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Chargers/Dischargers: What kinds should we use on Lithium cells?
#1
This is a very common question. The simple quick answer is: Well, that depends

Before getting into what types of chargers to use, let's discuss a little be about what the process involves.

We get the cells, clean them up a bit, do a quick test of voltage, then slap them in a charger to get them to 4.2V. Then we slap them into a discharger (this is either same as the charger, or a separate unit) to find out their capacity. Write down the mAh rating, and toss them into a bin depending on their rating.

This is all fine and dandy. However, there is one thing we should really ask here. Can we use different brands of dischargers to test our cells?

Well, you can, but should you? Depending on what kind of initial testing you are looking for will depend on the answer.

If you are testing the cells to see if they are good or duds, then no, it doesn't matter. If a cell is <1000mAh, it will test that regardless of which tester you use. If a cell gets hot during charging or discharging, it will do so regardless of which unit you use [There is a small variance here; if 1 unit chargers/discharges at 500mA and another at 1000mA, that can make a big difference in results. The 500mA unit will probably let through the heaters whereas the 1000mA units will show them].

This issue comes in when you are gathering numbers to build your packs. And then, this is still dependent on what your goal/method is.
Let's say you test the cells, and you group them by 200mAh differences (2000 - 2199mAh, 2200 - 2399mAh, etc for example). Then you take all cells that are in 1 group and make a batch of packs to make a full string of similar sized cells. This is fine. The whole string will be fairly well balanced.
However, if you group your cells by 100mAh variance, and you use all cells to make packs, this can make a huge difference in the end. Especially if parallel batches are 60, 80, 120p or more.

Opus units are notorious of being about 10% high in their capacity readings. This is due to the way they test, and can be influenced easily at times. So using an Opus and a LiitoKala could give you wide variances in cell results. Let's say you test a cell in the Opus, it comes back as 2200mAh. You toss that same cell into the LiitoKala and it comes back 2000mAh. A 200mAh doesn't seem that much, but what happens if you toss 60 cells together that have been tested by these two units? You could have 3000mAh difference from one pack to the next. 

So accuracy is key to building balanced packs. So, what kind of charger or discharger should you use?
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#2
What kind of charger or discharger should you use?

Keeping your apples to apples and oranges to oranges, we have several different types of chargers/dischargers that are common.

Charger(C.), Discharger (D)
TP4056 [eBay] (C.)
TP5100 [eBay] (C.) (This one is fairly new on the block)
Opus [eBay]  (C/D)
LiitoKala [eBay]  (C/D)
iMax [eBay]  (C/D)
BMax [eBay]  (C/D)
ZB2L3 [eBay]  (D)


[Note: Links are affiliate]
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#3
Generally what most users do is use the TP4056 or TP5100 to do bulk charging. Most of the time they charge right up to 4.2V (The 4056 units will need to be verified to make sure they are consistent. It is possible to have 1 charge to 4.18, and the other 4.23V. The datasheet says +/-1% tolerance, which is 4.158 - 4.242V)
Using these boards you also need to get cell holders if you didn't get a large pcb version that has them already made.

Then after weeding out any heaters, or those that take a super long time to charge (if a 2000mAh rated cell takes 4hrs to charge @ 1A, then it's probably toast), you can then move on to capacity testing. Using any of the Dischargers mentioned above can do this job. The key is to stick with one for a particular string. So all the cells that are going to make 1 Battery (7s60p, 14s 120p, etc), use 1 brand of discharger. Results are more of a guideline than a hard set fact. But, all the results will be biased in the same direction and by almost the same amount.

The iMax has the ability to also do capacity testing on the whole string, as well. It has balance leads that allow for each pack of parallels to be connected to the unit and it monitors all the packs in the string during the discharge test.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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