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Internal Resistance Testing: Ultra-Low Resistance 18650 holder accessory
#1
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Build guide with test results:


 
I appears testing IR with the typical 4 bay chargers isn’t repeatable, and the community seems to report inconsistent testing using an Opus, Lii-500 and the others…
 
My testing consists of using a single iCharger (hobby charger) for consistency along with a battery holder that was made out of quality silicone leads 12 AWG in size and good cell holder.  A good low resistance path of the cells.
 
The other ends were fitted with Anderson disconnects to increase functionality as a general charging and testing accessory.



I carefully cut away the insulation to expose some wire for each one of the cell holders tabs.  I decided to keep it towards the center of the leads in order to give me more position and flexibility for other uses as well.



All connections were soldered and then wrapped in Kapton tape.


After completion I went onto testing some cells...   
I moved the cell in and out of the cell holder rotating position on each test. I had 3 different cells that were tested randomly and here are the results.

Samsung 25R Cell 1
65
65
64
64
65
68
4 MAX DELTA



Samsung 25R Cell 2
64
67
63
65
63
63
4 MAX DELTA


Panasonic MH12210
89
86
86
83
86
6 MAX DELTA




Two Samsung 25R
38
38
37


Big Pack
23
25
25



All in all, I think it came out pretty good.  I get very consistent results testing IR, and as the cells come off the charger/capacity tester (Opus/Lii-500), its very easy to test each cell on the same equipment and get repeatable results for internal resistance.

It also allows me to connect it to other accessories I have like the 20 cell parallel cradle to use for testing and charging / discharging.

For my smaller power pack projects, I am able to match the capacity and internal resistance from series strings better to better my chances to keep things in balance.
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#2
The opus accuracy can be drastically improved by doing a little surgery. Solder a wire independent form the spring and the consistency rating goes way up. Also the capacity tests get more consistent and accurate as well.

Where did you get those cell holders? I like the wire crimp ability of them.
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#3
Cell holders came from https://www.mouser.com , I have had them for a while, ordered them a while ago. They were originally designed for surface mount on a PCB. I had soldered the leads directly to the cell holder.

Good to know on the Opus, but I have sold the one I had and bough (12) Lii-500 units for a good price. I plan to use the above contraption for IR testing moving forward as the results are good, and the added flexibility for different configurations are welcome.

I also plan on using this accessory for testing high drain cells. Its pretty amazing how much current two Samsung 25R cells can actually dish out. You need the beef.
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#4
A tips is to add wires from the other port so you get 4 wires. Then you get alot better accuracy than above even though you used thick wires. Then the iCharger will use those from the balancing port for measuring voltages and the others only for current.

And why did you cross the wires? Big Grin couldnt you have turned the holder around instead? Tongue
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#5
(04-09-2018, 08:10 PM)daromer Wrote: A tips is to add wires from the other port so you get 4 wires. Then you get alot better accuracy than above even though you used thick wires. Then the iCharger will use those from the balancing port for measuring voltages and the others only for current.

And why did you cross the wires? Big Grin  couldnt you have turned the holder around instead? Tongue

Mental programming... positive on top.  Some of my other chargers have the +/- flipped.  Either way, it gave me an excuse to use silicone leads, flexy and friendly.

I can always add the other 2 wires into the balance port... I have everything to do it, I just didn't think that it would work on a 1s configuration, I figured it was for 2s and above.  Ill give that a try this evening.

So far so good, I have been testing cells randomly and the results are excellent.
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#6
On my Duo the balance leads works on 1s and give alot better accuracy on high load. Actually its needed on lets say 10-30A load that i run througfh high current 18650 cells. On lower like 0.5A its not really needed though Smile
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#7
daromer

Thanks for the tip! I can’t believe I didn’t think to try that myself for one cell, but it was pretty obvious after you mentioned it.

I added a 2 pin JST to the cradle for voltage measurement and it works perfectly. The actual cell voltage is 100% in line with my Fluke meter now and termination voltage is spot on when charging 1s.

I am going to re-test all of my IR measurements I did above using the same batteries and see if the results change at all.
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#8
Many people miss the 4wire fact since frankly its not that common that its needed. But if you want it to be spot on thats how you need to do it and if you check any high end voltage metering device with a load function they all have 4 wires.
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#9
Well, its painfully obvious now, but I used to wonder why my batteries with JST balance leads always used to terminate spot on at 4.1v, but my single cells used to have +/- variations near the termination voltage.  It’s because of the difference in internal resistance difference from cell to cell and how the charger interprets that charge voltage with only 2 wires.



I have also re-tested the IR for the 3 cells… all the results came out with a lower mOhms value.

Samsung 25R Cell 1 (53 mOhms)
Samsung 25R Cell 2 (54 mOhms)
Panasonic (64 mOhms)
Two 25R Cells (28 mOhms)

Thanks again for the tip haha.
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#10
Your so much welcome! Im here to share the findings and mistakes i done myself in an earlier life Tongue

And im also here to learn new stuff of course... You dont stop learning untill you die Big Grin
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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