Datalogging a battery tester

I am new in this forum. I am trying to make a li ion battery capacity tester with charger. like this "HTRC B6 V2 80W 6A Digital RC Battery Balance Charger Discharger for LiPo Battery" but only for one cell i need at least 5 of them for testing 18650 cells i want to use multiple resistors and mosfets per discharger for keeping the load at a constant current from 4.2 to 2.9 V therefore i could use the elapsed time for calculating the capacity. The test does not have to be super accurate but close because I will only compare my own readings among themselves.
My other idea would has been using a dc-dc step up down regulator to get a constant voltage across a single resistor and ignore the power taken by the dc dc step up down converter. maybe adjust it to 3.5 v at the resistor so it would not have to boost the voltage 2 times if the battery is getting low.

Lastly i want to use an arduino to control everything and log the data ( V min, V max, mAh, Temp max( during charging and discharging each)) and to cycle each cell about 5 times to get an average.
I need to test a bunch of cells about 100 to 200.
as you see i am at the beginning of my project so if you see a fatal design flaw or know a better system please feel free to make suggestions

If you want a fixed constant-current sink, you could try this circuit:


I had to check in LTSpice, it looks like it works fine but needs a 12v supply to bias the transistors.

The example shows it sinking 0.5A from 4.2v down to below 1v.

The MOSFET would dissipate around 2W, so it needs a heatsink.


No need to use a constant current load.

You can also measure the capacity by measuring the current (i.e. the voltage drop across the load resistor), multiply by the time increment and sum up the result to get Ampere hours.