How do I design a constant current load circuit?
Designing a constant current load circuit to discharge a battery?


The basic math behind a CC load is well presented here in a TI application note.

 

In this application a DAC is used to set a precise current value, but of course this could equally be a simple pot across VCC to Ground. The integrator ramps up or down the gate drive to achieve the desired current through Q1 by adjusting the Ohmic value of RDS.

It's difficult to offer much help beyond that above since you don't specify any parameters you need to meet. (hopefully your EE education will eventually make you aware of things like specifications and requirements documents!).   

This you need to think about as you scale the above circuit to your needs:


Current range and power to be dissipated by this CC load. For example, discharging a 10AH 12V battery vs a 500AH 12V battery in say an hour could produce power dissipation levels in the 100W to 6kW range.
How do you control starting and stopping (turning off and on) this CC load. Especially at the end of the discharge period, is this to be a purely manual process (jumper cables) or do you have some automation that will run for a fixed time or to a defined voltage endpoint?
Are there to be defined voltage endpoints?
Is the CC load powered by the battery it is discharging or power separately
Does this CC load understand a range of batteries (terminal voltage and chemistry). For example is this for discharging a single LiIon cell or a battery pack made up of many cells?


And lastly....


Do you really want CC or would Constant Power be the best solution. For example, the current to discharge a 12V battery pack compared to a 50-170V battery pack. The battery packs may all be the same cell type and size ….but it would be much better to use constant power in most cases where your load device is limited in dissipative capability. 


Now that you have updated your question to include some specifications some advice can be given. 


With a load current of 20A and a terminal voltage range of 2.5-4.2V you can make decisions about where to dissipate the energy. 


In this case you could elect to dissipate as much energy as possible in a simple resistive load. For example you could use a 0.1 Ohm resistor and regulate the voltage on it to 2V. This would dissipate about 40W. While a power resistor is expensive, it is MUCH cheaper than extruded heatsink for an active device. 


You could then choose an active device to dissipate the rest of your maximum required load (4.2V @20A). The maximum dissipation is about (4.2 *20) - 40 = 44W. This is well within the range of a single FET devices with an appropriate heatsink. I showed this device below.
You might also consider that you may want a series diode in the circuit so that you have some polarity protection.


So a partial schematic might look like this:







simulate this circuit &ndash; Schematic created using CircuitLab