[RIPPERTON]

All my new Enerland 4350 LiPO's have their Internal resistance written on a spec sheet. Most of them range from 0.009 to 0.012.
should I group them in series or parallel.
Im guessing in series ?

[EVfun]

?? I'm a bit confused. You will need to put cells in series to reach the voltage you need. In series the voltage adds up but the amp hours remain the same as each single cell. In parallel the voltage stays the same but the amp hours add up.

[major]

Originally Posted by RIPPERTON

All my new Enerland 4350 LiPO's have their Internal resistance written on a spec sheet. Most of them range from 0.009 to 0.012.
should I group them in series or parallel.
Im guessing in series ?

Hi RIP,

I assume you are talking about how to sort and group cells by the spec sheet internal resistance. My opinion would be to keep the parallel groups as close as possible in the cells' resistance. That way they will current share more equally.

Series connected cells (or groups of paralleled cells) would not be sensitive to differences in resistance since the current will be equal by virtue of the series connection. So they should charge and discharge equally.

If you intend to use the battery hard, you might try to keep cells having higher resistance on the outer edge of the pack as they would heat more.

Even though the resistance values look small, there is actually a plus/minus 14% spread.

Did you purchase individual cells or multicell packs?

Regards,

major

[Tesseract]

I think, EVFun, he is asking how the cells should be grouped according to internal resistance.

Ideally, you want the equivalent resistance of each "cell" in series to be identical. So you parallel cells with various internal resistance figures to result in the same overall resistance as every other bank of cells that are paralleled together to make up a series string (ie - a battery).

The spread of 9 milliohms to 12 milliohms is pretty wide - but very typical of these Chinese cells, regardless of chemistry or manufacturer. Parallel cells so that each group has the same mean internal resistance as all of the cells considered together. Ie - if you have 10 cells at 0.009 ohms, 20 at 0.01 ohms and 10 at 0.012 ohms then the average (mean) resistance of all 40 cells is 0.01025 ohms. Aim to make each paralleled cell group have that resistance and you'll have the best possible balance w/r/t voltage drop under load for each "cell".

An additional consideration is to group by capacity, but that's sorta making things even more complicated. Might want to use a computer to sort that mess out. Not even gonna try it here.

[major]

Originally Posted by Tesseract

Ideally, you want the equivalent resistance of each "cell" in series to be identical. So you parallel cells with various internal resistance figures to result in the same overall resistance as every other bank of cells that are paralleled together to make up a series string (ie - a battery).

Hi Tess,

You know I hate to disagree with you And Woodsmith says we should not resort to insults, namecalling and threats. So let's look at a parallel set of cells. Take 4 for example. Use a simple model for the cell of an ideal capacitor and series resistance. Draw the circuit of the 4 cells connected. Use the same C value for each, but different resistance values.

When you put a load on the parallel group, each capacitor will see a different current depending on the individual resistor. The voltage on each capacitor will decrease as charge is removed. Once the load is removed from the parallel group, the voltage on the 4 capacitors are different. So, current will continue to flow within the group of 4 parallel cells until all 4 reach equilibrium.

This is called charge redistribution. It is undesirable because it is current flow within the system which does no useful work for you. Worst case; it can cause excessive heating. Lithium cells have high Columbic efficiency, so there will be little loss of capacity, just the energy lost in resistive heating.

And I guess I don't see a good reason why series connected cells need to be R matched. There you should match capacity.

Regards,

maj

[Tesseract]

Originally Posted by major

You know I hate to disagree with you And Woodsmith says we should not resort to insults, namecalling and threats.

Who are trying to kid, you crusty curmudgeon! You love to disagree with me!?

Originally Posted by major

This is called charge redistribution. It is undesirable because it is current flow within the system which does no useful work for you...

Yep, I do agree, but I saw this as the lesser of two evils at really high currents (relative the the cell's capacity) because if a paralleled group consisted of all high internal resistance cells it might trip LVC prematurely. I was keeping in mind what RIPPERTON here would likely be using the cells for - namely, in racing motorcycles. Of course, you have some practical experience in this matter whereas mine is entirely fabricated, so...

[RIPPERTON]

pack will be 32s 12p. 52ah 128v 6.6 kWh
4800 peak Amps which is irrelevant because that will never be used
controller can draw 500A cont which is 41Amps per cell
the pack will be ventilated with 1mm air gap between each cell
be used over 20 minute stint to complete 30km at race pace (90kmh av ?)
counting on burning 200Wh/km
theres going to be some very aggressive regen going on too

Just did a cell count out of the 450 cells I bought (need 384 for the pack)
57 cells are 0.009 ohms
234 cells are 0.010 (52%)
71 cells are 0.011
14 cells are 0.012
makes 376. theres a few 0.008's that I will throw in

there were a few freaky ones with 0.004 and up as high as 0.034 which
were sold to a friend for destruction testing.

I also thought to place matched cells in series to minimise disparity during charging and discharging. theoretically all cells in series would charge equally but balance out in parallel

[RIPPERTON]

corrections
missed 1 sheet
264 cells are 0.010 = 59%
63 are 0.009
77 are 0.011
making 404 usable cells
I can chuck the 0.012's and make my entire pack out of 9's 10's and 11's

[tomofreno]

If the cells are used for racing, so high discharge currents, I wonder how relevant the Rs measurements are, since the effective resistance at higher currents will be very different due to concentration polarization, as described by the Warberg factor, according to the CM video. In that case it seems what would matter is how much the W factors differ between cells, which you would have to determine yourself by discharging them at high rate. The Warberg factor appears to be the "effective" series resistance at high discharge rate, and is much larger than Rs, resulting in much more cell heating, and larger drop in cell voltage.

[BMI/LiFeTech]

Daniel, I am surprised if your Enerland cells have an IR as high as 9-12 milliohms. For racing you should get considerably higher discharge power from your LiFeTech X1P cells since they all should be under 3 milliohms. After you ordered your LiFeTech X1P Power cells the factory started labelling the IR of the cells on the the top left corner of each cell tray as part of the QC cell grading process. This makes it much easier to match cells for packs used for racing applications. I would be suurprised if any of your cells have an IR of higher than 3 milliohms since all the the LiFeTech X1P cells are all typically in the range of 2.5-3.0 milliohms.