A spacecraft's Power Processing/Distribution Unit (PPU/PDU) regulates a single DC bus that a solar array charges and every subsystem draws from through its own current-limiting breaker:
Battery open-circuit voltage: Voc = Vmin + (Vnom-Vmin)·SOC/100
Discharging (Iload > Igen): Vbus = Voc − Rint·(Iload − Igen)
Charging (Igen ≥ Iload): Vbus = min(Vnom, Voc + Rint·(Igen − Iload))
SOC update: dSOC/dt = (Igen − Iload) / Capacity_Ah · 100
Each load is fused: if its instantaneous current exceeds the breaker's trip threshold Itrip, the breaker opens and permanently disconnects that load (Ohm's-law short-circuit fault: injecting a fault multiplies a load's normal draw by ~9×, which is what pushes it over the threshold). The bus regulator's droop resistance Rint means the more current is pulled beyond what the array supplies, the further the bus sags — exactly the electrical behaviour that under-voltage load-shedding logic watches for.
- Eclipse slider — fraction of the orbit spent in Earth's shadow; solar current falls toward zero, forcing the battery to cover the load.
- Breaker trip threshold — the fuse/current-limiter setpoint per load; set it low and even a healthy load can nuisance-trip, set it high and a real short survives too long.
- Inject short circuit — simulates an insulation or component failure on the selected subsystem; watch whether the breaker isolates it before the bus collapses.
- Auto load-shedding — when bus voltage drops below the under-voltage threshold, the PDU sheds non-critical loads first (Payload, then Propulsion), then essential ones (Comms, Thermal), and restores them in reverse order once the bus recovers — ADCS (attitude control) is never shed.
- Sim speed — scales how fast the schematic and strip-chart advance, so slow eclipse cycles or fast fault transients are both easy to watch.
This is the real fault-protection philosophy flown on satellites and the ISS: current-limiting breakers isolate a fault electrically before it can collapse the whole bus, and priority load-shedding trades non-critical function for survival of attitude control and comms. This 2D build renders the same electrical model as a schematic — a hub-and-spoke power diagram, battery/solar gauges and a live bus-voltage strip chart — instead of a 3D spacecraft mockup, so every number on screen maps directly onto the equations above.