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+# Issue #505: 24V Charging Dock Hardware Design
+
+**Agent:** sl-mechanical
+**Status:** In Progress
+**Date Started:** 2026-03-06
+**Related Issues:** #159 (5V dock), #489 (docking node)
+
+---
+
+## Design Overview
+
+Upgraded charging dock system for 24V DC power delivery with improved reliability, higher power capacity, and integrated ArUco marker (ID 42) for precision alignment.
+
+### Key Specifications
+
+| Parameter | Specification | Notes |
+|-----------|---------------|-------|
+| **Voltage** | 24 V DC | Upgrade from 5V (Issue #159) |
+| **Power capacity** | 480 W (20 A @ 24V) | Supports battery charging + auxiliary systems |
+| **Contact type** | Spring-loaded brass pads (Ø12 mm, 2 pads) | 20 mm CL-to-CL spacing |
+| **Alignment method** | V-channel rails + ArUco marker ID 42 | Precision ±15 mm tolerance |
+| **Docking nodes** | Compatible with Issue #489 (ROS2 docking node) | MQTT status reporting |
+| **Frame material** | PETG (3D-printable) | All parts exportable as STL |
+| **Contact height** | 35 mm above dock floor (configurable per robot) | Same as Issue #159 |
+
+---
+
+## Subsystem Design
+
+### A. Power Distribution
+
+#### PSU Selection (24V upgrade)
+
+**Primary:** Mean Well IRM-240-24 or equivalent
+- 240W / 10A @ 24V, open frame
+- Input: 100-240V AC 50/60Hz
+- Output: 24V ±5% regulated
+- Recommended alternatives:
+  - HLK-240M24 (Hi-Link, 240W, compact)
+  - RECOM R-120-24 (half-power option, 120W)
+  - TDK-Lambda DRB-240-24 (industrial grade)
+
+**Specifications:**
+- PCB-mount or chassis-mount (via aluminum bracket)
+- 2× PG7 cable glands for AC input + 24V output
+- Thermal shutdown at 70°C (add heatsink if needed)
+
+#### Power Delivery Cables
+
+| Component | Spec | Notes |
+|-----------|------|-------|
+| PSU to pogo pins | 12 AWG silicone wire (red/black) | 600V rated, max 20A |
+| Cable gland exits | PG7, M20 thread, 5-8 mm cable | IP67 rated |
+| Strain relief | Silicone sleeve, 5 mm ID | 150 mm sections at terminations |
+| Crimp terminals | M3/M4 ring lug, 12 AWG | Solder + crimped (both) |
+
+#### Contact Resistance & Safety
+
+- **Target contact resistance:** <50 mΩ (brass pad to pogo pin)
+- **Transient voltage suppression:** Varistor (MOV) across 24V rail (14-28V clamping)
+- **Inrush current limiting:** NTC thermistor (10Ω @ 25°C) or soft-start relay
+- **Over-current protection:** 25A fuse (slow-blow) on PSU output
+
+---
+
+### B. Mechanical Structure
+
+#### Dock Base Plate
+
+**Material:** PETG (3D-printed)
+**Dimensions:** 300 × 280 × 12 mm (L×W×H)
+**Ballast:** 8× M20 hex nuts (4 pockets, 2 nuts per pocket) = ~690 g stabilization
+
+**Features:**
+- 4× M4 threaded inserts (deck mounting)
+- 4× ballast pockets (underside, 32×32×8 mm each)
+- Wiring channel routing (10×10 mm), PSU mounting rails
+- Cable exit slot with strain relief
+
+#### Back Wall / Pogo Housing
+
+**Material:** PETG
+**Dimensions:** 250 × 85 × 10 mm (W×H×T)
+**Contact face:** 2× pogo pin bores (Ø5.7 mm, 20 mm deep)
+
+**Features:**
+- Pogo pin spring pre-load: 4 mm travel (contact engage at ~3 mm approach)
+- LED status bezel mount (4× 5 mm LED holes)
+- Smooth contact surface (0.4 mm finish to reduce arcing)
+
+#### V-Guide Rails (Left & Right)
+
+**Material:** PETG
+**Function:** Self-aligning funnel for robot receiver plate
+
+**Geometry:**
+- V-channel depth: 15 mm (±7.5 mm from centerline)
+- Channel angle: 60° (Vee angle) for self-centering
+- Guide length: 250 mm (front edge to back wall)
+- 2.5 mm wall thickness (resists impact deformation)
+
+**Design goal:** Robot can approach ±20 mm off-center; V-rails funnel it to ±5 mm at dock contact.
