Lifepo4 cells won’t charge to 3.6 volts

[Dave (Boog)]

I am new to this forum and new to Solar. I have a basic knowledge of electronics though.
I am trying to get an off grid solar installation going for my house. I have purchased solar panels that I will install in the Spring.
For now I am trying to build my own 24 volt battery pack. I recently purchased 8 120 amp Lifepo4 cells and have 4 of them hooked up in parallel and am trying to charge them up to 3.6 volts each. I have had them charging with my power supply - 3.6 volts @ 3 amps for about 2 days and can’t get them to go above 3.36 to 3.37 volts.
These cells were 3.2 volts each when I got them from AliExpress. I arranged the 4 cells in series and put them on a charger in Lithium mode over night and the charger stopped saying they were full.
It’s my understanding that these cells are 3.2 volts nominal but 3.6 fully charged.
Can someone tell me why I can’t get them to charge to 3.6 volts?
Thanks

 

[Dzl]

You probably just need to be patient

 

[Dzl]

8 x 120 = 960Ah total capacity in parallel
If your batteries were at 3.2V when received, they are starting out at a pretty low SOC
You are adding 3Ah per hour, or 72Ah per day, and you need to get to 960Ah total.

 

[Bob B]

What Dzl said ... and get rid of the junky alligator clips and put some ring terminals on there .... you should be able to get more current.

Resist the temptation to increase the voltage to get more current. Set the current as high as it will go ... and always adjust the voltage with the leads disconnected from the battery.

 

[Dave (Boog)]

8 x 120 = 960Ah total capacity in parallel
If your batteries were at 3.2V when received, they are starting out at a pretty low SOC
You are adding 3Ah per hour, or 72Ah per day, and you need to get to 960Ah total.

Thank you. Makes sense. So it would take over 10 days to reach a SOC of 3.6 volts. I appreciate you answering my question.

 

[Dave (Boog)]

What Dzl said ... and get rid of the junky alligator clips and put some ring terminals on there .... you should be able to get more current.

Resist the temptation to increase the voltage to get more current. Set the current as high as it will go ... and always adjust the voltage with the leads disconnected from the battery.

Thanks I’ll do that. I have some good #14 wire I can use. And hopefully I can get more current flowing.

 

[John Frum]

@Boogc I've got a much faster way if you are interested.

 

[Dave (Boog)]

@Boogc I've got a much faster way if you are interested.

Sure, let me hear what you think.

 

[John Frum]

Sure, let me hear what you think.

Read this doco from start to finish https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/
and then look at the verbiage below.
I just did pretty much the same procedure and got very good results.

------------------
Top balancing should be renamed to initial cell charging.

q: What is top balancing?
a: {
Top balancing is really just charging each cell until it is full.
We do this to ensure the capacity of the battery is equal to that of the weakest cell.
Which is as good as it gets.
}

q: What is bottom balancing?
a: {
Its not applicable to our application(low cost energy storage).
Google is your friend.
}

q: How should lifepo4 cells be prepared for use in a serial battery?
a: Charge each cell to precisely 3.625 volts as measured at the cell terminals.

q: What is a lifepo4 cell?
a: {
For the purpose of this writing a cell is an indiviual battery component.
Its minimum voltage is 2.5 volts.
Its nominal voltage is 3.2 volts.
Its maximum voltage is 3.65 volts.
NOTE: Always check the manufacturers documentation, there may be exceptions.
}

q: What is a lifepo4 battery?
a: {
For the purposes of this writing a lifepo4 battery is a collection of serially attached cells.
Typical configurations are 4s, 8s and 16s.
These configurations have nominal voltages of 12.8, 25.6 and 58.4 volts respectively.
The battery charge voltages are 14.6, 29.2 and 58.4 volts respectively.
NOTE: For the purposes of this writting a lifepo4 battery includes a bms.
}

q: What is the classic method for initial cell charging?
a: {
The classic method involves connecting cells in parralel and charging them as a large "logical" cell to 3.65 volts.
Parralel charging these huge cells with a typical bench charger takes a very long time.
Cells are usually delivered at aproximately 50% state of charge.
To parralel charge 16 cells for a 48 volt battery...
280 amp hours * .5 state of charge * 16 cells / 5 charge amps = 448 hours.
Cells have dramaticcally increased in capacity of the last few years but benchtop chargers have stayed relatively static.
}

q: What is your preferred method for accomplishing initial cell charging?
a: {
Assemble cells into a battery complete with bms.
Configure bms to disconnect when the first cell gets to 3.60 volts.
Charge at battery charge voltage until the bms disconnects the battery.
Wait overnight for the cells to settle.
With a bench charger set to ~5 amps and 3.65 volts charge each cell until it reaches 3.625 volts as measured at the cell terminals.
Be careful to stop the charge precisely at 3.625 volts.
NOTE: Don't leave this process un-attended for even a minute.
}

q: why is this your preferred method?
a: {
Like many on this forum, I'm using the big blue prismatic cells.
These cells are a great value but the terminals are usually tapped aluminum and prone to stripping.
I prefer to meticulously prepare all the battery hardware exactly once to minimize circuit resistance and wear&tear.
}

q: But wait, how do I charge individual cells that are already serial connected to form a battery?
a: {
Current flows from its origin to its polar opposite.
So just attach the charge leads via alligator clips to the cell's terminal posts such that the current goes through just that cell.
* a picture here would really be good.
}

q: Is it really that simple?
a: Yes.

q: Is there anything else I can do for bonus points?
a: Enable bms cell balancing but only during charge and only above float voltage.

 

[Dave (Boog)]

Thanks. I’ll read the document and try those steps.

 

[John Frum]

Thanks. I’ll read the document and try those steps.

My part is draft quality so if you need any clarification just holler.

 

[Dave (Boog)]

My part is draft quality so if you need any clarification just holler.

Will do. Thanks again

 

[Capt Bill]

Thank you. Makes sense. So it would take over 10 days to reach a SOC of 3.6 volts. I appreciate you answering my question.

A Faster way to get batteries charged is put the 8 cells in series w close to matched voltage reading and a BMS, and charge close to full with your 24v system charge (even if only solar), then go back to parallel for the 3.2v top balance charge to full charge. ... just a thought on options ;+)