The curve is a straight line because the load is constant current, not just a resistor.
Oh yeah, that's right. I didn't notice that. THe load is constant current and not a resistor. So the reasoning of the ESR being combined with the load resistance does not apply.
But still, since it is constant current the voltage drop across the ESR should be constant too which would just offset the entire discharge curve, not make a two segment curve. If this was an *actual* measurement rather than a theoretically drawn graph, I would attribute it to some startup current pulse required by the constant current load.
Hmm... I think I understand your explanation of the ESR causing a voltage drop. Its more of a practical thing, but should not happen theoretically, correct?
What exactly does "constant current load" mean? And how does it cause straight-line discharge?
Do you know what a capacitor-resistor discharge curve looks like? As the capacitor discharges, the capacitor voltage drops. Since this voltage drops, the discharge current across the resistor also drops. THe result is that the discharge rate of the capacitor continuously decreases as the capacitor gets and the discharge voltage and current of the capacitor reaches an assymptotic zero.
But in your graph the voltage decrease linearily. That means that as the voltage is decreasing, the "so-called resistive load" that is discharging the capacitor is increasing (the current load is increasing which means that the resistance is decreasing) to maintain the voltage drop at a constant rate and stop it from slowing down.
In reality, other devices and physical phenomena are used to produce current sources that behave the same way rather than combining a variable resistor with a self-adjustment mechanism controlled by a current sensor.
In short, a constant current load is simply a load that adjusts the voltage across it to maintain the same amount of current flowing through it. A current sources adjusts the voltage across it to do the same thing except it is supplying current. A constant voltage load would be something like a diode that "for the most part" keeps the same voltage across it regardless of the current flowing through it.