+
+#### ArUco Marker Frame
+
+**Design:** 15 cm × 15 cm frame (150×150 mm outer), marker ID 42
+
+**Frame mounting:**
+- Material: PETG (3D-printed frame + acrylic cover)
+- Marker insertion: Side-slot, captures 100×100 mm laminated ArUco label
+- Position: Dock entrance, 1.5 m height for camera visibility
+- Lighting: Optional white LED ring around frame for contrast
+
+**Marker specs:**
+- Dictionary: `cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_250)`
+- Marker ID: 42 (uint8, fits DICT_4X4_250: 0-249)
+- Printed size: 100×100 mm
+- Media: Glossy photo paper + 80 µm lamination (weather protection)
+
+#### PSU Bracket
+
+**Material:** PETG
+**Attachment:** 4× M4 SHCS to base rear, bolts through PSU flanges
+
+**Features:**
+- Mounting pads for PSU feet
+- Cable routing guides (AC input + 24V output)
+- Thermal airflow clearance (30 mm minimum)
+- Optional DIN-rail adapter (for rackmount variant)
+
+#### LED Status Bezel
+
+**Material:** PETG
+**Function:** 4× LED indicator display (charging state feedback)
+
+**LEDs & Resistors:**
+
+| LED | Color | State | Vf (typ) | Resistor | Notes |
+|-----|-------|-------|----------|----------|-------|
+| L1 | Red | SEARCHING | 2.0 V | 180 Ω | No robot contact |
+| L2 | Yellow | ALIGNED | 2.1 V | 180 Ω | Contact made, BMS pre-charge |
+| L3 | Blue | CHARGING | 3.2 V | 100 Ω | Active charging |
+| L4 | Green | FULL | 2.1 V | 180 Ω | Trickle/float mode |
+
+**Current calculation (for 24V rail):**
+- Red/Yellow/Green: R = (24 − Vf) / 0.020 ≈ 1000 Ω (use 1.0 kΩ 1/4W)
+- Blue: R = (24 − 3.2) / 0.020 = 1040 Ω (use 1.0 kΩ)
+
+**Control:**
+- Jetson Orin NX GPIO output (via I2C LED driver or direct GPIO)
+- Pulldown resistor (10 kΩ) on each GPIO if using direct drive
+- Alternative: TP4056 analog output pins (if in feedback path)
+
+---
+
+### C. Robot Receiver (Mating Interface)
+
+**Cross-variant compliance:** Same receiver design works for SaltyLab, SaltyRover, SaltyTank with different mounting interfaces.
+
+#### Contact Pads
+
+- **Material:** Bare brass (10-12 mm OD, 2 mm thick)
+- **Pressing:** 0.1 mm interference fit into PETG housing
+- **Polarity marking:** "+" slot on right side (+X), "-" unmarked on left
+- **Solder lug:** M3 ring lug on rear face (connects to robot BMS)
+
+#### V-Nose Guide
+
+- **Profile:** Chamfered 14° V-nose (30 mm wide)
+- **Function:** Mates with dock V-rails for alignment funnel
+
+#### Mounting Variants
+
+| Robot | Mount Type | Fastener | Height Adjustment |
+|-------|-----------|----------|------------------|
+| SaltyLab | Stem collar (split, 2×) | M4 × 16 SHCS (2×) | Tune via firmware offset |
+| SaltyRover | Deck flange (bolt-on) | M4 × 16 SHCS (4×) | 20 mm shim if needed |
+| SaltyTank | Skid plate (bolt-on) | M4 × 16 SHCS (4×) | 55 mm ramp shim recommended |
+
+---
+
+## 3D-Printable Parts (STL Exports)
+
+All parts print in PETG, 0.2 mm layer height, 40-60% infill:
+
+| Part | File | Qty | Infill | Est. Mass | Notes |
+|------|------|-----|--------|----------|-------|
+| Dock base | `charging_dock_505.scad` (base_stl) | 1 | 60% | ~420 g | Print on large bed (300×280 mm) |
+| Back wall + pogo | `charging_dock_505.scad` (back_wall_stl) | 1 | 40% | ~140 g | Smooth face finish required |
+| V-rail left | `charging_dock_505.scad` (guide_rail_stl) | 1 | 50% | ~65 g | Mirror for right side in slicer |
