The Electrical Power System (EPS) is the backbone of the space station, responsible for generating, storing, regulating, and distributing power to all onboard subsystems.

This wiki page covers two perspectives:

1. Real ISS EPS Architecture – how the system works on the International Space Station.

2. Simulation in Space Station OS – how these components are modelled in ROS 2 nodes for your EPS demo.

The ISS EPS works as a primary–secondary power distribution network. It ensures that solar energy is efficiently harvested, stored in batteries, and routed to station modules at regulated voltage.

1. Solar Array (PV Module)

   • Main source of power when the ISS is in sunlight.

   • Output depends on sun angle and beta angle (station’s tilt relative to the sun).

2. SSU – Sequential Shunt Unit

   • Maintains array output voltage between 138–173 V.

   • Shunts excess current to keep the bus regulated.

3. DCSU – DC Switching Unit

   • Acts as a switching hub for solar array power.

   • Works with the BCDU to charge or discharge batteries.

4. BCDU – Battery Charge/Discharge Unit

   • Charges batteries during sunlight.

   • Supplies power from batteries during eclipse (when no sunlight).

   • Protects against overcurrent, overvoltage, and undervoltage.

5. MBSU – Main Bus Switching Unit

   • Combines inputs from multiple solar channels.

   • Selects healthy buses and routes them downstream.

   • Outputs 126–173 V primary power bus.

6. DDCU – DC-to-DC Converter Unit

   • Converts high-voltage primary bus to 124.5 V ±1.5 V regulated secondary bus.

   • Ensures stable delivery to station loads.

7. PDU / PDB – Power Distribution Units & Boxes

   • Deliver power from DDCUs to specific loads (computers, pumps, ECLSS systems, payload racks, etc.).

• In Sunlight:
  Solar arrays generate power → bus regulated by SSU → DCSU/BCDU direct power + charge batteries → MBSU selects channels → DDCU regulates to 124.5 V → PDUs distribute power.

• In Eclipse:
  Solar arrays inactive → BCDU discharges batteries → MBSU routes battery power → DDCU regulates → PDUs distribute.

Key voltages:

• 138–173 V from solar array via SSU.

• 126–173 V after MBSU routing.

• 124.5 V ±1.5 V regulated secondary power.

In Space Station OS, the EPS is abstracted into ROS 2 nodes. Each node represents one subsystem in the real architecture.

• File: sarj_mock.cpp

• Role: Simulates Solar Alpha Rotary Joint tracking of the sun.

• Outputs:

  • Publishes /solar/voltage (solar array voltage estimate).

Equivalent to: PV Module + SSU/DCSU behavior.

• File: battery_health.cpp

• Role: Models each Battery ORU (Orbital Replacement Unit).

• Interfaces:

  • Publishes /battery/battery_bms_<ID>/health (sensor_msgs/BatteryState).

  • Responds to /charge and /discharge services.

• Features:

  • Simulates voltage rise/drop with charge/discharge.

  • Enforces safety rules: prevents <70 V (undervoltage) or >120 V (overcharge).

Equivalent to: Battery ORUs with BMS protections.

• File: bcdu_device.cpp

• Role: Coordinates all battery operations.

• Interfaces:

  • ROS 2 Action Server on /bcdu/operation.

  • Subscribes to /solar/voltage and all battery health topics.

  • Publishes /bcdu/status and /eps/diagnostics.

• Features:

  • Parallelizes /charge and /discharge service calls to all healthy ORUs.

  • Fault isolation if current >127 A or voltage unsafe.

  • Automatic safe mode entry on fault detection.

Equivalent to: BCDU + DCSU logic.

• File: mbsu_distributor.cpp

• Role: Selects and routes channel power.

• Interfaces:

  • Subscribes to /mbsu/channel_<N>/voltage.

  • Publishes /ddcu/input_voltage.

• Features:

  • selectHealthyChannels() chooses the top 2 channels with voltage >120 V.

  • Averages their voltage and publishes to /ddcu/input_voltage.

  • Warns via diagnostics if <2 healthy channels.

Equivalent to: MBSU bus combiner and selector.

• Future Node

• Converts /ddcu/input_voltage (126–173 V) → regulated 124.5 V.

• Would eventually supply loads like ECLSS, Thermal, Comms.

Equivalent to: DDCU voltage regulator.

• All EPS nodes publish to /eps/diagnostics (diagnostic_msgs/DiagnosticStatus).

• BCDU: Overcurrent or out-of-range voltage.

• Battery Manager: Overcharge or undervoltage.

• MBSU: Insufficient healthy channels.

Diagnostics can be viewed in GUI dashboards or logged for offline analysis.

To bring up the EPS simulation:

ros2 run demo_eps sarj_mock
ros2 run demo_eps battery_health
ros2 run demo_eps bcdu_device
ros2 run demo_eps mbsu_distributor