Testing 18650 Internal resistance using resistor and multimeter

Raylinguo

1 mW
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Nov 13, 2022
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Hi there, I'm wondering if anyone can help me verify I'm doing the right method and using the appropriate resistor to test my 18650s liion internal resistance.

I have been following a method shown by 'Tom Donnelly' on YouTube, where you first measure the cell voltage, then put the cell across the resistor and re measure voltage. Then divide that voltage by the ohm rating of the resistor to get the current. Then subtract the resisted voltage from the open voltage to get the voltage drop. And finally divide the voltage drop by the current to get the resistance. (sorry that's long winded..)

What I'm unsure about is if I'm using the appropriate resistor, it's a 1.5 ohm and 5 watt resistor. Does watts change anything in this scenario? Also the resistor states it's 1.5 ohm, but my multimeter reads 2 ohms. So I put the positive and negative of the multimeter together and it reads .5 ohm. Does that mean I should be using 2ohm in the equation instead of 1.5?

The reason I'm asking is the results I got for 10 cells I tested were all relatively high, about 130 to 220 milliohms. So im hoping I've done something wrong and don't have 160 useless cells..

The other parameters of the cells all seem pretty good, most are around 3000mah capacity, and have been sitting for a few months according to the person I bought them from, and they all read around 4.07 to 4.1 volts before any other testing was done, so don't seem to self discharge. Can a cell have good capacity, and hold its charge well but still have high internal resistance?

Internal resistance should be below 65mOhm ish right? Would anything above that render them useless for an 1500w ebike, even if the capacity and voltage seem good?

One final point is the resistor I used got warm to the touch within 5 seconds or so, is this also normal?

Do you think I am using the appropriate resistor, might I need a new multimeter, or am I doing the equation incorrectly?

Sorry for the ignorance and appreciate any answers!

Cheers
 
Hi there, the best way to test cell IR is to use a resistor load , could be a 4ohm/5W resistor for about 1A discharge current to future calculations. If U don't have a 4ohm resistor try a speaker (P>5W) impedance should be 4 or 8ohms. measure R with multimeter to use at calculations (a 8ohm speaker could have 10ohm dc R).

make sure use different contact points for multimeter and load connections to avoid cable resistance.
the ideia is to have two contacts at positive and negative terminal of the cell. one contact for voltmeter and other to connect load. independent contacts for precise measure.

After this check initial volts of cell without load and write it. than apply load for 5seconds reference and measure voltage drop after this time.
Now you need ohms law formula. example V initial= 4,00V , Vfinal=3,95V means 50mV drop with a 4ohm load
Than IR=(Vinit-Vfinal)/4 = 0.05V/4 =12,5miliOhm Done
good new cells could go from 10mOhm to +30mOhm, example Sony VTC5A 10mOhm, LGMG1 27mOhm, Samsung 35E 23mOhm
 
The DC method is unreliable in my opinion. It probably works better with lower current, but I think you want to know the effective resistance at operating currents. Then it depends on what current you draw and how long you pull it, as the cell voltage is changing during the test. If checking at 1 amp, the resolution of most multimeters at +/-01V on the 20V scale is pretty sloppy..

I bought a YR1035+ AC impedance meter, about $40-45 ordered from China. Came in 10 days last year.s. The meter also is a high resolution voltmeter, useful for balance voltages. I would say the AC resistance comes in about a third of what I would compute using DC resistance.. Most new cells spec AS impedance on their data sheet.

New cells are 12-24 milliohms. I wouldn't go much above 35 milliohms on a used cell. If the battery is a large P, you can get away with a higher IR, but it all relates to higher voltage sag for an ebike application. I think anything above 50 milliohms really would be a poor performer. You would have to make a battery much larger than it needs to be to combat voltage sag,
 
Thanks for the replies, makes a bit more sense now. I'm still struggling with the testing using the resistor. I have tested multiple different cells, I have also linked both 2 and then 3 of the resistors in series to try measure a different ohms (although to be honest Im not 100% sure what this does apart from multiply the ohms) and i have also now used a 6ohm speaker. Im using thick wires and multimeter at the battery terminals, and with each different ohm resistance used to test I get a large discrepancy of internal resistance values even using the same cell. Sometimes 150mohms apart.

But always getting at least 120mohm + so Im thinking that is probably a sign they are not going to be suitable. Unless I'm doing the math wrong, but I've made sure its the right formula, or the resistors I'm using aren't appropriate. Or as mentioned perhaps it's just not a very accurate method.

I would get an internal resistance tester/meter but the project has been dragging on for months now, and being in New Zealand I may have to wait another month. So impatience might rule that out, and if i buy it and the cells are no good then it's another $50 on top of having to buy new cells also.

Might start looking at new cells.. Cheers!
 
howzit Ray, being a fellow kiwi (auckland, don't judge) I feel your pain with wait times. i thought 2020-2022 taught me about patience but where the hell did 2023 go? anyway merry xmas g
 
rackalhahn said:
howzit Ray, being a fellow kiwi (auckland, don't judge) I feel your pain with wait times. i thought 2020-2022 taught me about patience but where the hell did 2023 go? anyway merry xmas g

Yeah it can be a bit painful when your limited to just what's on trademe ay! 2023 has flown by for sure, let's hope 2024 slows down a bit. Merry Xmas to you too hope it's a goodie 🤙
 
Raylinguo said:
rackalhahn said:
i thought 2020-2022 taught me about patience but where the hell did 2023 go? anyway merry xmas g

Yeah it can be a bit painful when your limited to just what's on trademe ay! 2023 has flown by for sure, let's hope 2024 slows down a bit. Merry Xmas to you too hope it's a goodie 🤙
I think y'all might want to reset your clocks and calendars; 2022 isn't even over yet, and 2023 has not yet started (almost a month until that happens). ;)
 
Haha we're a year ahead down in NZ here 😂 oops went right over my head, all just blurs together at the moment! Came here to learn about internal resistance, get reminded what year it is - 2022 right, thank you 😂
 
You must have a real IR meeter with Kelvin probes, resistance is not the same thing and IR cant be measured with a common multimeter:
Does Internal Resistance have anything to do with Battery Capacity and how to test it? https://www.youtube.com/watch?v=-NKGx8u2tWY

Read the comments as well.
 
The Battery Doctor said:
You must have a re

you r still doing it wrong
 
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