+| V-rail right | *(mirror of left)* | 1 | 50% | ~65 g | — |
+| ArUco frame | `charging_dock_505.scad` (aruco_frame_stl) | 1 | 30% | ~35 g | Slot accepts 100×100 mm marker |
+| PSU bracket | `charging_dock_505.scad` (psu_bracket_stl) | 1 | 40% | ~45 g | — |
+| LED bezel | `charging_dock_505.scad` (led_bezel_stl) | 1 | 40% | ~15 g | — |
+| **Receiver (Lab)** | `charging_dock_receiver_505.scad` (lab_stl) | 1 | 60% | ~32 g | Stem collar variant |
+| **Receiver (Rover)** | `charging_dock_receiver_505.scad` (rover_stl) | 1 | 60% | ~36 g | Deck flange variant |
+| **Receiver (Tank)** | `charging_dock_receiver_505.scad` (tank_stl) | 1 | 60% | ~42 g | Extended nose variant |
+
+---
+
+## Bill of Materials (BOM)
+
+### Electrical Components
+
+#### Power Supply & Wiring
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| E1 | PSU — 24V 10A | Mean Well IRM-240-24 or Hi-Link HLK-240M24 | 1 | ~$40–60 | ~$50 | Digi-Key, Amazon |
+| E2 | 12 AWG silicone wire | Red + black, 600V rated, 5 m spool | 1 | ~$15 | ~$15 | McMaster-Carr, AliExpress |
+| E3 | PG7 cable gland | M20 thread, IP67, 5–8 mm cable | 2 | ~$3 | ~$6 | AliExpress, Heilind |
+| E4 | Varistor (MOV) | 18–28V, 1 kA | 1 | ~$1 | ~$1 | Digi-Key |
+| E5 | Fuse — 25A | T25 slow-blow, 5×20 mm | 1 | ~$0.50 | ~$0.50 | Digi-Key |
+| E6 | Fuse holder | 5×20 mm inline, 20A rated | 1 | ~$2 | ~$2 | Amazon |
+| E7 | Crimp ring terminals | M3, 12 AWG, tin-plated | 8 | ~$0.20 | ~$1.60 | Heilind, AliExpress |
+| E8 | Strain relief sleeve | 5 mm ID silicone, 1 m | 1 | ~$5 | ~$5 | McMaster-Carr |
+
+#### Pogo Pins & Contacts
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| C1 | Pogo pin assembly | Spring-loaded, Ø5.5 mm OD, 20 mm, 20A rated, 4 mm travel | 2 | ~$8–12 | ~$20 | Preci-Dip, Jst, AliExpress |
+| C2 | Brass contact pad | Ø12 × 2 mm, H68 brass, bare finish | 2 | ~$3 | ~$6 | Metal supplier (Metals USA, OnlineMetals) |
+| C3 | Solder lug — M3 | Copper ring, tin-plated | 4 | ~$0.40 | ~$1.60 | Heilind, Amazon |
+
+#### LED Status Circuit
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| L1 | 5 mm LED — Red | 2.0 V, 20 mA, diffuse | 1 | ~$0.30 | ~$0.30 | Digi-Key |
+| L2 | 5 mm LED — Yellow | 2.1 V, 20 mA, diffuse | 1 | ~$0.30 | ~$0.30 | Digi-Key |
+| L3 | 5 mm LED — Blue | 3.2 V, 20 mA, diffuse | 1 | ~$0.50 | ~$0.50 | Digi-Key |
+| L4 | 5 mm LED — Green | 2.1 V, 20 mA, diffuse | 1 | ~$0.30 | ~$0.30 | Digi-Key |
+| R1–R4 | Resistor — 1 kΩ 1/4W | Metal film, 1% tolerance | 4 | ~$0.10 | ~$0.40 | Digi-Key |
+| J1 | Pin header 2.54 mm | 1×6 right-angle | 1 | ~$0.50 | ~$0.50 | Digi-Key |
+
+#### Current Sensing (Optional)
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| S1 | INA219 I2C shunt monitor | 16-bit, I2C addr 0x40, 26V max | 1 | ~$5 | ~$5 | Adafruit, Digi-Key |
+| S2 | SMD resistor — 0.1 Ω | 1206, 1W | 1 | ~$1 | ~$1 | Digi-Key |
+
+### Mechanical Hardware
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| M1 | M20 hex nut | Steel DIN 934, ~86 g | 8 | ~$0.80 | ~$6.40 | Grainger, Home Depot |
+| M2 | M4 × 16 SHCS | Stainless A4 DIN 912 | 16 | ~$0.30 | ~$4.80 | Grainger |
+| M3 | M4 × 10 BHCS | Stainless A4 DIN 7380 | 8 | ~$0.25 | ~$2.00 | Grainger |
+| M4 | M4 heat-set insert | Brass, threaded, M4 | 20 | ~$0.15 | ~$3.00 | McMaster-Carr |
+| M5 | M3 × 16 SHCS | Stainless, LED bezel | 4 | ~$0.20 | ~$0.80 | Grainger |
