I will begin by explaining my use case then I will present the beginner problem I have. First let me suffice this with that I am rank beginner and I know there are far better designs than this simple thing I have. So I wish to drain an 18650 battery and I wish to sense the voltage of the battery by using a point just after the 0.1ohm sense resistor I have chosen. Now The battery drain is through a mosfet and no connection from that circuit to the control circuit bar the high impedance input that an op amp provides. I have the shutdown after low voltage part done as a test circuit. we'll see after connection. the part I am having a bit of trouble with is the Mosfet allowable current which can vary between 100mA and 1.5A with the ability to go to 2A. Being the type that likes to learn through struggling I am not looking for a drop in solution but rather some pointers on direction. So for the current control circuit it I think will start with a difference amplifier between the top and the bottom of the current sense resistor. from there at 1.5A there will be a constant 150mv drop across that resistor. Feeding that to the difference amplifier would give me 150mv at the output. That voltage must be boosted to gate voltage likely between 2 and 5 volts or in the linear control part of the Mosfet. So some circuit advice here would help can the difference amplifier be boosted to a more apropriate voltage for the Mosfet or must I use another op amp to get the voltage correct. I know this seems simple to most of you but I'm retired and I have really nothing better to do other than what little brain I have to play with junk I have had in drawers since I was a young teen.

 

Welcome to AAC!

You could use an opamp to make a current sink, using a potentiometer to set the current. You could use a comparator to control a MOSFET to switch the battery out of the circuit when it gets down to 2.5V.

In a pinch, you could use a comparator as a slow opamp so you can use something like LM393 (dual comparator). You want the current sense voltage to be low (so use something like a 0.2 ohm resistor). You'll need the common mode input voltage to be low. LM393 allows 0V on the inputs.

When I was testing Li-ion cell capacity, I ended up buying a 4-channel board that charged and discharged at around 1A. From AliExpress: