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IR for cells in parallel
#1
Has anyone ever confirmed how far off IR is for cells in parallel vs individual?

Meaning, if you pull two cells in parallel from a laptop battery, do he they follow 1/R=1/R1+1/R2

I know 4 wire is only way to go, but how well does 4 wire follow the formula

Same for Other tester
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#2
The formula is valid but the actual calculated value is less relevant than resistance matching. Matching internal resistance of a parallel pack is important in ensuring the long cycle life of the battery pack. As example, a 20% difference in cell internal resistance between two cells cycled in parallel can lead to approximately 40% reduction in cycle life when compared to two cells parallel-connected with very similar internal resistance.
That means internal resistance mismatch leads to higher than normal currents during cycling, which leads to abnormal temperatures, which leads to thermal damage therefore capacity reduction.
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#3
Yes, I have tested it, at the cell level and at the pack level.  It appears it would be similar to how you would calculate it for resistors.  The difference being IR is a variable value on a battery in that it will change based on SoC and temperature.

Here are some examples of a Samsung 25R - from cold.  It just about got cut in half when I added the 2nd cell in parallel.





There comes a point when some testers cant output a reliable number when the IR is too low.  I have encountered this with some of my lower end chargers when testing EV modules. That said, I typically do a manually calculated IR with a CC load or known resistance.  I have found that to be the best / most practical method because it takes into account your application discharge rates as well.
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#4
You need to add the balance Wires doing ur testing om that tester. So you get 4 Wires. Then you get better and more even results
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#5
I recently bought a iCharger X8 and have been discharging my (new) packs down to 3.6v in prep for adding the powerwall.    Have sense leads attached as well as power leads.   The packs are 50cells / 130ah.  

The software (via USB) reports 8milliOhm IR for the pack...  which seems crazy to me as the individual cells report 80millOhm via OPUS.

Is this 8mOhm IF for pack somehow 'reasonably correct'? or bad info?
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#6
It is most likely correct. The more cells the lower total resistance
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#7
(12-31-2019, 06:48 PM)daromer Wrote: It is most likely correct. The more cells the lower total resistance
OK.  But i don't understand how more cells make lower resistance and related - a lot of discussion on individual cell IR but not as much on pack IR.

My sense (so far) is that under low charge / discharge per cell - e.g. 300ma/cell - IR is just not a big deal as heat won't be an issue?     Or maybe the key question is pack IR?     

For example, 14packs in series for 48v - with each pack at 8mOhm  vs 20mOhm (for example) - what does this affect?
- Is it a matter of heat at higher amps / pack?  or what?
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#8
Its a matter of heat generated över a certain current.

Ir per cell is important since you design based on current per cell and then add the amount of cells needed for the max current you need.

2 rwsistors in parallel with same resistance half the resistance. Same with cells.

You can Read more here https://www.swtc.edu/Ag_Power/electrical...rcuits.htm
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#9
(12-31-2019, 08:13 PM)daromer Wrote: Its a matter of heat generated över a certain current.

Ir per cell is important since you design based on current per cell and then add the amount of cells needed for the max current you need.

2 rwsistors in parallel with same resistance half the resistance. Same with cells.

You can Read more here https://www.swtc.edu/Ag_Power/electrical...rcuits.htm

Perfect, appreciate the link / formulas.  I can now answer my own questions Smile
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#10
(12-31-2019, 08:47 AM)daromer Wrote: You need to add the balance Wires doing ur testing om that tester. So you get 4 Wires. Then you get better and more even results

We have had this conversion before... everything I do now has a 4 wire.

The photos I shared were pretty old, right before I started adding balance leads to all my 1s accessories.  I just wanted to show that similar to resistors in parallel, batteries in parallel will reduce the resistance in a similar fashion.

The net results being a lower IR and better current handing as more cells are added in parallel.  This is why pack IR matters as well, because it will help determine the final discharge C rates.
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