+| M6 | M3 hex nut | DIN 934 | 4 | ~$0.10 | ~$0.40 | Grainger |
+| M7 | M8 × 40 BHCS | Zinc-plated, floor anchors (optional) | 4 | ~$0.50 | ~$2.00 | Grainger |
+| M8 | Rubber foot | Ø20 × 5 mm, self-adhesive | 4 | ~$0.80 | ~$3.20 | Amazon |
+
+### ArUco Marker & Frame
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| A1 | ArUco marker print | 100×100 mm, ID=42, DICT_4X4_250, glossy photo paper | 2 | ~$1.50 | ~$3.00 | Print locally or AliExpress |
+| A2 | Lamination pouch | A4, 80 µm thick | 2 | ~$0.40 | ~$0.80 | Amazon, Staples |
+| A3 | Acrylic cover sheet | Clear, 3 mm, 150×150 mm | 1 | ~$3 | ~$3.00 | McMaster-Carr |
+
+### Consumables & Assembly
+
+| # | Description | Spec | Qty | Unit Cost | Total | Source |
+|---|---|---|---|---|---|---|
+| X1 | Solder wire | 63/37 Sn/Pb or lead-free, 1 m | 1 | ~$3 | ~$3.00 | Digi-Key |
+| X2 | Flux paste | No-clean, 25 mL | 1 | ~$4 | ~$4.00 | Digi-Key |
+| X3 | Loctite 243 | Thread-locker (medium strength), 10 mL | 1 | ~$4 | ~$4.00 | Grainger |
+| X4 | Epoxy adhesive | Two-part, 25 mL | 1 | ~$6 | ~$6.00 | Home Depot |
+
+---
+
+## Assembly Procedure
+
+### Phase 1: Preparation
+
+1. **Print all PETG parts** (see STL export list above)
+   - Base: 0.3 mm layer, 60% infill (heavy/stable)
+   - Back wall: 0.2 mm, 40% infill
+   - Rails & brackets: 0.2 mm, 40-50% infill
+   - Support removal: slow, avoid pogo bore damage
+
+2. **Prepare ballast nuts**
+   - Sort 8× M20 hex nuts (stack in 4 pockets, 2 per pocket)
+   - Optional: fill pockets with epoxy to prevent rattling
+
+3. **Press brass contact pads**
+   - Apply 0.1 mm interference press-fit into receiver housing bores
+   - Use arbor press @ ~2 tons force
+   - Or use slow manual press (avoid chipping brass edges)
+
+### Phase 2: Base Assembly
+
+4. **Install heat-set M4 inserts** into base plate
+   - Back wall attach points (3×)
+   - Guide rail attach points (4× each side)
+   - ArUco mast feet (4×)
+   - PSU bracket mount (4×)
+   - Use soldering iron (350°C) or insert tool, press vertically
+
+5. **Ballast installation**
+   - Insert M20 hex nuts into base pockets (from underside)
+   - Verify pockets are flush, no protrusions into wiring channel
+   - Optional: epoxy-lock nuts with 5-minute epoxy
+
+6. **Install pogo pins** into back wall
+   - Press spring-loaded pins from front face into Ø5.7 mm bores (20 mm deep)
+   - Flange seats against counterbore shoulder at 1.5 mm depth
+   - Apply small drop of Loctite 243 to bore wall (prevents rotation)
+
+### Phase 3: Electrical Assembly
+
+7. **Solder wires to pogo pin terminals**
+   - 12 AWG red wire → POGO+ pin
+   - 12 AWG black wire → POGO- pin
+   - Solder both in & out of lug for redundancy
+   - Add ~50 mm strain relief sleeve over each joint
+
+8. **Route pogo wires through base wiring channel**
+   - Guide down channel (10×10 mm trough)
+   - Exit through cable gland slot on rear
+
+9. **Assemble PSU bracket**
+   - Bolt Mean Well IRM-240-24 (or equivalent) to bracket pads
+   - 4× M4 fasteners through bracket to base rear
+   - Orient PSU exhaust away from dock (for ventilation)
+
+10. **Connect 24V wiring**
+    - Pogo+ wire (red) → PSU V+ terminal
+    - Pogo- wire (black) → PSU COM/GND terminal
+    - Observe polarity strictly (reverse = short circuit)
+
+11. **Install power protection**
