I want to discharge a battery at a constant C rate (0.2C) the battery is 3.7V and 380mAh. I did some calculations and found out that the current the circuit will need to take is 76mA. I am planning to build this with LED's. I am not sure but from what i know LED's take a constant current, i am not sure if this is 20ma or 15ma or something else. Also I am not sure if I should connect them in series or in parallel. Can someone help me come up with a circuit I could make that takes up a constant current of 76mA. Any suggestions of LED's i could buy off amazon do to this would also be helpfull
The most you should discharge a Li-ion cell to is about 2.8 to 3.0V to prevent damage to the cell.
Here is a very simple circuit that maintains a fairly constant discharge rate from 2.8 to 4.2V (and beyond).
R2 biases D1 (actually an IC shunt reference that regulates at 1.24V) and provides a small amount of base current for Q1. Q1 provides the remainder of the base current Q2 requires.
R1 is chosen so that 1.24V/R1 ~= 76mA. A bit of current that is not metered goes through R2 so the current is not precisely constant.
Here is a quick LTspice simulation:
The circuit will typically work well below 2.8V-3.0V so you need to provide some means of cutting off the current before the cell is damaged.
Just for comparison, as I already had it set up, here is @Michal's circuit simulated:
It's possible that he got different results depending on the models used.
In real life, it would be much nicer to make a circuit that operates from an independent regulated power supply such as 5.0V which would eliminate both the low voltage limit and most of the slope in the current-voltage line. It would also be easy to incorporate a low voltage cutoff.