Most cost effective, timely method of matching 350 M1 cells.

So I'm geting a bunch of A123 cells all having similar cycle life but very low use (<10 cycles). I have two hyperion 1420i. What is the easiest method to match all the cells into groups so I can create 28 11p modules with similar capacities?

I figure I can charge all the cells together, then discharge them in pairs with each hyperion... taking 15 minutes plus setup for each pair. At that rate, though, we're talking upwards of 42 hours.

Maybe it would be worth making some sort of arduino kit that would record capacity of 8 cells being discharged at once. How hard would it be to calibrate, though? Could I do this with pwm to achieve the same discharge current with 5 or 10% tolerance resistors?

What about charging them all up, then hooking maybe a dozen up simultaneously to an LED which will turn off at say 2.5V. Then I could simply time them near the last 30 critical seconds.

You can just charge them and match them after their resting voltage. Is supposed to be good enough according to several posts on ES.

If I'm going to do it right, I should probably charge them up to 3.65, wait one day?

I will have to linearize the graph of the results in order to make best combination to achieve even capacity. Does this sound right, guys?

Bear in mind I'm not combining cells of the same capacity..

Here is what I would do:

1) charge all cells to 3.65 until current drops to C/20. You can do this cell by cell or in a big parallel group if you have good temporary connetions. Let cells rest for 24 hours. Measure voltage again. Good cells will still be 3.45 to 3.5 while
marginal cells will have dropped to 3.3v. Set aside any cells that fail this test.
2) pick random cells and put into groups of 11 cells in parallel (temporary but good connections).
3) discharge each 11 cell group to cutoff voltage (discharge rate not critical) and record capacity.
4) Take least capacity group apart and charge/discharge each cell to find least capacity cells
5) Take most capacity group apart and charge/discharge each cell to find most capacity cells
6) swap around cells from most/least groups to even out their group capacity
7) repeat 4,5,6 until the variance between most and least capacity of all the 11 cell groups is small enough.
8) Build up pack with cell groups and you are done.

Not a bad idea. I will try to mill some flat blocks to use as good connections to bulk charge 40 or so cells at a time. I'll give it a shot when they come in. Hopefully 12 extra cells will be enough to fill the voids.

Most likely the groups are going to be within 50mAh of each other.. given the large pool of cells.

Milling a pair of flat blocks will result in the tallest cells connecting.

Scotchbrite one side of an aluminum strip for one side, flexible metal braid and some rubber bands for the other side.

K. I'll try that. good idea... question is how many have you done before at once?

hillzofvalp said:

K. I'll try that. good idea... question is how many have you done before at once?

Click to expand...

However many 26650's fit against a 4ft piece of aluminum strip, I don't remember.

The can itself is both aluminum and the positive terminal of an A123 26650 M1 cell. I recomend having the negative button side press against the scotchbrite'd aluminum strip face. You could add a second strip of wood with holes drilled in it to mount against the side and form a little zip-tie pocket to stabilize them if you like, or a pair of zip tie pockets to avoid them twisting. Make the zip ties kinda loose and you can re-use them as little sockets to drop cells into.

Dealing with a zillion round cells to make a large pack becomes something labor intensive pretty much no matter how you do it.

I think you mentioned using All-cell material somewhere, keep in mind that stuff conducts electricity (which stopped me from using it for a project), and once it gets to it's melting point, it holds that melting point temp (like 55degC) for the whole time it's cooling off until all the wax that energy went into melting re-crystallizes. No free lunches in the cooling world, the energy it stores it still has to release back in the form of holding the cells at high temps for a long period. (which is better than having thermal runaway of course, but with an A123 M1 round cell you're not going to enter thermal runaway unless you radically short something or radically overcharge something anways).

Yeah.. They told me that they would recommend 37C melting point for these cells... But this is 55C is all they have for discounted student options. I doubt I will ever get to that temperature at 3C continuous and 8-10C bursts. I run 8C on one of my a123 packs and it hardly gets warm. This pack will be 18mOhm

Luke, the nicest thing to me about the material is the rather uniform temperature achieved for the pack. Regarding, conductivity, you do have to be a little cautious (put positive of a123 in first). I am still exploring if I will separate into 28 blocks. Dfinitely leaning towards no. I am using 50mm stuff so that leaves 7.5mm of the cells sticking out each side. Also, we did put series connections in the same blocks with 1200 18650s for our go kart packs... And haven't had any issues yet.

As long as you keep aluminum in contact with the blocks, I think most long duration over temperature scenarios are rare. I would guess that this method is better than spacing out the cells for air cooling

edit: I am understanding that all cell even recommends double shrinking the cells if they are in a block which has series connections. Is that really necessary? I know some cylindricals have nearly exposed edges.. but m1s have like 1mm overlap. Do you have an opinion as to whether the original factory green shrink would be adequate?