Strange charging behaviour on LiFePo4

[Thodoris]

Hello everyone!
I recently bought a 51.2V / 150Ah LiFePo4 battery from a certain seller on aliexpress, with numerous positive reviews (not fake).
The battery looks very well made and my first tests were positive, the battery communicates perfectly with my inverter (Growatt SPF5000ES) and in general it looks like a quality item. Please note that i have set the battery as Li in the inverter and communication is normal and successful.

I have not installed my solar panels yet, so i tried to test charge the battery from utility.
I started from 50% SOC. When the battery arrived at 99% (154.8Ah) it was stuck there and remained at this percentage for more than half an hour! Charging current was 30A (0.2c) all the way from 50% SOC. At some point the current dropped to 18A and then to 7A. It eventually reached 100%SOC but that was more than 1hour AFTER it reached 99% SOC! So i have spent around 5kWh in total to go from 50% SOC up to 100% SOC.... This is not normal, don't you think? For a battery with a theoretical capacity of 7.68kWh....

Maybe you could help me on this? Is this normal? Is it normal for a LiFePo4 battery to delay so much to reach 100% SOC?

 

[RCinFLA]

When the battery arrived reached at 99% (154.8Ah) it was stuck there and remained at this percentage for more than half an hour!

Just means Columb counter in BMS was not accurate. It is waiting for BMS's full charge reference voltage and possibly charge taper off current to reset Columb counter.

When a battery sits on shelf for several months its self-discharge current is not captured by BMS's Columb counter.

 

[Thodoris]

Just means Columb counter in BMS was not accurate. It is waiting for BMS's full charge reference voltage and possibly charge taper off current to reset Columb counter.

When a battery sits on shelf for several months its self-discharge current is not captured by BMS's Columb counter.

So do you think it will correctly reset after the 1st full charge?

 

[RCinFLA]

So do you think it will correctly reset after the 1st full charge?

Yes, if BMS shows 100%. Their firmware apparently will not allow display to go to 100% until it detects a full battery condition (voltage, and possibly charge current taper down). Once it trips full criteria it will reset Columb counter amp-sec count to zero and show 100% SoC.

Some BMS's just hold at 100% readout, some stupid BMS's keep climbing above 100% until battery full criteria is tripped at which point the display pops back to 100%.

There are some BMS's that don't reset Columb counter until the charger drives a cell overvoltage trip. Not a great way to reset Columb counter.

An external battery monitor, like Victron 712, can only reset its Columb counter based on total battery stack voltage. A BMS has access to each cell voltage which creates a more complicated decision on what full charge is, based on condition of SoC balance between cells. It should require all cells be greater than about 3.5vdc.

SoC readout is register stored value of AH size of battery minus the Columb counter value in AH's. If either is wrong the SoC readout will be wrong.

Columb counter amp-hour cumulation will accumulate errors over time due to inaccuracy in current sense readings, particularly during lower current levels that are not very accurate. You need to fully charge periodically to reset Columb counter to clear out errors. (also need full charging to do cell balancing).

Many BMS's have a 'feature' that will modify the battery AH size register when a low battery voltage is tripped. This can actually screw things up if a true full charge condition was not achieved before full discharge of battery or cells are out of balance.

 

[Texas-Mark]

I started from 50% SOC. When the battery arrived at 99% (154.8Ah) it was stuck there and remained at this percentage for more than half an hour!

What was telling you that it was 50%? The battery BMS or the inverter? As noted above, once fully charged (by cell voltage), things will be in sync. However, unless using an external coulomb counter (or if the inverter has an accurate one) things can eventually get out of sync. That is why it's best to invest in some external unit that measures exactly what goes in and out of the battery.

 

[Thodoris]

"You need to fully charge periodically to reset Columb counter to clear out errors. (also need full charging to do cell balancing)."

Do you mean that i have to charge each cell individually so i top balance the battery?
I also found out that when i reached 100%, cell voltage difference is somewhere like 200mV, so an active balancer could be needed too...

 

[Thodoris]

Another strange thing i found out: I stopped charging and added a small load. The battery went from 155Ah (100%) to something like 153.5Ah (99%). I then turned off the battery and when i turned it on again, the screen was showing 155.0Ah and 100%SOC. At this point the average cell voltage were only 3330mV (with 20mV difference between cells), nothing close to 100% i think.

Maybe it's my inexperience and ignorance on LiFePo4 batteries, but it seems that this BMS is screwed up...

 

[Thodoris]

Another update. In the BMS settings the overcharge voltage alarm point is at 3650mV, and the point where the alarm goes off is 3450mV.
So, i tested again to fully charge the battery and the result was that cell #15 went over 3650mV and then i got the overcharge alarm. The battery stopped charging at that point. Problem is that only cell #15 went overcharged, the others were around 3450mV...

 

[Ampster]

At this point the average cell voltage were only 3330mV (with 20mV difference between cells), nothing close to 100% i think.

That resting voltage looks normal. A more informative cell voltage would be when they are charging and above 3400 mv (3.4 volts) per cell.
EDIT: As you subsequently found our the delta will increase during charging. Set your max charging lower to give that cell time to balance.

 

[TomC4306]

T

Maybe it's my inexperience and ignorance on LiFePo4 batteries,

This.
There is nothing wrong with the BMS

 

[Thodoris]

T
This.
There is nothing wrong with the BMS

My concern is that when the battery is charging and reaching full SOC, only one cell (#15) gets overcharged at 3650mV, the other are 200+mV below.

 

[TomC4306]

My concern is that when the battery is charging and reaching full SOC, only one cell (#15) gets overcharged at 3650mV, the other are 200+mV below.

Full state of charge, 100%, is achieved above 3.45 volts per cell for a duration of about an hour anything above that is unnecessary.
IMO

About twice per year I do charge to 3.5 volts and absorb for 3 hours and turn on the active balancer.

 

[Thodoris]

Full state of charge, 100%, is achieved above 3.45 volts per cell for a duration of about an hour anything above that is unnecessary.
IMO

About twice per year I do charge to 3.5 volts and absorb for 3 hours and turn on the active balancer.

Understood, but why only one cell gets overcharged?
Is that a red flag that the some of the cells are damaged, used or simply of bad quality?

 

[RCinFLA]

Understood, but why only one cell gets overcharged?
Is that a red flag that the some of the cells are damaged, used or simply of bad quality?

The cells are out of balance. This is common when a battery sits for some time unused due to variance in their self discharge rate.

You have to hold absorb charging for several hours to allow BMS to balance out cells. You may get some BMS charging shutdowns along the way. It will bleed high cell and reset back to charging so it can continue balancing. Most all BMS's do not balance dump current until a cell exceeds 3.4v.

If your BMS balancing current dump is only 100 mA then it takes 10 hours to balance out a 1 AH (1% for 100 AH battery) SoC difference.

New cells have a self-discharge rate of 0.5% to 1.5% of their total capacity per month at about 25 degs C. If battery is shipped in a shipping container in the summer, exposed to higher ambient temperatures, the self-discharge rate is greater.

 

[Tomthumb62]

Understood, but why only one cell gets overcharged?
Is that a red flag that the some of the cells are damaged, used or simply of bad quality?

Not necessarily. It usually means that that cell is out of balance with the others. This happens a lot with the cheaper budget batteries. They save money by not taking the time to pick cells are are already balanced with each other. The BMS can often balance them over time, but it can take weeks or months to get there.