When charging and discharging the capacitors it is best practice to never discharge them below 0.0 volts. Typically a safety factor would be used here, for example only discharging them down to no less than 0.1 volts.
When discharging a capacitor with a constant current discharge the change in voltage will be linear.
I = C dv/dt
I = Current
C = Rated Capacitance
dv = change in voltage
dt = change in time
Below are two graphs showing a typical charge discharge cycles (down to 0.01 Volts) with the potential (V) and Current (A) graphed.