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voltage difference before soldering
#1
Hey,
before soldering a parallel pack together (wire cell level fuses)
should I:

1. discharge all to maybe 3v (much work because my cells are stored at about 4v after testing)?
2. not discharge but keep the voltage difference within max 0.2v that means max 2A for a very short time when internal resistance is max. 0.1Ohm ?
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#2
I solder them at full voltage or the voltage they are resting at. I dont see the problem as such. Yes the potential is higher but the time to discharge once more take to much time since i run the full charge/discharge

If the difference is 0.2v at 3V or at 4V you dont get 2A but perhaps 50mA at most. Thats because the potential vs voltage is low.
On other hand betweeen 3.6 and 3.4 you will see 2A easy!

Thats how I do it but of course you can always do it better but above works fine so far.
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#3
My process is....
1) Test the cells to figure out their capacity - leaving them at full charge at the end of that process
2) After several weeks, assemble the cells into packs so the cells+capacity are evenly distributed
-----------
3) THEN, as a 1st step of soldering things up - test the voltage of each cell and discard self-discharged cells (e.g. cells < 4.10v after several weeks sitting) and replace them with extras of similar capacity.

From your description, you may want to consider setting up a process where one of the steps is to check for self-discharge after several weeks Smile
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#4
I usually do them fully charged, but that has high risk in case of +/- accidental contact, so I don't recommend it.
At 3.3 V it should be quite safe. You should be able to discharge them fine to have that value +/- 0.05V.
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#5
After the ~30 day resting V check the cells are usually between a minimum 4.12V and a maximum of 4.18V.
A difference of 0.06V,  at an average cell IR of 50mΩ the max amps between 2 cells at that V difference would be 1.2A for a relatively short period of time. Well within the  max charging spec of most cells. 
Even with a max difference of 0.1 V the amperage interchange is at 2A, again for a very short time. 
At 0.5V difference between cells now we are talking about getting to some serious amperage. 10A is what that calculates out to, so personally I would try to keep my cell voltages(whether fully charged or with a storage charge) to <0.09V difference and you should have no problems.
For high drain batteries such as power tool and such that have generally a lower IR you want to tighten your max V difference up a bit < 0.05 as that V difference with a 20mΩ IR will produce 2.5A.

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#6
The current you calculated is only valid at 3.6v. at 4.1+v you dont get ftaction of that current due to the discharge curve Smile
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#7
(10-26-2020, 06:43 PM)daromer Wrote: The current you calculated is only valid at 3.6v. at 4.1+v you dont get ftaction of that current due to the discharge curve Smile
???
At 4.1+V  I dont get fraction of that current?
I don't understand. Sorry.

Plus I have put an amp meter between 2 cells and there is an instantaneous spike between the 2 cells with different V which dissipates very quickly even at 4.1+V.
But you are correct it is far more pronounced at ~3.6V.
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#8
The spike at 4.1V is fraction so you cant measure it with a normal volt/amp-meter.
If you have 1 cell at 4.1V and another at 4.15 you wont see anything compare to if it would have been 3.65 vs 3.6V.

This due to that the potential does not carry alot of stored energy at 4.1v compare to 3.6. Just hook a scope to it and look at it and you see. A normal amp-meter wont be able to show you much at all unless you can save 100s of samples per second to look at. I dont remember the numbers when i tested it now but i can of course go back when back home if wanted to.

Yes there will be current flowing but its not something to "worry" about. If so i would be more worried over people even handling lithium cells Smile
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#9
Yep the amp meter I was using was a INA260 set to 1024 samples at 1.1ms.
Also all I was trying to accomplish in the previous post is to show that a 0.05V difference is not much to worry about as you are saying.
So we are on the same page.   Wink

Wolf
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