. No "float." Many are concerned with low current over-charge at voltages as low as 3.40V, but THIS is how many builders manage their batteries... Clearly low current over-charge is not a concern.

 

[trig2]

What's all the fuss over 3.65 then?
Does it come down to cell manufacturer?

My understanding is they recommend charging to 55.5 which is what I did the first day. Which is 3.46 per cell. The BMS charged to 56 which would be 3.5. 3.9 is just the safety threshold to prevent damage, at least I hope it prevented damage.

 

[trig2]

Okay, if I set bulk and float to 54.2 then that most problematic cell gets to 3.679 Pretty close to the ol 3.65. I will try leaving it like that for a day and see what happens and if it starts improving instead of getting worse I'll start upping the voltage.

 

[RCinFLA]

Unused, self-leakage variance between cells will result in imbalance in several months.

The BMS likely only balance dumps 100 to 200 mA, maybe even less. That means it will take 5 to 10 hours to correct 1 AH difference in state of charge which would be only 1% imbalance for 100 AH battery pack.

You will start to have issues with one cell tripping overvoltage when attempting a full charge at about 1% imbalance, but it also depends on bulk current charging rate. Higher bulk charging current will be more prone to tripping cell overvoltage.

Leave it soak at highest absorb voltage possible without tripping a cell overvoltage for five hours then raise absorb voltage a little higher. When you can get charge current to taper off with absorb at about 56.0 - 56.8 vdc you are good to go with 100% charge on all cells.

With lowest cell at 3.35vdc rested, no-load voltage you are imbalanced by 3-4% so it is going to take a while to get it back in balance.

 

[trig2]

Thank you for your help everyone, does anyone know what the cutoff is for the BMS to start balancing? I've heard above 3.4 or 3.45?

 

[RCinFLA]

Thank you for your help everyone, does anyone know what the cutoff is for the BMS to start balancing? I've heard above 3.4 or 3.45?

It is typically above 3.40v per cell when a cell balance dump starts. BMS's sometimes have a limit on how many simultaneous cells are dumping current to limit the total heat build up within BMS.

 

[trig2]

In the last 30 minutes 0 charging has been done according to solar assistant and in fact there's been an occasional discharge. The high cell has actually gone up in voltage... I'm seeing zero evidence of balancing. ?


Edit: Under battery current it actually says 2 amps going to the batteries so I guess I'll wait and see.

 

[trig2]

Ok, so I think I figured it out, again, hopefully for real this time. I turned float down and it's no longer applying the 2 amps and after a few minutes the batteries started discharging .1 amps each and dropping that top cell. So they apparently won't enter balance mode if they're not in standby mode.

 

[Zapper77]

In the last 30 minutes 0 charging has been done according to solar assistant and in fact there's been an occasional discharge. The high cell has actually gone up in voltage... I'm seeing zero evidence of balancing. ?


Edit: Under battery current it actually says 2 amps going to the batteries so I guess I'll wait and see.

Check the inverter page for charging amperage when under 20 amps. I've noticed on LP4 the BMS only reports amperage over 1.5-2 for a pack total of 12 amps and SA will report zero while the inverters are actually pushing 10 or more amps.

 

[trig2]

So in the manual when it says that cell balancing is done automatically at all times what they meant to say was it's only done when you're not using the battery and it's fully charged... That would have been nice to know.

 

[RCinFLA]

In the last 30 minutes 0 charging has been done according to solar assistant and in fact there's been an occasional discharge. The high cell has actually gone up in voltage... I'm seeing zero evidence of balancing. ?


Edit: Under battery current it actually says 2 amps going to the batteries so I guess I'll wait and see.

Many BMS's do not enable balancing unless BMS detects charging current. Some have option to allow balancing during discharge. It is cryptically called 'charge balancing' check box on many BMS's.

Problem is on many BMS's when you disable 'charge balancing' it also disables any balancing during charging which is totally stupid design.

In reality, to get a cell greater than 3.4v balance trigger voltage you pretty much need to have some charging current through cell.

A cell topped out at 3.65v will quickly drop below 3.40v with a short period of relatively low discharge current.

BMS low current detection is not great for accuracy, so they typically require a few amps of charging current to enable balancing when they must see charging current. Only an amp or two of charging current may not be enough to enable BMS balancing.

 

[Zapper77]

So in the manual when it says that cell balancing is done automatically at all times what they meant to say was it's only done when you're not using the battery and it's fully charged...

I really need a LL V2 for testing. I've been able to initiate passive balancing on the LP4, if charging is I pushed above 30 amps per battery. So a 6 pack would need a current above 180 or you could try to balance your problem batteries by turning off the others.

 

[trig2]

Many BMS's do not enable balancing unless BMS detects charging current. Some have option to allow balancing during discharge. It is cryptically called 'charge balancing' check box on many BMS's.

Problem is on many BMS's when you disable 'charge balancing' it also disables any balancing during charging which is totally stupid design.

In reality, to get a cell greater than 3.4v balance trigger voltage you pretty much need to have some charging current through cell.

Well I certainly do not know what is going on inside this thing but I didn't seem to get any balancing until it had no input or output.