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Large delta at the end of the charge....200mv.

On_The_Road

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I am trying to figure out why Cell 4 is falling so far behind at the very end of charging. Deltas under 8mv until the very last Ahs to go then it ends up at 200mv at absorb which is set to 14.00. If I discharge after the charge the deltas come down below 10mv quickly.

Two 280AH packs 4S. Cells from 18650 Battery Store
Top balanced as per this PDF https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/
Two Overkill 120A BMSs
Polished and cleaned all contacts
All BMS voltages verified with a voltmeter
Using 45in lbs for the bolts on the battery.
2/0 positive wires / double 2awg negative wires all crimped with a 5 ton hydraulic hand crimp tool. Copper wire.

I did discharge it yesterday and it seemed well behaved at the end and the capacity seems right (See attached pictures). During that discharge I was pulling 180A at times and the deltas stayed under 8mv.

Cell #4 on each pack is where the positive battery cables land and those are the cells that seem to fall behind the furthest. It almost seems like Cell #4 in each pack stops charging 3.38v while the other cells keep on going.

Suggestions so far.....Swap Cell #4 to a different position on each pack to see if it follows the cell. If it does follow the cell top balance again.

Could it be the fact that the battery terminals that have my positive wires have less surface area contact with the lugs than a battery terminal with a bus bar on them?

Just looking for any additional suggestions before I take it all apart and try the above? I can't help but think it's something with my setup as it's the same cell in each pack falling behind. I have ordered some silver paste and I will give that a try on all of the contact points.

Thank you :)
 

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Thought I would see if it was the uneven contact on the positive battery pads and added on some extra bus bar to both batteries. Didn't change anything. Although, I do like this way to land the positives better.
 

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You said you did a top balance so you have a bench power supply. Charge the pack to where you start getting the delta. Set the power supply to 3.6 volts and connect it to the low cell only with everything as is. It should come up in voltage fairly quickly. I have done this myself.

Alternatively you could use the power supply to charge the pack through the BMS this would insure a minimum amperage at the top of charge allowing the balance feature of the BMS time to do its job.

If this is a brand new assembly I wouldn’t worry much the BMS will probably even them out over a month or so. Lite cycles will help this along.
 
You said you did a top balance so you have a bench power supply. Charge the pack to where you start getting the delta. Set the power supply to 3.6 volts and connect it to the low cell only with everything as is. It should come up in voltage fairly quickly. I have done this myself.

Alternatively you could use the power supply to charge the pack through the BMS this would insure a minimum amperage at the top of charge allowing the balance feature of the BMS time to do its job.

If this is a brand new assembly I wouldn’t worry much the BMS will probably even them out over a month or so. Lite cycles will help this along.

Thanks for the post I will give that a try tomorrow.

I'm curious.... raising cell 4 in each pack to 3.60...what does that accomplish inside of the cell? This won't throw the pack out of balance?

You mentioned you've done this before....and it brought things back into line?
 
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Lower your maximum charging voltage to 3.50V/cell. You are at the point where some cells are reaching saturation, and small SOC changes result in large mV changes.

My absorb was set to 14.0 / 3.5 already....granted at this setting I didn't hit the high voltage cutoff of 3.65.

I know I'm chasing such an small amounts of capacity but it bothers me it's not straight. I've tried really hard to have the best possible outcome.
 
My absorb was set to 14.0 / 3.5 already....granted at this setting I didn't hit the high voltage cutoff of 3.65.

I know I'm chasing such an small amounts of capacity but it bothers me it's not straight. I've tried really hard to have the best possible outcome.
You need to set the cell maximum voltage, not the average. No single cell should go over 3.50V. It doesn’t matter if at the end of your charge cycle if one cell is 3.50, and one is 3.40 (as long as you are getting the required capacity from your pack)

Letting a cell go significantly over 3.5V on a regular basis will result in early cell failure.
 
Thanks for the post I will give that a try tomorrow.

I'm curious.... raising cell 4 in each pack to 3.60...what does that accomplish inside of the cell? This won't throw the pack out of balance?

You mentioned you've done this before....and it brought things back into line?
No just bring it to the highest cell voltage not 3.6 volts.

I would keep the cell high cutout at 3.65 on the BMS. unless you want BMS cutting out regularly.

If the imbalance persists it may be a cell capacity mismatch. At that point you will need to either use a active balancer of reduce charge end amps and loose a couple of AH. Also remember you can top balance or bottom balance but not both. So only balance during charge.
 
No just bring it to the highest cell voltage not 3.6 volts.

I would keep the cell high cutout at 3.65 on the BMS. unless you want BMS cutting out regularly.

If the imbalance persists it may be a cell capacity mismatch. At that point you will need to either use a active balancer of reduce charge end amps and loose a couple of AH. Also remember you can top balance or bottom balance but not both. So only balance during charge.

Ah, that makes sense - I will give that a try thank you.
Would there be any benefit to top balancing each cell to 3.65 individually in the pack? I would guess to get the best results I would need to remove everything from the battery?
 
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Unless this has been asked before, what are your Overkill BMS settings?

A skilled Overkill user might spot an issue.
 
Unless this has been asked before, what are your Overkill BMS settings?

A skilled Overkill user might spot an issue.

HVC is 3.65.

I don't see how BMS settings could influence this last bits of charging outcome since it's occuring before the high voltage cutoff?

I did have the balancer in the BMS set to run at all times for two days prior to the charging. Could that have kicked the cells out of balance? Even though the balancer in the Overkill BMS is so low current?

I've since turned off that setting.... changing it to balance only while charging.
 
Huh, wonder if that's the default behavior out of box for the Overkill?

I'm old school - for initial balancing, I only balance during charge when well into the upper knee, and the bleeder trigger voltages (if adjustable) are at at 3.5 maybe 3.55v. I go way up there.

If there are problems, I take a tip from balance chargers (that is, the balance is in the charger box, not the battery!), and when they see a cell reach a high voltage, like 3.6v, they start to reduce charge current rather than just trip off. This reduction in charge current helps give the bleeders a chance to do something. An internal bms doesn't have control over the user's charge current, so this never happens. It just trips out.

Anyway, glad you changed to balance during charge. If the bleeder trigger if adjustable is at a ridiculously low value like 3.4v, raise that to 3.5v.

And if you still have runners tripping the bms, use a lower-amperage charger, even though it might be a snooze-fest until things get balanced. And no need to cycle very far like back down to full discharge! Just take 10-20% of the top with a discharge, and recharge with your low-amperage charger.
 
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Batteries sat around for over two weeks slowly discharging....0.4A. Charged them back up and it's is better now. Not sure what changed. Deltas under 31mv at over 14.5V.

Added a Heltec 5A active balancer to both batteries and it's even happier. I was able to charge up to 14.55V. I'll stick around 13.9/3.45 going forward.
 

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Lower your maximum charging voltage to 3.50V/cell. You are at the point where some cells are reaching saturation, and small SOC changes result in large mV changes.
This exactly. I had (have) the same issue on all 4 of my packs and this corrected it completely. I can get very near 100% SOC with 3.5/cell, it doesn’t take much longer than 3.6 and I have zero issues with large imbalances.

I did a top balance on them as well, but small differences in cell resistance cause this to happen from what I’ve read. I even run an active balances on one pack and it can’t keep up.
 
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