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Managing a single cell repeatedly hitting HVD - 16S

UV-PWRD

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Jan 13, 2021
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ETC 277ah 48v pack
JK 2a active balancing BMS.
MPP 7248

Bulk Charge 54.4, float is 53.8v. Pack was top balanced in parallel for a good 2 weeks (3.6v/10a and let it drop to almost 0a draw) before it was put in service.

Been in service a month and going well, but I have noticed 1 cell hitting HVD (3.55v I have it set to) It's cell 2, second after main bat neg.

I assume it just doesn't quite have the capacity of the others so it's hitting full charge earlier. What I am seeing is that cell hitting HVD, charging stops, but the pack keeps balancing as I have balancing kick in from 3.35v, then charging recommences as that 1 cell settles below the recovery voltage (3.45v) and repeat while the other cells catch up.

Cell wire resistances are all almost spot on if that matters.

Aside from replacing that cell, what management strategies are people using? I've lowered the float and widened the balancing voltage window, I was contemplating dropping the bulk charge further also, say 54v bulk, 53.2 float as 53.2 is where the pack likes to settle, but we are coming in to Autumn/Winter here so I'll have less time per day to fill (currently no issues there).

Thoughts welcome as I assume it's not ideal to have that 1 cell bouncing off the top end of its knee constantly.
 
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I am far from an expert - I have built a couple of batteries and read a bunch here.

I am not sure you have a problem - one cell is always going to hit HVD before the others. Have you measured your capacity? If I was getting good capacity, I wouldn't worry about it. I am interested in others' opinions about this.

What are your other cells voltage when the one hits HVD?
 
The others are all approaching the 3.35-3.4v range roughly in unison.

Capacity to me seems quite good, but no formal test so take with a grain of salt.

I have no "issues" only the warning from the BMS.

That 1 cell will keep bouncing off the HVD to recovery voltage as the others catch up/balancing sorts them.
 
I'll try and watch it more closely this morning and see at what total pack percentage it starts to happen.
 
so up to a .2 volt spread?

My only suggestion would be to bleed off some power from that cell by using a resistor and see how it goes or getting an active balancer that will transfer more power to the other cells faster than the BMS balancer.
 
Not quite, and only at the very upper knee.

The BMS is active 2a now.

I'll watch it more today
if I can.
 
Recheck your terminal/bus bar connections. Before you do anything, when the battery is under a load, put your finger on each terminal (one at a time, of course) and check the temperature. If you come upon one that is hotter than the others, you may have found your problem. Bad connections account for probably 50% of the cell balance issues we see here.
 
Did some experimenting, bulk was actually originally set at 56.4 and float at 54.4, so turning it down to 54.8 bulk and 54.2 for float saw a much nicer and more even charge curve with no HVD.

See how it goes in winter with shorter days and less time to fill cells.
 
Recheck your terminal/bus bar connections. Before you do anything, when the battery is under a load, put your finger on each terminal (one at a time, of course) and check the temperature. If you come upon one that is hotter than the others, you may have found your problem. Bad connections account for probably 50% of the cell balance issues we see here.
Each cell is double stacked busbar (basic ones you get with cells from Ali) and "feels" fine. Though I do have an IR gun if needed. I'll keep this mind.
 
get a high wattage resistor and short it across the #2 cell as it approaches HVD until it falls back in line with the rest
pick a resistance value that will draw a few amps
hopefully you just have to do this once
 
So the plot thickens, as well as hitting HVD first (which it's no longer doing with lower charge voltage) it's also showing signs of dropping first/sagging furthest.

Wondering if this cell had capacity issues and assuming the next step is swap two cells and be sure the problem follows that cell.

Screenshots show what I mean.

Cell 14 is the culprit.

Balancing is now on sooner also.
 

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