+    - Fuse holder in-line on PSU V+ output (25A slow-blow)
+    - Varistor (MOV, 18–28V) across V+/COM rails (clamp transients)
+    - Optional: NTC thermistor (10Ω @ 25°C) in series for soft-start
+
+12. **Wire AC mains input** (if not pre-assembled)
+    - Route AC input through cable gland on PSU bracket
+    - Connect to PSU AC terminals (L, N, PE if applicable)
+    - Ensure all connections are soldered + crimped
+
+### Phase 4: LED Assembly
+
+13. **Install LED bezel into back wall**
+    - 4× 5 mm LEDs press-fit into bezel holes (bodies recessed ~2 mm)
+    - Solder resistors (1 kΩ 1/4W) to LED anodes on rear
+    - Connect all LED cathodes to common GND line (black wire to PSU COM)
+    - Wire LED control lines to Jetson Orin NX GPIO (via I2C expander if needed)
+
+14. **Connect LED header**
+    - 2.54 mm pin header (1×6) plugs into LED control harness
+    - Pin 1: LED1 (red, SEARCHING)
+    - Pin 2: LED2 (yellow, ALIGNED)
+    - Pin 3: LED3 (blue, CHARGING)
+    - Pin 4: LED4 (green, FULL)
+    - Pins 5–6: GND, +24V (power for LED feedback monitoring)
+
+### Phase 5: Mechanical Assembly
+
+15. **Bolt back wall to base**
+    - 3× M4×16 SHCS from underside of base
+    - Tighten to ~5 Nm (snug, don't overtighten plastic)
+    - Back wall should be perpendicular to base (verify with level)
+
+16. **Attach V-guide rails**
+    - Left rail: 4× M4 fasteners into base inserts (front & rear attach)
+    - Right rail: Mirror (flip STL in slicer) or manually mirror geometry
+    - Verify V-channels are parallel & symmetrical (±2 mm tolerance)
+
+17. **Mount ArUco marker frame**
+    - Bolt 4× M4×10 fasteners to frame feet (attach to base front)
+    - Insert laminated 100×100 mm ArUco marker (ID 42) into frame slot
+    - Verify marker is flat & centered (no curl or shadow)
+
+18. **Attach rubber feet** (or floor anchors)
+    - 4× self-adhesive rubber feet on base underside corners
+    - OR drill M8 holes through base (optional: permanent floor mounting)
+
+### Phase 6: Robot Receiver Assembly
+
+19. **Assemble robot receiver** (per variant)
+    - **SaltyLab:** 2-piece stem collar (M4×16 clamps Ø25 mm stem)
+    - **SaltyRover:** Single flange piece (4× M4 to deck underbelly)
+    - **SaltyTank:** Single piece w/ extended nose (4× M4 to skid plate)
+
+20. **Press brass pads into receiver**
+    - Ø12 mm pads press into 0.1 mm interference bores
+    - Apply Loctite 603 retaining compound to bore before pressing
+    - Manual arbor press @ ~1-2 tons force; pads should be proud 0.2 mm
+
+21. **Solder receiver wires**
+    - 12 AWG wires (red/black) solder to M3 solder lugs on pad rear
+    - Route wires through wire channel on mount face
+    - Terminate to robot BMS/charging PCB input
+
+---
+
+## Wiring Diagram (24V System)
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                       MAINS INPUT (AC)                       │
+│                        110/220 V AC                          │
+└────────────┬────────────────────────────────────────────────┘
+             │
+             ▼
+      ┌──────────────┐
+      │  IRM-240-24  │  24V / 10A out (240W)
+      │   PSU        │  ±5% regulated, open-frame
+      └──┬───────┬───┘
+    +24V │       │ GND
+         │       │
+    ┌────┴┐   ┌─┴────┐
+    │ [F] │   │ [F]  │  Fuse holder (25A slow-blow)
+    │     │   │      │
