This is about how to use a normal CV-CC power supply as a controlled load for discharging batteries with a constant current, and thus measure a battery's capacity C (mAh).
A normal power supply can control the current through its load, and the top voltage, too. However, a normal power supply can only source current, it can not sink current and it does not support negative voltage applied to its output.
The goal is to connect a battery in such a way that we can keep current flowing out of the power source, and voltage on the power source to stay positive, while the current through the battery under test acts as a discharging current. Else said, the battery needs to be connected in reverse, with the positive terminal to GND. Then, we will need a drop voltage bigger than the battery's open voltage, so to create a positive voltage for the power supply's positive terminal.
That can be done with a series resistor and a big enough current set from the source, so the U
Rs = Rs * Is >= Vbatt, or with a Zener diode, or an active circuit to drop a voltage >= Vbatt, etc.
Some batteries can deliver huge currents, so special measures
must be taken to protect the output of the power supply. Here, diode D1 is connected in reverse at the output of the power supply, so to avoid negative voltages. Diode D2 protects in case V outside is bigger than V generated by the power supply.
The set current in the power supply will be the discharging current. The set voltage of the power supply will limit the min discharge voltage for the battery. For example, if the battery has to be discharged with 1A until it reaches 10V, then the power supply must be set for 1A and 5.5V (-10V on battery, 15V on Rs@1A and 0.5V on D2).
Note that the actual load is the resistor R, not the power supply. The power supply only helps with keeping a constant discharging current, and with monitoring the current and the voltage (also with logging, in case of a DP832).