Runner cell "trigger" voltage

[Hedges]

One cell will always be closer to full than any other, so will jump first.

I think a "bad" cell would be one that self-discharges much faster than the others, or has much less capacity.
If you built a battery with 7, 280 Ah cells and 1, 200 Ah cell, should still be able to use it for 200 Ah capacity. The odd cell might diverge a bit in voltage from the others in use, but shouldn't be a runner at 3.5V per cell pack voltage if pack previously top-balanced to 3.65V.

 

[sunshine_eggo]

One cell will always be closer to full than any other, so will jump first.

I think a "bad" cell would be one that self-discharges much faster than the others, or has much less capacity.
If you built a battery with 7, 280 Ah cells and 1, 200 Ah cell, should still be able to use it for 200 Ah capacity. The odd cell might diverge a bit in voltage from the others in use, but shouldn't be a runner at 3.5V per cell pack voltage if pack previously top-balanced to 3.65V.

and Bad cell =
big voltage swings in both directions during normal operation.
Triggers high voltage cut off AND low voltage cut off on discharge.

@Don B. Cilly Please take care to read the following copied from the first page:

You are observing the typical behavior of gray market cells. THAT cell just happened to hit 100% before the others.

They are not matched or balanced based on SoC. This costs a LOT extra.

The cells likely shipped at a 30-50% SoC, and they all needed slightly different input to get to 100%. In series, they all get exactly the same amount of input.

There is ZERO evidence of a bad cell. What you are experiencing is normal. If this doesn't happen, you are among the lucky few.

This behavior is literally that described in the guide. Don't bother Amy until you have evidence of an issue.

 

[Don B. Cilly]

OK, I'll (re)top-balance and see how it goes.



Churning away at 10A (best I can do). It'll probably take a few days :·/
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[sunshine_eggo]

Disconnect immediately and follow the guide.

 

[Don B. Cilly]

They are disconnected. I only charge during the day (9:45 PM here).
I am following this guide.
Except, there it says to set the voltage at 3.65. If I do that, I can only get some 3.5A out of the supply. I want 10 if I can, don't I?
The top cell (where the + charge lead is) got to 3.36 during charge, 3.33 now (after about two hours' rest).

[EDITED] because the pdf wasn't linkable.
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[Pim57]

The 4.2 V will kill (Lithium plating) the connected cells. 3.65 Volts is the maximum for LFP chemistry!

You don’t need max amps. A few amps at 3.65V is perfectly fine. It’s all about the voltage.

 

[Don B. Cilly]

But the cells are a long way from 3.65. It'll be days before they get even close to that. At 10A. At 3 or less, weeks.
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[Pim57]

If you have such a big voltage drop between the power supply and the cells your power leads are too thin.

Better increase the size of these power leads, than taking the risk of mistreating of even killing your cells.
Several DIY guys went down this road and ended up with destroyed cells.

 

[acdoctor]

But the cells are a long way from 3.65. It'll be days before they get even close to that. At 10A. At 3 or less, weeks.
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Small cables and poor connections are the cause of the low current. 10 awg wire with ring terminals on both ends will allow 9.xx amps at 3.65 volts.

Their is probably a hundred threads on here of people doing exactly what you are then they have expensive blue pillows.
When the voltage starts to climb it goes fast. One fellow was charging one at a time and ruined 4 in a row and he was “ watching “ them.
Fallow the guide, it works.

 

[Don B. Cilly]

0.9V is a big voltage drop? I checked the leads - after hours of charging, they're barely warm.
I am watching the whole process like a hawk, you know. One of the reasons I only charge during the day.
I see both the supply and the voltmeter from my chair.

Small cables and poor connections are the cause of the low current.

I get 10A - which is all that supply does, the leads are reasonably thick, they're not hot...
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[sunshine_eggo]

the guide instructs you to fabricate quality leads. Please follow the guide. Disconnect the power supply and read the entire thing.

