charging - Ultracapacitor initial voltage drop during discharging

I've been building a test stand for ultracapacitors, with very simple schematic looking like this:

^{simulate this circuit – Schematic created using CircuitLab}

The resistor on the bottom is to enable the source to push the load through the cap while it's discharged as it's internal resistance is too low for short-circuit-protected source to work. Programmable load is also further connected to a PC that lets me read the current and voltage values during discharge. The ultracap is Maxwell's BCAP3000 2,7V.

I discharged the ultracap with stable 10 Amps and what I've noticed is that there is a voltage drop immediately when discharge starts. My question is what is the cause of that voltage drop? Voltage drops from 2,7 to about 2,45 V. The characteristic of discharge looks like this:

Also, should I be concerned about the linearity of the characteristic? I think it is linear because I used the CC method rather than CV where it would drop exponentially.

Answer

Using only the ESR from the datasheet (ideal wires...):

Current is practically zero before discharging and jumps to 10A, hence the drop. Initial linear discharge is expected due to constant current.

The larger drop you see is certainly due to wires, switches etc.

To be more specific, let's consider 3 steps:

• charging: capacitor practically reached its max. voltage, current through ESR, from the voltage source, is mainly due to leakage (max 5.2 mA in the datasheet)


• both switches opened: current through ESR is zero but capacitor is self discharging due to leakage (could be modeled by a 520 Ohm in parallel with the cap)



• discharging: current through ESR follows the load current