+    │ +24 │   │ GND  │  12 AWG silicone wire to back wall
+    │     │   │      │
+    └────┬┘   └─┬────┘
+         │     │
+    +24V│     │GND
+        ▼     ▼
+    ┌─────────────────┐
+    │  Back wall      │
+    │  ┌───────────┐  │
+    │  │  POGO+    │  │ Spring-loaded contact pin (+24V)
+    │  │  POGO-    │  │ Spring-loaded contact pin (GND)
+    │  └────┬──────┘  │
+    │       │         │
+    │ ┌─────┴─────┐   │
+    │ │  LED 1-4  │   │ Red, Yellow, Blue, Green indicators
+    │ │  Resistors│   │ 1 kΩ limiting resistors (×4)
+    │ │  [GPIO]   │   │ Control from Jetson Orin NX I2C
+    │ └───────────┘   │
+    └─────┬───────────┘
+          │
+     ═════╧════════ DOCK / ROBOT AIR GAP (≤50 mm) ═════════════
+          │
+          ▼
+    ┌──────────────────┐
+    │  Robot Receiver  │
+    │  ┌────────────┐  │
+    │  │ Contact +  │  │ Brass pad (Ø12×2 mm) [+24V]
+    │  │ Contact -  │  │ Brass pad (Ø12×2 mm) [GND]
+    │  └──┬──┬──────┘  │
+    │     │  │         │
+    │   12 AWG wires   │ Red/black to BMS
+    │     │  │         │
+    │  ┌──▼──▼──┐      │
+    │  │ Robot  │      │
+    │  │ BMS    │      │
+    │  │Battery │      │ Charging current: 0–15A (typical)
+    │  └────────┘      │
+    └──────────────────┘
+
+OPTIONAL — CURRENT SENSING (Diagnostic)
+         │ +24V
+    ┌────┴────┐
+    │[INA219] │ I2C current monitor (0.1Ω sense resistor)
+    │ I2C 0x40│ Jetson reads dock current → state machine
+    └────┬────┘
+         │ GND
+
+LED STATE MACHINE CONTROL (from docking_node.py):
+    State          GPIO/Signal    LED Output
+    ─────────────────────────────────────────
+    SEARCHING      GPIO H         Red LED ON (20 mA, 1 kΩ)
+    ALIGNED        GPIO H         Yellow LED ON (pre-charge active)
+    CHARGING       GPIO H         Blue LED ON (>1 A charging)
+    FULL/COMPLETE  GPIO H         Green LED ON (float mode)
+
+    GPIO driven via Jetson Orin NX I2C LED driver (e.g., PCA9685)
+    or direct GPIO if firmware implements bitbang logic.
+```
+
+---
+
+## Integration with ROS2 Docking Node (#489)
+
+**Docking node location:** `./jetson/ros2_ws/src/saltybot_docking/docking_node.py`
+
+### MQTT Topics
+
+**Status reporting (outbound):**
+```
+saltybot/docking/status  → { state, robot_id, contact_voltage, charge_current }
+saltybot/docking/led     → { red, yellow, blue, green }  [0=OFF, 1=ON, blink_hz]
+```
+
+**Command subscriptions (inbound):**
+```
+saltybot/docking/reset   → trigger dock reset (clear fault)
+saltybot/docking/park    → move robot out of dock (e.g., after full charge)
+```
+
+### Firmware Integration
+
+**State machine (4 states):**
+1. **SEARCHING** — No robot contact; dock waits for approach (ArUco marker detection via Jetson camera)
+2. **ALIGNED** — Contact made (BMS pre-charge active); dock supplies trickle current (~100 mA) while robot capacitors charge
+3. **CHARGING** — Main charge active; dock measures current via INA219, feedback to BMS
+4. **FULL** — Target voltage reached (≥23.5 V, <100 mA draw); dock holds float voltage
+
+**Current sensing feedback:**
+- INA219 I2C shunt on 24V rail monitors dock-to-robot current
+- Jetson polls at 10 Hz; state transitions trigger LED updates & MQTT publish
+- Hysteresis prevents flickering (state valid for ≥2 sec)
+
+---
+
+## Testing Checklist
+
+- [ ] **Electrical safety**
