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robbymax

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Dec 22, 2020
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just bought a RC3563 and i have tested a single cell and a 12 volt lithium battery and got readings but where is the info to tell me what is good and what is bad
 

Korishan

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Jan 7, 2017
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Datasheets for the cells
 

Wolf

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Sep 25, 2018
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@robbymax
A 12 volt Li based battery is probably a LiFePo4 Battery and the IR of the whole battery will be skewed by the BMS system.
The best way to test the IR of cells is without any connections to them.
IR values are dependent on cell chemistry so the results can vary greatly.

Wolf
 

robbymax

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Dec 22, 2020
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so the BMS screws with I R so to get a reading across 288 x 3.2 cell battery go straight to the neg and pos before the B M S to get an overall reading will this work ?
 

robbymax

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Dec 22, 2020
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all recovered from modems i bought from battery hook up
1643748719701.jpeg
 

Wolf

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Sep 25, 2018
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Well the battery above looks like a 10p14s and is already built. Looks quite good. (y)
What we usually do is check the IR of the cells before the assembly and as first touch to determine the quality of the individual cell.
Once all the cells are put together into a battery you cannot check each individual cell for IR.
Wolf
 

robbymax

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Dec 22, 2020
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i have got 6 of these each 48v with a bluetooth B M S so i can monitor them but i wanted to check the overall IR of the 6 48volt batteries
 

robbymax

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Dec 22, 2020
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this is one of the big batteries i have been making to replace my 16 year old lead acid bank that has run my house but now are starting to fail
these are K2 lfp 26650 3.8AH new cells 18P/16S with JBD smart 100amp BMS with bluetooth and i thought i wouldbe able to read the IR ?
small solar 2.JPGIMG_0989.JPG
 

Oberfail

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Jun 22, 2021
Messages
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No, a BMS wont allow you to read the IR as it would be very very inaccurate. To read the IR, you'll need an proper 1khz AC IR meter like the RC3563.
 

floydR

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Aug 23, 2017
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1,481
Some smart bms's do individual cell IR. Not sure which ones. Most likely would be IR by dc method which may be inaccurate. You can measure the IR of the whole pack with most Ac IR meters. just use the most positive and the most negative of your pack before the BMS and if your IR meter can handle the voltage it should give you an IR reading. but individual cell IR doubtful.
Later floyd
 

Wolf

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Sep 25, 2018
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i thought i wouldbe able to read the IR ?
Sure you can read IR (Internal resistance) on anything. Same as you can read IR on a capacitor in a circuit or the impedance of a choke but if they are in a circuit especially if they are powered up the readings are more than likely not valid.
The issue is once you have anything in parallel the rules change. Also if you add electronic components, passive or not it will change as everything has impedance/resistance even the 'balancing" wires have a capacitance/resistance/impedance. Mostly too little to worry about but nevertheless its there. Therefore it is recommended that the balance wires on a BMS should all be the same length.
Here is an example of how resistance works in parallel. Fig.1 and Fig. 2 have essentially the same end result but vastly different individual readings.
So you see you cannot judge the individual cells IR by the end result.
Wolf
1643893283334.png
 

Redpacket

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Feb 28, 2018
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To get accurate & meaningful readings, like said above, you need to:
a) be testing a single cell separately one at a time
b) test at the cells terminals not after the BMS. (BMS FET & wiring resistance affects readings)

With a) sure, yes you can test an assembled battery pack like in your pics (nice, like them!). Trick is if there's a bad/high IR cell in the pack, you wouldn't know.

Re what is a good IR vs bad IR number, best to find the manufactures spec. I'm totally guessing but <40mOhms would be a starting suggestion for your LiFeP04 26650.
I googled "26650 lifepo4 datasheet" & found this example spec showing <20mOhms:
 
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