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Cell Placement - When building a Battery Pack... Thoeries?

Zoodles

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Apr 1, 2022
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Building a 2P4S battery with 280AH cells and after capacity checking individual cells I have two that are moderately different in capacity and top/bottom points...

It got me thinking if there is any theory on where would be the best position in the battery pack for cells of certain characteristics...???
 
Could it be because your battery supplier didn't match the battery before shipping it to you? It is normal for the same batch of batteries from the same factory to have slightly different capacities. Therefore, a reliable battery supplier should adapt the battery capacity for you before sending it to you. Regarding the placement issue you mentioned... In fact, just make sure not to stack it horizontally. It is not recommended. Taking this way of placing the battery for a long time may have a certain impact on the battery performance and even affect the battery cycle life.4.jpg
 
@Zoodles what did you decide? I am in same situation buiding two packs of 2p4s with 16 x 50AH cells. They are B grade but I capacity tested the one with the highest IR of 0.76 and it still gave me 50.64AH ? I have the IR for them all and have grouped them into two batches of 8.

batch 1 is 0.57 to 0.76 and batch 2 is 0.45 to 0.48. More worried about batch 1. After top balance and add BMS I will keep them between an equivalent 3.0v and 3.4v per cell so I don't plan on ever taking them below 3v. I will be happy with whatever capacity is between 3.4v to 3.0v.

Big question is cell matching and placement within the 2p4s packs with only IR as the value to use. Wondering if I should mix high and low IR in the parallel cells or just sort them in order and connect like that.
 
My cells arrived well-matched (at 60% level). There was only one cell with a .005 V difference, top balance showed two cells about .05-.07 V higher, the rest were fairly close, so I decided to just place wherever.
Have installed and so far so good.
 
How did you connect the doubles up cells? Are all + and - connections doubled up?
 
How did you connect the doubles up cells? Are all + and - connections doubled up?
Yes they will be four x 2p then series but had a dumb moment and realised I should use top balanced IR to sort them. Doing things in wrong order. Did the planned top balanced yesterday but stopped at 3.5v last night before I ran out of time to watch them. I let them settle overnight and today will take them to 3.6v. Then will let them settle for an hour then take their IR.

I have a YR1035+ so confident in the IR.

Also have ET5410 so after I build them in 2p4s config with their JK BMS I will take them from top balance down to cell cut off 2.5v (10.0v) in a 0.2C test and measure capacity while noting 3.4v and 3.0v capacity during the test.

Once the cells are in their 2p4s config I will operate them 3.0v to 3.4v. They are for a UPS and being B grade I will take it easy on them.

The ET5410 pack capacity test should tell me if my sorting and grouping by IR worked or not. Each paralleled cell voltage should be pretty close after settling after end of the test.

Top balance set up:
20220613_184901.jpg

Pic below shows my method of sorting. This is the 8 worst cells I plan for one pack but need to update for top balanced IR later. mΩ after top balance is only correct for cell #1, that one I top balanced and capacity tested at 0.2C. The rest are from about 70% SOC so TBD for sorting.
20220614_102143.jpg

I could capacity test them all individually but i have 16 total cells for two packs of 2p4s. At 5hrs per cell that's too long for my impatience. Even testing paralled groups of two is 40hrs testing time so 3 days real world since I wont wake up to start new tests at night. If the method described above doesnt work I will go the long route.
 
Nah it’s good to share your experience and set up.

I was mainly concerned with inter cell connections. Here is my current 2p4s set up with 240ah cells.

I did have double bus bars from the “cell bank” 1 to “cell bank” 2 same with “cell bank” 3 to “cell bank” 4 (it formed a box). But I noticed “cells banks” 1 and 4 charged much quicker than 2 and 3, cell bank 1 hit HVD when cells 2 and 3 were still at 3.48ish. The BMS screen shot was after they tested a while.

So I pulled one bus bar connection per bank connection and have the cells diagonally connected, I’ll try drawing the bank down and charging again
 

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I'm doing about the same setup 2p4s 230ah cells. I don't trust the ir readings of my tester. But I recently got an EBC-A40L and I've been capacity testing the cells individually and matched in pairs. My plan is to pair lower capacity cells with higher capacity cells to create as close to matched Pairs as I can. When it comes to placing the matched pairs in series I have the same questions. I tend to think that you will want to put the lower capacity cells in the middle of the 4s configuration due to buss bar and connection resistance.

I'm only testing my cells to the range I'm planning on using them that being 3.5v-3.0v. FWIW I tested a couple cells from 3.6-2.5v and I reach around 94.5% capacity with my testing below. My test procedure on my EBC-A40L is as follows: Charge @40A to 3.5v with a cutoff of 23A (.05 of C), Wait 1 hour, Discharge of 40A to 3.0v. Below is my testing thus far. 1655209611091.png
 
Docan cells I take it? Average capacity of 213 for 230 labeled cells, ouch.

What are the manufacture dates of them?
 
Docan cells I take it? Average capacity of 213 for 230 labeled cells, ouch.

What are the manufacture dates of them
Read everything I wrote. The test was how I was going to use them from 3.5-3.0v NOT 3.65-2.5v my cell tests from 3.6-2.5 were all over 230ah. Yes docan. I also think temp has a large part in cell testing. If I ever do more testing I will most likely set up a temp controlled box (cooler) to have stable temps for each test as it seems that some of the colder days I was testing showed lower capacities.
 
Interesting that 3.0V to 2.5V is ~10% capacity in your case. My experience with the 280's is that 3.0V to 2.5V is under 2% of capacity.

I wonder if Docan has cells with "weak knees" that explains the much lower price point?
 
Interesting that 3.0V to 2.5V is ~10% capacity in your case. My experience with the 280's is that 3.0V to 2.5V is under 2% of capacity.

I wonder if Docan has cells with "weak knees" that explains the much lower price point?
I also didn't charge to 3.65v so the capacity loss is not all from 3.0-2.5v range. I think the temp played more into my numbers than I first thought as I'm testing in an uncontrolled garage and had outdoor temp ranges from 80-20*f for them two weeks.
 
Read everything I wrote. The test was how I was going to use them from 3.5-3.0v NOT 3.65-2.5v my cell tests from 3.6-2.5 were all over 230ah. Yes docan. I also think temp has a large part in cell testing. If I ever do more testing I will most likely set up a temp controlled box (cooler) to have stable temps for each test as it seems that some of the colder days I was testing showed lower capacities.

Thanks for the info, you also stated few tested 94% with full voltage range, maybe that’s the temp impacting it as well.

Did you try 20a discharge rate?
 
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