overland-controller/docs/decisions.md

Decisions

This document records important design and architecture decisions for Overland Controller.

The goal is to preserve why choices were made, not just what was implemented.

Decision: ESP32 Owns System State

The ESP32 controller is the source of truth.

It owns:

• Sensor readings
• Relay states
• BMS state
• Configuration
• Alarm state

Reason:

The ESP32 is installed near the electrical system and should continue operating even if the dashboard is disconnected.

Decision: Pico Is a Dashboard, Not the Controller

The Pico dashboard is responsible for user interaction.

It should not own critical control state.

Reason:

The dashboard may reboot, disconnect, or be removed without disabling the controller.

Decision: UART Is the Primary Dashboard Link

UART is the preferred Pico-to-ESP32 communication path.

Reason:

• Simple wiring
• Deterministic
• No WiFi dependency
• Works offline
• Easier to debug electrically

HTTP remains useful for setup, debugging, and future web UI.

Decision: HTTP and UART Should Share a Data Model

The same status/config model should be used across:

• UART
• HTTP
• Simulator
• Future MQTT/Home Assistant integrations

Reason:

Maintaining multiple data models creates unnecessary rework and bugs.

Decision: Use Generic IDs Internally

Firmware uses generic identifiers:

relay_1
relay_2

temp_1
temp_2
temp_3
temp_4
temp_5
temp_6
temp_7
temp_8

Reason:

The project should not be tied to one vehicle or one layout.

Decision: User Names Belong in Configuration

Installation-specific names are saved in config, not hardcoded.

Examples of allowed config names:

Fridge
Starlink
Cabin
Outside
House Battery

Reason:

The same firmware should work in different vehicles, trailers, campers, and power systems.

Decision: BMS Address Is Configurable

The BMS BLE address is stored in configuration.

Reason:

Different users will have different BMS devices.

The firmware should not require source code changes to connect to a battery.

Decision: BLE Setup Mode Exists

BMS discovery runs in setup mode.

Reason:

BLE scanning can conflict with an active BMS connection.

Setup mode pauses normal BMS reconnect behavior and allows repeated scans.

Decision: Keep USB Serial as a Service Interface

USB serial remains available for:

• Recovery
• Troubleshooting
• Factory reset
• Manual configuration
• Debug logging

Reason:

A physical service path is valuable when WiFi, UART, or dashboard software is broken.

Decision: Relays Should Drive Automotive Relays

ESP32 relay outputs should not directly power high-current loads.

Recommended pattern:

ESP32 output
  -> relay module
  -> automotive relay
  -> fused load

Reason:

Loads like refrigerators, inverters, compressors, and Starlink should use proper fused automotive circuits.

Decision: DS18B20 Sensors Need Pull-Up Resistor

The 1-Wire bus requires a pull-up resistor.

Recommended value:

4.7k ohm

Reason:

Long DS18B20 leads are unreliable without the required pull-up.

Decision: Local-First Operation

The system should not require internet access.

Reason:

Overland and remote-use scenarios may have no connectivity.

Decision: Keep APIs Human-Debuggable

Messages use JSON.

Reason:

JSON is easy to inspect in serial logs, HTTP responses, simulators, and future tools.

Decision: Avoid Premature Cloud Features

Cloud dashboards and remote access are not current priorities.

Reason:

The core controller/dashboard system should be reliable locally before remote features are considered.