You can buy 4x 20mA CC chips , if that's what you mean but diodes aren't unless you use current limiting resistors. e.g. to a 2V LED and then it's approximate from 3.8 to 3.0 as the current changes from the voltage drop across the R.
How you measure capacity depends on your test method. If it differs from the OEM manufacturer's datasheet, you better have a good reason for it.
- If you want to validate their quality, use their method.
- If you want to improve lifespan or compromise lifespan and use the capacity faster, understand the parameters which affect capacity and also lifecycles.
- raising CV from 4.1 to 4.2 to 4.28 improves capacity but the time spent during CV also degrades lifecycles and depends on the cutoff current of 10% CC to 5%CC to 1% CC
0.2C which is about 76mA.
This means 20% of the mA in the mAh rating and ONLY "Constant Current" and not constant R.
It also means 1/20% or about a 5 hr test if the result matches the rating but only for the CV charge and cutoff V in the mAh rating.
Note that using CV to 4.28CV and 1% CC maximizes capacity but if OEM still specifies 500 cycle lifetime and that is acceptable to you and you intead to use discharge it in the same rate as this regular charge time, then you ought to expect a long life of 500 cycles/
But if you intend to use it up in much 30 minutes, don't expect 500 cycle lifespan. Alternatively, if you would wish to have a lifecycle of 5x or 2500 full charge cycles and learn that reducing the CV maximum and raising the under V cutoff to say 3.5V instead of 2.85 or 3V gives allows you to use only 50% of its rated capacity, but then you can get 5000 charge cycles or 5x the total lifetime Ah storage.
What you are really doing is conducting a Design Validation Test or DVT to YOUR specifications with tradeoffs for best short-term capacity or best capacity for longer lifespan.
As professional Test Engineers we might do both to verify the lifespan improvement and OEM specs.
See an example OEM datasheet below.
Next you need a circuit to program a CC, CV which I assume you have chosen from available charger.
Then choose a design spec for your test.
using your specs for: CV, CC charge, %CC cutoff, CC discharge, UV cutoff & Time counter method.
How to design this depends on your parts available, your specs and your skills (copy or DIY design.)