 

[Pim57]

0.9 Volts is a BIG BIG BIG voltage drop!!

Follow the guide or join the club with blue bricks.

 

[sunshine_eggo]

OK, I'll (re)top-balance and see how it goes.

View attachment 103965

Churning away at 10A (best I can do). It'll probably take a few days :·/
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(re)top-balance? You have top balanced these cells before?

But the cells are a long way from 3.65. It'll be days before they get even close to that. At 10A. At 3 or less, weeks.
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This is the assumption made by those that brick their cells. You've already charged in series, and one has hit 100%. It's likely that the others are not that far behind.

 

[conquistador]

I am watching the whole process like a hawk, you know. One of the reasons I only charge during the day.

Be very careful when charging with a voltage of more than 3.6V.
I had a similar event a couple of days back. As you can see, the runaway cell was still ok at 8am, just when the charging starts. 30min later it was already out of spec and 1 hour later hopelessly at 3.8V.
Once the voltage runs away, it goes very fast.
This is like running a LED without resistor to limit the current. After a certain voltage the current rises exponentially.

 

[SparkyJJO]

Please Just follow the guide and DO NOT FRY your cells with 4.2 volts! Don't have us saying "we told you so" while you stare and dead cells.

If you're not getting 10A at 3.65V, then your leads totally suck (even 14 AWG can handle it I noticed), or your connection is terrible, or you're not all that far away from full already.

PATIENCE. Do it right. Don't be silly.

 

[Don B. Cilly]

You have top balanced these cells before?

Yes, when I got them. To 3.60.
Still, it'll take ages to get to 3.65 if I charge at 10A. If I charge at 3... and they will be even less as voltage goes up...
Look, just now, with cells at 3.330:



[EDIT] Oh all right then. At that amperage, I can leave it on all night.
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[conquistador]

Yes, when I got them. To 3.60.
Still, it'll take ages to get to 3.65 if I charge at 10A. If I charge at 3... and they will be even less as voltage goes up...
Look, just now, with cells at 3.330:

View attachment 103975

If your cells are at 3.3V and your power supply is at 3.65V and current is only 2.6A, then you have a big issue with the cables and connectors.
With a correct electrical connection, your cells should be a few 10mV below the power supply voltage. Or one of your instruments is wrong.

 

[sunshine_eggo]

Yes, when I got them. To 3.60.
Still, it'll take ages to get to 3.65 if I charge at 10A. If I charge at 3... and they will be even less as voltage goes up...
Look, just now, with cells at 3.330:

View attachment 103975

You indicated you got them on 7/15, but you got them "balanced" by the time you posted the next day after cells were at 3.282V? I'm not saying you're lying, but if you're not, you currently hold the fastest record for someone top balancing their cells.

Guide, Page 1, Item 1, Note 3.

 

[Don B. Cilly]

Well, I got half of them two days before that... but then. I did "fast" balance" them - got them up to voltage individually - and didn't fry them.
I'll check my leads, make new ones. But if I can get 10A by turning up 0.9V... and they're barely warm... oh well. I'll see. Late enough now.
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[Hedges]

The only way to safely balance cells is with power supply set to a max voltage which is OK for the cells, like 3.65V. For most supplies, this must be set with leads disconnected, never adjusted again while connected.

If you don't like the voltage drop from supply to cells, if you don't like the low current, use fatter wires, ring terminals not banana plugs, ring terminals not alligator clips.

The only way to safely series-charge cells is with a BMS, which monitors individual cell voltage and disconnects battery pack if any gets too high.

With large voltage drop from 3.65V supply to cells, you can alternatively just exercise patience.
If connects between cells are poor, the farthest cell will get charged later. We recommend positive at one end, negative at other, to better match voltage on cells during parallel top balancing.

After balancing, separate the cells and allow to settle a few hours. Check voltage when first separated, and after settling. If any are/were much lower than the rest, it may have had poor connection. Hook it up individually and bring up to target voltage (e.g. 3.65) then disconnect and let settle.