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Capacity test many 26650 LiFePo4 cells?
I need to test a bunch of cells, right now about 500 but i will be getting 1000 more. How can i test these cells as quick as possible ? Are there any solution that i can use ? 

The only solution i have found is to buy few OPUS chargers and solder some 26650 holders to them so i can have 4 cells tested at the same time but are there other solutions ? I am open for DIY if there are some modules i can use. I know about TP500 that can handle 3.6V but what about capacity testing, are there any ready module or something i can build using ESP8266 or Arduino ?

test 1 cell at a time at around the max amps you expect to pull from the cells. There is no "quick" way to test capacity. It takes a certain amount of time pull a certain amount of power from a battery/cell.

You "could" parallel a batch of cells together and pull a huge amount of current. But the problem with this is if there is a weak or low capacity cell, it won't show up in this test. Think of having a tug of war match. You have 10 Body builders pulling and 1 skrawny guy as well, you won't know the skrawny guy is there if you have them all pull the rope at the same time. But you'll quickly find the weak link if you have them pull the rope 1 at a time.

Needing to test that particular size does have its draw backs as most chargers are for 18650 size. What you could do is build your own holder. There are several 3D print models on thingiverse (if you have 3d printer) or something out of wood perhaps using those as the guide.
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There is the ZB2L3 if you want to go the DIY route. It is also has a 3a maximum load.

Another place to look is Google USB load tester. According to some YouTube reviews some of them are quite good.
I used 2x opus and 8x 26650 holders to test about 200 of the BAE 26650. I test them at 1 amp so it took me several days to do them all. I thought it went quicker then the 18650 li-ion I have tested.  

I didn't solder any wires to the opus, I used a spacer that was shaped like a 18650 and connected wire to those leading to the 26650 holders and placed into the opus. It worked good enough.  I tried to solder some wires to the opus but my solder iron wasn't hot enough to solder it.

You have to open up the opus to move the micro switch to lifepo4.

If I had to test more 26650, thats how I would do it, it worked good for me.
The trouble with using opus to test lifep04 cells is the discharge still cuts off at 2.80 so you don't get an accurate capacity.
But it still gives some indication of cell health as a comparison.

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