+  - [ ] 24V output isolated from mains AC (< 2.5 kV isolation @ 60 Hz)
+  - [ ] Fuse 25A blocks short-circuit (verify blow @ >30 A)
+  - [ ] Varistor clamps transient overvoltage (check 28V limit)
+  - [ ] All crimps are soldered + crimped (pull test: no slippage @ 10 lbf)
+
+- [ ] **Mechanical**
+  - [ ] Base level on 4 rubber feet (no rocking)
+  - [ ] V-rails parallel within ±2 mm across 250 mm length
+  - [ ] Back wall perpendicular to base (level ±1°)
+  - [ ] Pogo pins extend 4 mm from back wall face (spring preload correct)
+
+- [ ] **Contact alignment**
+  - [ ] Robot receiver pads contact pogo pins with ≥3 mm contact face overlap
+  - [ ] Contact resistance < 50 mΩ (measure with multimeter on lowest ohm scale during light press)
+  - [ ] No visible arcing or pitting (inspect pads after 10 charge cycles)
+
+- [ ] **Power delivery**
+  - [ ] 24V output at PSU: 23.5–24.5 V (under load)
+  - [ ] 24V at pogo pins: ≥23.5 V (< 0.5 V droop @ 10 A)
+  - [ ] Robot receives 24V ± 1 V (measure at BMS input)
+
+- [ ] **LED status**
+  - [ ] Red (SEARCHING) steady on before robot approach
+  - [ ] Yellow (ALIGNED) turns on when pads make contact
+  - [ ] Blue (CHARGING) turns on when charge current > 500 mA
+  - [ ] Green (FULL) turns on when current drops < 100 mA (float mode)
+
+- [ ] **ArUco marker**
+  - [ ] Marker ID 42 is readable by Jetson camera from 1.5 m @ 90° angle
+  - [ ] No glare or shadow on marker (add diffuse lighting if needed)
+  - [ ] Marker detected by cv2.aruco in < 100 ms
+
+- [ ] **MQTT integration**
+  - [ ] Dock publishes status every 5 sec (or on state change)
+  - [ ] LED state matches reported dock state
+  - [ ] Current sensing (INA219) reads within ±2% of true dock current
+
+---
+
+## Firmware/Software Requirements
+
+### Jetson Orin NX (Docking controller)
+
+**Python dependencies:**
+```bash
+pip install opencv-contrib-python  # ArUco marker detection
+pip install adafruit-circuitpython-ina219  # Current sensing
+pip install rclpy  # ROS2
+pip install paho-mqtt  # MQTT status reporting
+```
+
+**Key Python modules:**
+- `cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_250)` → ArUco ID 42 detection
+- `Adafruit_INA219` → I2C current monitoring @ 0x40
+- GPIO library → LED control (via I2C LED driver or direct GPIO)
+
+**ROS2 node:** `saltybot_docking/docking_node.py` (already present, Issue #489)
+- Subscribes to `/docking/approach_request`
+- Publishes to `/docking/status`, `/docking/led_state`
+- MQTT gateway for legacy systems
+
+---
+
+## Files to Commit
+
+**New files for Issue #505:**
+```
+chassis/
+├── charging_dock_505.scad              [Main dock 24V design]
+├── charging_dock_receiver_505.scad     [Robot receiver 24V variant]
+├── ISSUE_505_CHARGING_DOCK_24V_DESIGN.md  [This file]
+├── charging_dock_505_BOM.csv           [Excel-friendly BOM export]
+└── charging_dock_505_WIRING_DIAGRAM.md [Detailed wiring guide]
+
+docs/
+└── Issue_505_Assembly_Guide.md  [Step-by-step assembly photos + text]
+```
+
+---
+
+## Revision History
+
+| Date | Version | Changes |
+|------|---------|---------|
+| 2026-03-06 | 1.0 | Initial design (24V upgrade from Issue #159) |
+
+---
+
+## Next Steps
+
+1. ✅ Design specification (this document)
+2. ⏳ OpenSCAD CAD files (`charging_dock_505.scad`, `charging_dock_receiver_505.scad`)
+3. ⏳ BOM export (CSV format for procurement)
