Typical Internal Resistance of 18650 Pack

hias9

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When calculating internal resistance of my 14S10P Sanyo GA pack based on voltage drop it is around 90-95mOhm.
I read that the internal resistance of one GA cell should be <38mOhms, so I expected the internal resistance to be 14/10*38mOhm + 5mOhm (wire resistance) = <58mOhm.
Are the 35mOhms caused by the connections inside the pack? Or does it have a higher internal resistance on the first cycles?
Weather was cold, but the bike+battery was inside before.
 
what are you referong to... DC or AC IR ?
how are you measuring it ?
IR changes with temperature and SOC significantly (+_ 50%)...
what voltage are you measuring at ?
but you only need one poor connection in the series links to screw up the theoretical figure.
 
My controller logs battery current and battery voltage. I am simply calculating it resistance = voltage sag divided by current.
The 38mOhm was probably AC resistance. But shouldnt DC resistance be lower?
Battery temperature was about 15 degrees Celsius. There was no significant difference between 90% and 50% SoC (I only discharged from 90 to 50 on that ride). The result is always around 90mOhm.
 
There's no such thing as a typical internal resistance for an 18650 pack because the number of cells in parallel, the thickness of the bus wiring, and the ratings of the cells have a huge variance. 18650 continuous discharge rates vary from 0.5C to 20C depending on the cell.

The GA cells perform best at 1C or less, per the cell's data sheet. What C rate are you running your pack at, peak?
 
Of course there is no typical voltage for all 18650 packs. I am just asking if there are typical values for certain packs (like 13s5p mj1 or 14s5p GA or what ever) to see if it is normal that the internal resistance is 50% higher than the calculated value (without resistance of connections inside the pack).

This pack has only had 2 cycles so far and maximum discharge is <0.4C (low power, max range).
 
For the overall IR to be so different from the calculated value, there's really only 4 places for error:
1. Cell resistance: Your estimation for cell IR is wrong. Where did you get your data ?
2. Wiring resistance: Your estimation for interconnects is wrong (or you have a connection problem). How did you estimate this?
3. Other resistance: Other things not considered, including resistance through BMS MOSFETs. Do you have a BMS in the circuit ?
4. Measurement error: How do you know if your equipment is accurately measuring the parameters ? Have you verified with another instrument?

You can't control sag in the cells directly. They are what they are. But you can reduce sag in wiring. Thicker and shorter, with larger and less connectors.

You're asking for help but haven't shared any details on the construction. Why don't you upload some photos and details of the materials used.
 
Depending on how the pack is built, the resistance of nickel strips can be pretty significant. If you can see inside it, you could measure the dimensions of the nickel strips and calculate the resistance. 12 inches of 10ga copper wire is about 1mOhm.
 
I did not build the pack myself, so unfortunately there are no fotos.
I was just asking if anybody can post the internal resistance of his pack. If I assume an internal resistance of 38mOhm (lygte-info.dk writes <38mOhm), so for a 14S10P pack combined cell resistance would be 58mOhm. The resistance of the wiring outside the battery to the controller can be easily calculated and would be 4-5mOhms in this case. So what is missing are the resistance of all the connections inside the battery + resistance of the BMS. If it is realistic that this would cause the additional 30mOhm or more, everything is fine with the pack.
Measurements of voltage and current are done by the ASI controller. I assume the error (maybe +- a few percent) is not significant in this case.
 
I have a 16s, 12p pack that measures 38mOhm on a Cycle Analyst. I'd expect your pack resistance to be sort of in that ballpark but mine has very heavy nickel sheet interconnects.

The first few cycles on a pack will have higher resistance. It "breaks in" after a few good cycles. Colder temps definitely increase the resistance too, so your pack may not be terribly bad. Different meters will give different numbers too. I don't trust those numbers too much, but you can see how much voltage sag you get under load, which is the bottom line.
 
hias9 said:
Thanks for the info! What cells do you use in your 16s12p pack?
They are pretty tame Panasonic CGR18650DA cells. And pretty old at that. I do see quite a bit of sag under heavy load (180A peak) but I don't notice much heating.

Here's a build pic:
Img_1098A.jpg
 
The <38mOhm value for one cell I used is for 1kHz AC, for DC it's more like 50mOhm, and MOSFET resistance of the BMS could also be about 10mOhm and it was a bit cold so I guess everything is fine.

Are you not using a BMS for this battery and do you also measure it (voltage sag) divided by (battery amps) ?
 
I'm using a basic BMS and I have a Cycle Analyst that does all the measurements. I'm not exactly sure how it calculates the number, but I pretty much trust it. I know it works by measuring the voltage sag. AC measurements are pretty meaningless.
 
fechter said:
I have a 16s, 12p pack that measures 38mOhm on a Cycle Analyst. I'd expect your pack resistance to be sort of in that ballpark but mine has very heavy nickel sheet interconnects.
Actually, isn't 38mOhm IR on your 12p pack on the high side? Just imagine how much higher the IR would be on just one of the cells (or even across 3 parallel cells) of your aging 16s12p pack.
 
eMark said:
Actually, isn't 38mOhm IR on your 12p pack on the high side? Just imagine how much higher the IR would be on just one of the cells (or even across 3 parallel cells) of your aging 16s12p pack.

The cells I'm using aren't really great. They do sag quite a bit under full load. I haven't done the math on the resistance of the nickel strips, BMS and wiring but it all adds up.
 
fechter said:
I have a 16s, 12p pack that measures 38mOhm on a Cycle Analyst.
eMark said:
Actually, isn't 38mOhm IR on your 12p pack on the high side? Just imagine how much higher the IR would be on just one of the cells (or even across 3 parallel cells) of your aging 16s12p pack.
Was under the DCIR impression that the more cells in parallel the lower the overall DCIR (e.g. 12p=38mΩ versus 6p=76mΩ), but actually the opposite may be true or does the DCIR remain the same when using a Cycle Analyst no matter how many cells are in a parallel group (e.g. 16s12p vs 16s6p) ?

There are other variables, but you get the general idea ... that being does overall DCIR increase or decrease (using Cycle Analyst) depending on the number of cells in the parallel group (e.g. 12p group vs 6p group. For example if one were going to reconfigure a 16s12p pack to a 16s6p pack (new cells) would the DCIR be the same, higher or lower when reconfigured from a 16s12p pack to a 16s6p pack (all other variables aside like temp, SoC, etc) ?
 
fechter said:
I have a 16s, 12p pack that measures 38mOhm on a Cycle Analyst.
fechter said:
They are pretty tame Panasonic CGR18650DA cells. And pretty old at that. I do see quite a bit of sag under heavy load (180A peak) but I don't notice much heating.
The CGRDA data sheet lists the IR of this cell as either 83mΩ or 145mΩ depending on industry test criteria … https://www.nsnam.org/wiki/Li-Ion_model_fitting … assuming type of test is an industry norm, whereas the Cycle Analyst test is apparently based on some ebiking IR norm.

If my 16s12p figuring is correct (with newer cells) that would mean an IR somewhere between 111mΩ (83 ÷ 12 = 6.9 x 16 = 111mΩ) or 193mΩ (145 ÷ 12 = 12 x 16 = 193mΩ) going by the above data sheet IR. Apparently the Cycle Analyst has become somewhat of a more practical ebiking standard as a general IR reference point … even with variables.

Because there are so many variables for measuring IR we assume 38mOhm is high, not knowing how much lower the Cycle Analyst IR reading would have been on a newer CGRDA 16s12p pack as a beginning IR reference point.
____________
NOTE: edited previous post on 1/3/21 to hopefully clarify.
 
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