Batteries matched? How close is "close?"

[sailingharry]

I have just received a dozen EVE MB31 from 18650. Well packed, look good, fingers crossed. I also have a FNIRSI HRM-10 resistance tester. I've tested all cells, and now don't know what to think of the data. Looking for advice! The meter on resistance would dance around, I recorded high and low values (around 0.02mh variation) and tabulated the average of each cell.

My voltages ranged from a low of 3.2594 to a high of 3.2599. I know that voltage is a very poor indicator of SOC, but these numbers seem to imply 50% SOC, and also seem to imply they are all very close to that.

My resistances range from a low of .125mOhms to .215. I realize that's almost a factor of 2, but also realize they are all VERY small numbers and might well be errors in measurement.

So, any comments on these numbers? Good, bad, no comment? The resistance especially -- I'm concerned about the differences. I'll be doing 3P4S -- do I group them in the 3P area (lowest 3 in one group, next 3 in the next group, etc) so each cell in a 3-pack is the same, or do I group them so that the average of each 3-pack is the same (each set of 3 gets one from the high, middle, and low resistances)?

Thanks!

 

[timselectric]

The cells seem great.
But I would not

be doing 3P4S

The additional fuses and connections are just asking for trouble.
BMS's are designed to monitor and balance individual cells, not groups of cells.
You should be making 3 X 4s batteries , with a BMS for each.

 

[GXMnow]

I agree, make 3 separate 4S batteries with a BMS on each. That way every cell is being monitored. And if one battery goes into shut down, you still have the two others working for you.

Each string of 4 cells should be the closest match to each other. Then you will have the 4 best cells together, the 4 worst cells together, and the 4 middle cells all together.

Measuring cell internal resistance is a bit tricky. The leads that come with the tester are actually 4 wires, 2 going to each clip. It should have the current source on one side of the clip, and then it measures the voltage on the other side of the clip. This way, it should only measure the cells resistance and ignore any resistance in the test leads. But for it to get a proper reading, both sides of the clip need to make a solid connection to the battery terminal, while not touching each other. You may need to clean the battery terminals carefully to get a solid reading.

 

[timselectric]

That's why I have never purchased one of those testers. I don't think that I would trust the readings that I would get.
I just charge them, balance them, and capacity test them.
And move on to the next thing on my list.
As long as you buy from a reputable seller, that's all that you need to do.
(In my opinion)

 

[robbob2112]

YR-1035 meter... $35 to $50 on aliexpress or $70 off amazon. There are versions with probes that have 2 pins each, or alligator clip where each jaw is a lead. It uses a 1khz square wave to meassure impedance and blocks dc so it can meassure the IR of a battery or cell