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Wiring Schematic for EG4 BMS ESPHome Component

Standard Wiring (Most Common)

┌─────────────────────────────────────────────────────────────────┐
│                                                                 │
│  EG4 BMS RJ45 Port (looking into the socket, tab down)         │
│  ┌─────────────────────────┐                                    │
│  │  1  2  3  4  5  6  7  8 │                                    │
│  │  │  │  │  │  │  │  │  │ │                                    │
│  └──┼──┼──┼──┼──┼──┼──┼──┼─┘                                    │
│     │  │  │  │  │  │  │  │                                      │
│     │  │  │  │  │  │  │  └── RS485 B  (Brown)                  │
│     │  │  │  │  │  │  └───── RS485 A  (Brown/White)            │
│     │  │  │  │  │  └──────── GND      (Green)                  │
│     │  │  │  │  └─────────── (unused)  (Blue/White)            │
│     │  │  │  └────────────── (unused)  (Blue)                  │
│     │  │  └───────────────── GND      (Green/White)            │
│     │  └──────────────────── RS485 A  (Orange)                 │
│     └─────────────────────── RS485 B  (Orange/White)           │
│                                                                 │
│  Pins 1/8 (B) and 2/7 (A) are bridged — use either end.        │
│  Pins 4/5 are not RS485 on this port. Do not tap them.         │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘
                    │
                    ↓
┌─────────────────────────────────────────────────────────────────┐
│                                                                 │
│  RS485 to TTL Converter (e.g., MAX485)                         │
│  ┌────────────────────────────────────┐                        │
│  │  [VCC] [GND] [A+] [B-] [RX] [TX]  │                        │
│  │    │     │     │    │     │    │   │                        │
│  └────│─────│─────│────│─────│────│───┘                        │
│       │     │     │    │     │    │                            │
│       │     │     │    │     │    │                            │
└───────│─────│─────│────│─────│────│─────────────────────────────┘
        │     │     │    │     │    │
        ↓     ↓     ↓    ↓     ↓    ↓
┌───────────────────────────────────────────────────────────────────┐
│                                                                   │
│  ESP32 Development Board                                         │
│  ┌─────────────────────────────────────────────────────────┐    │
│  │                                                           │    │
│  │  3.3V ─────────────────────────┐                        │    │
│  │  GND ──────────────────┐       │                         │    │
│  │  GPIO17 (RX) ─────┐   │       │                         │    │
│  │  GPIO16 (TX) ──┐  │   │       │                         │    │
│  │                │  │   │       │                         │    │
│  └────────────────│──│───│───────│──────────────────────────┘    │
│                   │  │   │       │                               │
│                   │  │   │       │                               │
└───────────────────│──│───│───────│───────────────────────────────┘
                    │  │   │       │
                   TX  RX GND     VCC

Component Pinout Reference

RS485 Module Pins

Pin Function Connect To
VCC Power ESP 3.3V
GND Ground ESP GND (also tie to BMS RJ45 Pin 3 or 6)
A RS485 A BMS RJ45 Pin 2 or Pin 7
B RS485 B BMS RJ45 Pin 1 or Pin 8
RX UART Receive ESP TX Pin
TX UART Transmit ESP RX Pin
DE/RE Flow Control (optional) ESP GPIO (if needed)

Note: Pins 4 and 5 on this RJ45 are not RS485. Earlier versions of this guide listed pins 4/5 by mistake — if you wired to those pins you won't get RS485 traffic. Use pins 1/2 or 7/8 for this component.

ESP32 Pins (Default Configuration)

ESP32 Pin Function Connect To
GPIO16 UART TX RS485 RX
GPIO17 UART RX RS485 TX
3.3V Power RS485 VCC
GND Ground RS485 GND

ESP8266 Pins (Default Configuration)

ESP8266 Pin Function Connect To
GPIO1 (TX) UART TX RS485 RX
GPIO3 (RX) UART RX RS485 TX
3.3V Power RS485 VCC
GND Ground RS485 GND

Multi-BMS Setup

To connect multiple BMSes on the same RS485 bus:

┌──────────┐     ┌──────────┐     ┌──────────┐
│  BMS #1  │     │  BMS #2  │     │  BMS #3  │
│  Addr:1  │     │  Addr:2  │     │  Addr:3  │
└────┬─────┘     └────┬─────┘     └────┬─────┘
     │                │                │
    A+/B-            A+/B-            A+/B-
     │                │                │
     └────────┬───────┴────────┬───────┘
              │                │
         ┌────┴────┐      ┌────┴────┐
         │  120Ω   │      │  120Ω   │
         │Terminator│      │Terminator│
         └────┬────┘      └─────────┘
              │
         RS485 Bus
              │
         ┌────┴────┐
         │ RS485   │
         │  to TTL │
         └────┬────┘
              │
            ESP32

Important Notes for Multi-BMS:

  • Use separate eg4_bms entries with different address values
  • Add 120Ω termination resistors at both ends of the bus
  • Keep total cable length under 1200m (typical RS485 limit)
  • Use shielded twisted pair cable for longer runs

Power Supply Recommendations

Option 1: Power from USB (Recommended for Development)

  • Connect ESP32/ESP8266 via USB
  • RS485 module powered from ESP's 3.3V pin
  • Limit: Not suitable for permanent installation

Option 2: External Power Supply (Recommended for Production)

  • Use 5V power supply with voltage regulator
  • Or use ESP board with onboard voltage regulator
  • Provides stable power for reliable operation

Option 3: Power from Battery System (Advanced)

  • Use DC-DC buck converter (e.g., LM2596) if battery voltage > 5V
  • Add proper fusing and protection
  • Warning: Only if you understand battery system safety

Important Warnings

⚠️ Always use 3.3V for RS485 module VCC

  • Using 5V may damage the ESP's GPIO pins
  • If your RS485 module is 5V-only, use a logic level converter

⚠️ Check A+/B- polarity

  • If no communication, try swapping A+ and B-
  • RS485 is differential, so polarity matters

⚠️ Cable quality matters

  • Use twisted pair cable for RS485
  • Shielded cable for electrically noisy environments
  • Cat5e/Cat6 ethernet cable works well

⚠️ Grounding

  • Connect GND between all devices
  • Improper grounding can cause communication errors
  • In multi-BMS setups, ensure common ground

Testing Your Wiring

Before powering on:

  1. ✅ Check all connections with multimeter
  2. ✅ Verify no shorts between VCC and GND
  3. ✅ Confirm A+ and B- are not swapped
  4. ✅ Ensure RS485 module is powered from 3.3V

After powering on:

  1. Check ESP boots normally (look for WiFi activity)
  2. Check logs for "eg4_modbus" initialization
  3. Look for Modbus communication in debug logs
  4. Verify voltage readings make sense

Troubleshooting

No Communication

  • Swap A+ and B- wires
  • Check baud rate is 9600
  • Verify BMS address (default 0x10)
  • Try shorter cable

Intermittent Communication

  • Add termination resistors
  • Check for loose connections
  • Use better quality cable
  • Reduce cable length

Incorrect Data

  • Verify power supply is stable
  • Check for electromagnetic interference
  • Ensure proper grounding
  • Try lower polling frequency