Internal Resistance: What is acceptable?

[Storx]

I did search for this, but most of the threads that came up just talked about it and didnt really give numbers, just said the higher the internal resistance, the lower it will drop under load..

I have been trying to learn more about the imax b6 mini i have and decided to testa few of the cells i discharged tested and recharged back to 4.2v, i noticed after testing about 15 cells, they all were between 120-138miliohms, is this acceptable? seems like that is the normal for the cells i have atleast, i did test them without the magnets, just not as easy since the tester will error out often due to trying to hold 2 leads together securely as well trying to press the run button down... its with the magnets it seems to average about 80miliohms higher on the scores, but the true reading using the leads the tester came with was 120-138miliohms range for the 15 i randomly picked.

 

[Ykick]

What type of battery? Series strings or single cells?

I normally use a plain DC method which simply measures resting voltage, apply known Amp load for 10s, record voltage and do Ohm's law calculation x1000 for mOhm values.

RC Lipo cells I don't care for anything above 20mOhm for motive use.

 

[izeman]

My whole 12s4p lipo pack is 24mOhm. So the single cell resistance is irrelevant. It's the whole pack that counts.

 

[Storx]

they are 18650 cells

 

[Punx0r]

True, but testing the cells gives a good idea what the IR of a finished pack will be without having to construct it first...

120mohms per cell is pretty horrendous. Recycled laptop cells?

 

[Ykick]

Single cell IR is very relevant to me when assembling and later on troubleshooting.

 

[izeman]

@ykick of course it's mandatory to check individual cells of unknown source before pack assembly.
What i tried to say was that you can make a low ir pack with a lot of cells, so the single cell's ir itself is irrelevant - if all cells are the same of course. A single bad cell can kill a whole pack.

 

[Syonyk]

I have been trying to learn more about the imax b6 mini i have and decided to testa few of the cells i discharged tested and recharged back to 4.2v, i noticed after testing about 15 cells, they all were between 120-138miliohms, is this acceptable?

What cells are you testing? That's "junk" territory for anything modern and high power.

The 1khz IR for a Sony US18650V3 is 31mOhm (spec sheet), and the "ZB206+ Tester" resistance for one at storage voltage (~3.6v) is 40mOhm. A Samsung ICR18650-32A on the same tester turns in about 55mOhm.

So for any sort of power cells, yeah, those are terrible numbers. They might be within spec for 1C laptop cells, but you probably don't want to run a traction battery on those.

 

[Storx]

Well i am not sure if its the tester or not.. i tested another 47 cells tonight and the lowest one was 118miliohms

 

[Syonyk]

You steadfastly refuse to answer people's questions about which cells you're testing, and I'm not going to go searching for any other threads you've posted for clues.

If you're testing new, high amperage 18650s, those numbers are suspect.

If you're testing a selection of junked laptop cells, then your tester is probably correctly telling you they'll sag like none other under load.

 

[izeman]

Looks like salvaged laptop cells from most likely different vendors. Syonyk i'm with you. It's hard to help ppl if they don't answer the questions asked. But that's mostly because they think they already told all relevant stuff, so no bad intention.

 

[Hillhater]

At best the test you are doing is giving you ACIR figures, which are not very useful for battery analysis.
Better to search how to test for DCIR.