Top: spacecraft (ground) potential evolution in five SPIS simulations. The reference simulation with no biased surfaces in a warm T_ew = 10 eV plasma (blue line), with small charged surfaces of +75 V in a PIC simulation (black) or a fluid Maxwell-boltzmann simulation (yellow dot-dashed line). Also plotted, a simulation of the same plasma density but with 50 percent T_ec = 0.1 eV electrons with either no biased surfaces (red), or with biased surfaces using the fluid approximation (purple dot-dash line). The Maxwell-Boltzmann simulations with surfaces at +75 V are strictly not valid but serve to illustrate the first approximation from Orbital-Motion-Limited theory. Bottom: zoom-in of above, with the calculated mean (dashed black line) and a 1σ range (dotted black lines) of spacecraft potential from the PIC simulation in this interval.