+4. ⏳ 3D-printed prototype testing
+5. ⏳ Electrical integration with Jetson docking node
+6. ⏳ ArUco marker calibration & documentation
+7. ⏳ PR submission & merge to `main`
+
+---
+
+**Designer:** sl-mechanical
+**Date:** 2026-03-06
+**Status:** Design Specification Complete — Awaiting CAD Implementation
diff --git a/chassis/charging_dock_505_BOM.csv b/chassis/charging_dock_505_BOM.csv
new file mode 100644
index 0000000..f3a9f1b
--- /dev/null
+++ b/chassis/charging_dock_505_BOM.csv
@@ -0,0 +1,41 @@
+Item,Description,Specification,Quantity,Unit Cost,Total Cost,Source Notes
+E1,Power Supply,Mean Well IRM-240-24 / Hi-Link HLK-240M24 (24V 10A 240W),1,$50.00,$50.00,Digi-Key / Amazon
+E2,12 AWG Silicone Wire,Red + Black 600V rated 5m spool,1,$15.00,$15.00,McMaster-Carr / AliExpress
+E3,PG7 Cable Gland,M20 IP67 5-8mm cable,2,$3.00,$6.00,AliExpress / Heilind
+E4,Varistor (MOV),18-28V 1kA,1,$1.00,$1.00,Digi-Key
+E5,Fuse 25A,T25 Slow-blow 5x20mm,1,$0.50,$0.50,Digi-Key
+E6,Fuse Holder,5x20mm inline 20A rated,1,$2.00,$2.00,Amazon
+E7,Crimp Ring Terminals,M3 12 AWG tin-plated,8,$0.20,$1.60,Heilind / AliExpress
+E8,Strain Relief Sleeve,5mm ID silicone 1m,1,$5.00,$5.00,McMaster-Carr
+C1,Pogo Pin Assembly,"Spring-loaded Ø5.5mm 20mm 20A 4mm travel",2,$10.00,$20.00,Preci-Dip / Jst / AliExpress
+C2,Brass Contact Pad,Ø12x2mm H68 brass bare,2,$3.00,$6.00,OnlineMetals / Metals USA
+C3,Solder Lug M3,Copper ring tin-plated,4,$0.40,$1.60,Heilind / Amazon
+L1,5mm LED Red,2.0V 20mA diffuse,1,$0.30,$0.30,Digi-Key
+L2,5mm LED Yellow,2.1V 20mA diffuse,1,$0.30,$0.30,Digi-Key
+L3,5mm LED Blue,3.2V 20mA diffuse,1,$0.50,$0.50,Digi-Key
+L4,5mm LED Green,2.1V 20mA diffuse,1,$0.30,$0.30,Digi-Key
+R1-R4,Resistor 1kΩ 1/4W,Metal film 1% tolerance,4,$0.10,$0.40,Digi-Key
+J1,Pin Header 2.54mm,1x6 right-angle,1,$0.50,$0.50,Digi-Key
+S1,INA219 I2C Shunt Monitor,16-bit I2C 0x40 26V max (Optional),1,$5.00,$5.00,Adafruit / Digi-Key
+S2,SMD Resistor 0.1Ω,1206 1W (Optional current sense),1,$1.00,$1.00,Digi-Key
+M1,M20 Hex Nut,Steel DIN 934 ~86g,8,$0.80,$6.40,Grainger / Home Depot
+M2,M4x16 SHCS,Stainless A4 DIN 912,16,$0.30,$4.80,Grainger
+M3,M4x10 BHCS,Stainless A4 DIN 7380,8,$0.25,$2.00,Grainger
+M4,M4 Heat-Set Insert,Brass threaded,20,$0.15,$3.00,McMaster-Carr
+M5,M3x16 SHCS,Stainless,4,$0.20,$0.80,Grainger
+M6,M3 Hex Nut,DIN 934,4,$0.10,$0.40,Grainger
+M7,M8x40 BHCS,Zinc-plated floor anchor,4,$0.50,$2.00,Grainger
+M8,Rubber Foot,Ø20x5mm self-adhesive,4,$0.80,$3.20,Amazon
+A1,ArUco Marker Print,"100x100mm ID=42 DICT_4X4_250 glossy photo (qty 2)",2,$1.50,$3.00,Print locally / AliExpress
+A2,Lamination Pouch,A4 80µm,2,$0.40,$0.80,Amazon / Staples
+A3,Acrylic Cover Sheet,Clear 3mm 150x150mm,1,$3.00,$3.00,McMaster-Carr
+X1,Solder Wire,63/37 Sn/Pb lead-free 1m,1,$3.00,$3.00,Digi-Key
+X2,Flux Paste,No-clean 25mL,1,$4.00,$4.00,Digi-Key
+X3,Loctite 243,Thread-locker 10mL,1,$4.00,$4.00,Grainger
+X4,Epoxy Adhesive,Two-part 25mL,1,$6.00,$6.00,Home Depot
+P1,PETG Filament (3D Print),"Natural/White 1kg ±15% waste factor",2.5,$20.00,$50.00,Prusament / Overture
+,,,,,
+SUBTOTAL (Electrical + Hardware + Consumables),,,,,,$234.00,excludes 3D printing
+SUBTOTAL (With 3D filament @ $20/kg),,,,,,$284.00,all materials
+LABOR ESTIMATE (Assembly ~4-6 hrs),,,,,,$150-225,tecnico time
+TOTAL PROJECT COST (Material + Labor),,,,,,$434-509,per dock