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Internal resistance of cells
#1
Hello
Im at early stages of building a pack, how does internal resistance of each cell effect the makup of a pack ?
Obviously going to use a calc to balance all my cells equally, but unsure about internal resistance of each cell
Thanks
Battery is li iron using 18650s, 14s, 0p, and growing cell so far 300, untesteded.
Bms is a diy open source by stuartpittaway.
No solar yet, 
Building wind turbine, 48v im after a 48v motor if anybody has one for sale.
Cells 2000 to 2500ma.
Cells 1500 to 2000ma.
Cells 1000 to 1500ma.
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#2
It depends. Do you plan huge current per cell or low?

If you have run your test with a a current equal or above your planned load later on you also know that the resistance currently is low enough to suit that load. This will work great for loads up to 1A and you dont have to sort them.

As long as you randomize the packs they will work fine and will equalize over time. Some cells will have slightly higher current draw than other but this is not a problem.

High current packs like e-bikes you SHOULD check IR and make sure they are more equal.

There are threads where this is tested and you can see the behaviour.

Am I guessing? No Im not. I have around 29 000 Cells in my powerwall and followed a set rules when doing it. Currently after 4 years I have still not swapped out any cells due to issues of above.

For those interested the Ir is very much related to the tested current and the time frame you run the test and this also shows it.

if you plan and do want to know the IR you need a 4 wire tester. The testers on for instance Opus is useless.
You should have same amount of high and low capacity cells in each pack. You should also mix the resistance cells.

For instance if you sortr all your cells in 100mAh buckets and then randomize them this will be 100% true on larger packs like 80p or above based on statistics. Statistics can for sure be used doing low current powerwalls like most of us do. Smile

Wolf and the others will soon chime in on the importance of Ir and what they have done.
chuckp likes this post
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#3
Have a look at the excellent work that’s been done by Wolf.
He has done a lot of testing and documented his findings with regards to internal resistance and harvesting of cells.

I will certainly be adopting his methods on my next batch of cells.

(08-25-2019, 04:22 PM)daromer Wrote: Wolf and the others will soon chime in on the importance of Ir and what they have done.

Hi Daromer

You gotta love The Wolf.

I’d like to pick your brains at some stage about an ATS if that possible.
UK Southwest.

7 kWp Solar Panels (28 x 250Wp Shinetime Mono).
14 X APS YC500i Micro Inverters.
28 X 40P 18650 cell packs/modules configured as 14S 80P.
Sofar Mass Energy ME3000SP AC coupled charger/inverter.
Still sourcing and processing cells for powerwall.
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#4
Feel free to do so.

Yeah Wolf do all the testing and showing all the parts that I have during my time tried to explain but failed to get time to show properly Smile

Cheers for him and the others!
chuckp likes this post
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#5
(08-25-2019, 05:54 PM)daromer Wrote: Feel free to do so.

Thanks buddy will do.

I have no problem setting it up as a normal ATS. Just not sure of the easiest way to do it on my system.
I have an idea but it would mean using 3 contractors. I’ll start a new thread on it.
UK Southwest.

7 kWp Solar Panels (28 x 250Wp Shinetime Mono).
14 X APS YC500i Micro Inverters.
28 X 40P 18650 cell packs/modules configured as 14S 80P.
Sofar Mass Energy ME3000SP AC coupled charger/inverter.
Still sourcing and processing cells for powerwall.
Reply
#6
(08-25-2019, 11:17 AM)Jonisingt35r Wrote: Hello
Im at early stages of building a pack, how does internal resistance of each cell effect the makup of a pack ?
Obviously going to use a calc to balance all my cells equally, but unsure about internal resistance of each cell
Thanks

Jonisingt35r,

Internal resistance is the blood pressure of a battery. Just like with us if our BP is high it indicates that we have a health problem.
Same goes with 18650 cells (or any battery for that matter of fact) if the IR is high it does indicate a SOH (State of Health) issue.
One tool to put into your pack building toolkit should be a YR1030+ or YR1035+ IR Tester. Unless you can afford a Keithley DMM7510,
it is a 4 wire Kelvin 1kHz AC IR testers specifically designed to test the very low mΩ resistance of batteries and quite accurate. 
I have written about the importance of checking IR as the first touch of each cell harvested for quite some time now and some have found this advice and method very useful and enlightening. (It also saves a ton of time as you are not testing cells that wont give good results.)
I have built a chart that gives the optimal and max IR of each cell for it to be considered a good subject for testing. If you stick close to the optimal mΩ reading you will have good results. Notice there is no voltage requirement on any of these measurements. I have tested cells as low as 0.5 volt and found acceptable IR. I recharge anything below 2V at 50mA per cell till ~4V and then test them on Various charger/testers. If you want a good tester I highly recommend the Foxnovo or if you can afford it the SkyRC MC3000
You can find this chart here 
As you can see there is some variation as to the IR of individual cells, some are low and some are higher that others. This is due to the doping of the chemistries by manufactures and using slightly different materials etc.
In the final analysis any cell over 80 mΩ you would not want in a battery pack.
See my thread on High and low drain cells mixed were I am exploring the performance of cells in parallel with different IR values.
Another good thread is Does internal resistance matter? Lots of good info there.
My sheet with the testing results of 6198 (and growing) recorded cells is available here with all kinds of goodies attached.

If you have just started this journey I would highly recommend you download this sheet here. It will give you a good start.
Another sheet you can use is from Nemo and you can get it here.
These are all great tools to use.
Questions just ask.
Wolf

chuckp likes this post
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
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#7
Thanks got a foxnovo charger, think i will id every cell from now on and record everything like capacity and internal resistance.
Battery is li iron using 18650s, 14s, 0p, and growing cell so far 300, untesteded.
Bms is a diy open source by stuartpittaway.
No solar yet, 
Building wind turbine, 48v im after a 48v motor if anybody has one for sale.
Cells 2000 to 2500ma.
Cells 1500 to 2000ma.
Cells 1000 to 1500ma.
Reply
#8
foxnovo wont meassure ir good enough.... look at the links Wolf added. they are good
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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