Cells not accepting current when balancing

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GoneDiving

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I've received 48 280ah Eve Lifepo4 cells. I'm attempting to balance them all in parallel at 3.40v using my 20a power supply. However, the parallel string is only accepting ~8a at 3.40v.

The batteries were received at 3.30/.31v with two at 3.28v. Charging at 3.50v gives ~15a, 3.60v gives the full 20a.

I would have expected the paralleled cells to receive the full 20a when first connected to the power supply. Should I be concerned or is the low current due to the small voltage difference between the power supply (3.40v) and the pack voltage (3.31v)? Can I expect faster charging when the finished pack is connected to two PIP5048MKs with 120a charge capability?

FWIW: I've confirmed that my power supply can deliver 20a at 3.40v.

My intention is to balance all cells in parallel at 3.40v then charge in series to 3.60v max per cell, identifying/noting any cells that have different capacities, then top balance in parallel at 3.60v.

Many thanks.
 

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Most likely difference in voltage. Charging at a higher voltage opens the gate way up, sorta, allowing more current to flow.

However, because they are connected in parallel, the are already balanced. It is impossible (with normal means) to have cells not in balance when connected in parallel.
 
GoneDiving said:
I've received 48 280ah Eve Lifepo4 cells. I'm attempting to balance them all in parallel at 3.40v using my 20a power supply. However, the parallel string is only accepting ~8a at 3.40v.

The batteries were received at 3.30/.31v with two at 3.28v. Charging at 3.50v gives ~15a, 3.60v gives the full 20a.

I would have expected the paralleled cells to receive the full 20a when first connected to the power supply. Should I be concerned or is the low current due to the small voltage difference between the power supply (3.40v) and the pack voltage (3.31v)? Can I expect faster charging when the finished pack is connected to two PIP5048MKs with 120a charge capability?

FWIW: I've confirmed that my power supply can deliver 20a at 3.40v.

My intention is to balance all cells in parallel at 3.40v then charge in series to 3.60v max per cell, identifying/noting any cells that have different capacities, then top balance in parallel at 3.60v.

Many thanks.
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LFP cells at 3,4V with 20A divided to 48cells are already in resting stage. They are already perfectly balanced. You can only get a few mAhs of ions move around a bit when you push it into the final spike of the charging curve. With my experience with that chemistry, I would say you are perfectly set to build your pack now ;)
 
With thinner cables in the pics, it's not hard to get voltage drop across the cables (incl cell - cell & cell string links).

I'm hoping you already realize that the point of balancing isn't to force current in.
If you have them parallel connected & they have likely already reached equilibrium (ie minimal/no current BETWEEN cells) then they are already "balanced".
Since you can't do much about minor differences between cells (eg slight differences in batches, electrolyte chemistry, metallurgy, temperature) it's better to accept them as they are & work with that.
For your cells try to find the point where the voltage starts to climb faster when charging. Just before this is when to stop charging.
Balancing packs above this point becomes much harder, especially with larger cells/systems.

Sure, test each cell individually & weed out any obvious problems.

Also note that operating your cells at 3.6V is likely to shorten their life a lot. There is very little capacity in the 3.4 to 3.65V range. You may get swelling (start of "the end").
The manufacturers are of course eager to claim the highest capacity number they can. But longer cell life is had by operating a bit "inside" the specs they give eg not-so-low min, not-so-high max end-points.

When charging, if you note the point when the voltage starts to rise faster, this is the top edge of the "chemistry plateau" and IMHO, charging beyond this is not useful & degrades the cells.
For my LiFePo4 system, this is 3.45V. I think Daromer has his at 3.50V.
My chargers bring the cells to 3.45 & hold there & the current tapers away. After this, they are "full".
My packs voltages also start going out of balance above about 3.45 - 3.50V.
Testing shows I still have within a few % of the pack's rated capacity at 3.45V.

Chasing the last few % capacity at the cost of damaging your cells will be an expensive lesson.
Keep your cells on the "flat" part of the voltage curve. Off either end, low or higher, isn't good for them.

PS: I'm jealous! Nice cells!
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As already stated as soon as you go above 3.4v they are kind of full and its not much you gain going above that. Look at a charge curve of an LFP and you see it.
 
Thanks guys. Yes, my operating range will be around 3.00 to 3.40 per cell. Having said that, I would like to top balance all cells once at 3.60 so that I have a margin of safety knowing that one cell won't be over charged should pack voltages over shoot for what ever reason.

All cells have been in parallel now for 36 hours and cell deviation has dropped from 100 to 20mv. I'll call that close enough and start charging in series and monitor if any cells' voltage rise faster than the others. If there is a sizable difference as I approach 57.6v then I'll join them in parallel again to equalise voltages. Again, this is just for a safety margin, not for regular use. When finished I'll draw the pack down to ~53v and revert to the usual settings.
 
Redpacket said:
PS: I'm jealous! Nice cells!
Edit typos
Click to expand...
Thanks. The end use isn't actually a residential or Powerwall installation. It's to replace two 35 and 10kva generators on a boat.

The whole package is:
5kw solar panels
48*280ah Lifepo4 for 43kwh at 48v
2*PIP5048 inverters for 10kw 230v single phase
15kw 230v VFD to power a 3 phase hydraulic anchor winch
48 to 12v converter at 60a for low voltage ancillaries
A small 3kva 230v generator as backup power/charging.

Thanks to all😁
 
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