# Power Management > --- name: power-management description: 'Design robot power distribution. Use when sizing batteries, planning voltage rails, monitoring power rails, or charging systems.' --- # Robot Power Management and Distribution ## Power Architecture Overview A mobile robot's power system distributes energy from the battery to multiple subsystems at different voltages: ``` Battery (e.g., 4S LiPo 14.8V nominal) │ ├── Main Fuse (20-30A) │ ├──→ Buck Converter → 12V rail (motor drivers, high-power actuators) │ ├──→ Buck Converter → 5V rail (SBC: Raspberry Pi, Jetson, NUC) │ ├──→ Buck Converter → 3.3V rail (MCU, low-power sensors) │ └──→ Direct → Motor Controller input (accepts battery voltage directly) ``` ## Voltage Rails and Regulation ### Buck Converters Step-down (buck) converters are >90% efficient. Use these, not linear regulators (LM7805 wastes energy as heat). | Rail | Typical Source | Consumers | Notes | |------|---------------|-----------|-------| | Battery (14.8V) | Direct | Motor controller, high-power relay | No regulation needed | | 12V | LM2596 / MP1584 buck | Cooling fans, some actuators | 3A typical | | 5V | D24V50F5 (Pololu) / MP1584 | SBC, servos, USB devices | 5A+ for SBC with peripherals | | 3.3V | AMS1117 LDO from 5V | Teensy, I2C sensors | 1A sufficient, LDO is fine here | ### Critical: SBC Power Requirements - **Raspberry Pi 5**: 5V, 5A recommended (27W USB-C PD). Under-voltage causes throttling and SD card corruption. - **Jetson Nano**: 5V, 4A (through barrel jack or header pins). - **Intel NUC**: 12-19V barrel jack, 3-5A depending on model. Use a dedicated, high-quality buck converter for the SBC. Do NOT share its 5V rail with servos or motors—inrush current from servos causes voltage dips that reset the SBC. ## Power Monitoring with INA219/INA226 Place current/voltage monitors on each power rail for real-time monitoring. ### INA219 Wiring ``` Battery+ ─── [Shunt Resistor] ─── Load+ │ │ VIN+ VIN- (INA219 sense inputs) Battery+ ──── VBUS ─── GND (INA219 bus voltage input) ``` ### Multi-Rail Monitoring Code ```cpp #include Adafruit_INA219 ina_motor(0x40); // A0=GND, A1=GND Adafruit_INA219 ina_compute(0x41); // A0=VCC, A1=GND Adafruit_INA219 ina_sensor(0x44); // A0=GND, A1=VCC struct PowerRail { const char *name; Adafruit_INA219 *sensor; float voltage; float current_mA; float power_mW; }; PowerRail rails[] = { {"motor", &ina_motor, 0, 0, 0}, {"compute", &ina_compute, 0, 0, 0}, {"sensor", &ina_sensor, 0, 0, 0}, }; void readAllPower() { for (auto &rail : rails) { rail.voltage = rail.sensor->getBusVoltage_V(); rail.current_mA = rail.sensor->getCurrent_mA(); rail.power_mW = rail.sensor->getPower_mW(); } } ``` ## Power Budget Calculation List every component and its current draw at each operating state: | Component | Idle Current | Active Current | Peak Current | Voltage | |-----------|-------------|----------------|-------------|---------| | Raspberry Pi 5 | 0.6A | 1.5A | 5.0A | 5V | | Teensy 4.1 | 0.1A | 0.15A | 0.2A | 3.3V (USB powered) | | Motor driver (RoboClaw) | 0.05A | 2.0A | 15A | 14.8V | | LIDAR (LD19) | 0.35A | 0.35A | 0.5A | 5V | | IMU (ICM-20948) | 0.003A | 0.003A | 0.003A | 3.3V | | Servo × 2 | 0.01A | 0.5A | 1.5A | 5V | | **Total from battery** | **~1.5A** | **~5A** | **~20A** | **14.8V** | Total average power: ~5A × 14.8V ≈ 74W ## Battery Sizing $$ \text{Battery capacity (Wh)} = \text{Runtime (hours)} \times \text{Average power (W)} $$ For 2 hours of runtime at 74W average: 148 Wh needed. A 4S 10,000 mAh LiPo: $14.8\text{V} \times 10\text{Ah} = 148\text{Wh}$. Marginally sufficient — in practice, plan for 70% usable capacity (never discharge below 20% SoC for battery health): $$ \text{Effective capacity} = 148 \times 0.70 = 103.6\text{Wh} → \text{Runtime} \approx 1.4\text{hours} $$ ## LiPo Safety - **Minimum cell voltage**: 3.0V per cell (12.0V for 4S). Below this causes permanent damage, swelling, and potential thermal runaway. - **Storage voltage**: 3.8V per cell for long-term storage (not fully charged, not depleted). - **Charging**: Always use a balance charger. Charge at 1C maximum (10A for a 10Ah pack). Never charge unattended. - **Physical protection**: LiPo bags or fire-resistant enclosure on the robot. Inspect for swelling or damage after impacts. - **Wiring**: Use properly rated connectors (XT60 for up to 60A, XT90 for higher). Solder all connections—crimps and screw terminals add resistance and fire risk at high currents. ## Power Sequencing Incorrect startup order can cause issues: 1. **MCU first**: Powers up, initializes I/O pins to safe states (motors OFF, servos neutral) 2. **Sensors next**: LIDAR, IMU, cameras initialize and begin publishing 3. **SBC last**: Boots, starts ROS 2, begins sending commands If the SBC powers up before the MCU, the MCU's I/O pins may float, causing unpredictable motor behavior. Design the firmware so that **powers-up in safe state by default** — all outputs LOW/disabled until explicitly enabled by a command. ## Emergency Power Cutoff A physical kill switch must be accessible without tools: - **Location**: Rear of robot, top surface, marked with red - **Type**: Latching push-button (push to cut, twist to release) or key switch - **Circuit**: In series with battery positive line, before all electronics - **Rating**: Must handle peak battery current (e.g., 30A rated for a 20A peak system) For advanced setups, use a relay/MOSFET that the MCU can also trigger for software-initiated shutdown (e.g., critical battery level). ## Publishing Power Data as ROS 2 Diagnostics ```python from diagnostic_msgs.msg import DiagnosticArray, DiagnosticStatus, KeyValue class PowerDiagnostics(Node): def __init__(self): super().__init__('power_diagnostics') self.diag_pub = self.create_publisher(DiagnosticArray, '/diagnostics', 10) self.timer = self.create_timer(1.0, self.publish_diagnostics) def publish_diagnostics(self): msg = DiagnosticArray() msg.header.stamp = self.get_clock().now().to_msg() for rail_name, voltage, current in self.get_power_data(): status = DiagnosticStatus() status.name = f'Power: {rail_name}' status.hardware_id = f'ina219_{rail_name}' if voltage < self.min_voltages[rail_name]: status.level = DiagnosticStatus.ERROR status.message = f'{rail_name} voltage LOW: {voltage:.2f}V' else: status.level = DiagnosticStatus.OK status.message = 'Normal' status.values = [ KeyValue(key='voltage_V', value=f'{voltage:.2f}'), KeyValue(key='current_mA', value=f'{current:.1f}'), KeyValue(key='power_W', value=f'{voltage * current / 1000:.1f}'), ] msg.status.append(status) self.diag_pub.publish(msg) ``` View diagnostics with: ```bash ros2 run rqt_runtime_monitor rqt_runtime_monitor # or ros2 topic echo /diagnostics ``` ## Thermal Considerations - Buck converters generate heat proportional to current. Mount with heatsinks and airflow. - Motor drivers at sustained high current need heatsinking. RoboClaw 2x30A can thermal-throttle without airflow. - SBC (especially Jetson) needs active cooling under load. Monitor CPU temperature and throttle navigation if overheating. - In enclosed robot bodies, consider a small exhaust fan controlled by the MCU based on internal temperature readings.