An ESP32-C3 port of the Open Battery Information project by Martin Jansson.
This firmware provides a web-based interface for reading Makita LXT battery information, including cell voltages, temperatures, charge cycles, and error codes.
- Web Interface: Responsive single-page application accessible from any browser
- Real-time Battery Data: Cell voltages, pack voltage, temperatures, charge count
- mDNS Discovery: Access via
http://obi-esp32.local - OTA Updates: Wireless firmware updates after initial flash
- REST API: JSON endpoints for integration with other systems
- Dual Temperature Sensors: Cell thermistor and MOSFET temperatures
| Component | Quantity | Notes |
|---|---|---|
| ESP32-C3 Super Mini | 1 | Any ESP32-C3 board with native USB should work |
| 4.7kΩ Resistor | 2 | Pull-up resistors for data lines |
| Makita LXT Battery | 1 | 18V batteries (BL18xx series) |
| Connecting wires | 4 | For battery connections |
| Makita charger connector | 1 | See below for links to a source on aliexpress |
A 3D printed enclosure designed for the original Arduino-based OBI works well with this ESP32 version:
- Enclosure: Makita 18V Battery Unlock Device (OBI) on MakerWorld
- ESP32 Wedge: ESP32-C3 Super Mini holder - small wedge to secure the ESP32 in place
The enclosure was designed for the Arduino Nano but accommodates the ESP32-C3 Super Mini with the addition of the wedge adapter.
These parts were used in the OBI build video:
| Part | Link |
|---|---|
| Arduino Nano (or use ESP32-C3) | AliExpress |
| Makita charger connector | AliExpress |
| Spade terminals | AliExpress |
| 4.7kΩ 0.25W resistors | AliExpress |
If the link to the Makita charger connector doesn't work, try searching for something like "BL1830 Charger Connector Terminal for Makita".
| ESP32-C3 Pin | Makita Battery | Description |
|---|---|---|
| GPIO3 | Pin 2 (Data) | OneWire data line |
| GPIO4 | Pin 6 (Enable) | Enable signal |
| GND | Pin 5 (B-) | Ground reference |
| 3.3V | — | Power for pull-up resistors only |
Both GPIO3 and GPIO4 require 4.7kΩ pull-up resistors to 3.3V. These are essential for the modified OneWire protocol to function correctly.
Looking at the battery terminals from the front:
┌───────────────────────────────────────────┐
│ [1] [2] [3] [4] [5] [6] [7] │
│ B+ DATA EN B- │
└───────────────────────────────────────────┘
- Outside pin (B+): Positive terminal (18V) - DO NOT CONNECT
- Pin 2 (Data): OneWire data communication
- Pin 6 (EN): Enable signal (side terminal)
- Outside pin (B-): Ground/negative terminal
- PlatformIO (VS Code extension or CLI)
- USB-C cable
Copy the secrets template and add your WiFi credentials:
cp src/secrets.h.example src/secrets.hThen edit src/secrets.h with your WiFi credentials:
#define WIFI_SSID "YourSSID"
#define WIFI_PASS "YourPassword"The secrets.h file is gitignored, so your credentials won't be committed to version control.
# Build and upload via USB
pio run -e esp32c3_web -t upload
# Monitor serial output
pio device monitorAfter the initial flash, you can update wirelessly:
# Upload via OTA (update IP in platformio.ini if needed)
pio run -e esp32c3_ota -t uploadNote: If OTA fails with "Connect Failed", you may need to allow incoming connections from the ESP32's IP address in your firewall:
# Linux with firewalld
sudo firewall-cmd --add-rich-rule='rule family="ipv4" source address="192.168.x.x" accept'
sudo firewall-cmd --runtime-to-permanentThe ESP32 advertises itself via mDNS. After connecting to WiFi, access:
- mDNS:
http://obi-esp32.local - Direct IP: Check serial output for assigned IP address
The web interface provides:
- Connection Status: Current battery connection state
- Battery Information: Model, charge count, manufacturing date, capacity
- Cell Voltages: Individual cell voltages with colour-coded health indicators
- Green (≥3.5V): Good
- Yellow (3.0-3.5V): Warning
- Red (<3.0V): Critical
- Temperatures: Cell and MOSFET thermistor readings
- Error Code: BMS error status
- Actions: LED test and error reset functions
- Debug Log: Real-time operation log
Returns complete battery information including voltages.
{
"success": true,
"model": "BL1850B",
"locked": false,
"chargeCount": 42,
"mfgDate": "2021-06",
"capacity": 5.0,
"errorCode": 6,
"packVoltage": 16.52,
"cell1": 3.304,
"cell2": 3.305,
"cell3": 3.303,
"cell4": 3.304,
"cell5": 3.304,
"cellDiff": 0.002,
"tempCell": 29.5,
"tempMosfet": 28.2
}Returns voltage and temperature data only.
{
"success": true,
"packVoltage": 16.52,
"cell1": 3.304,
"cell2": 3.305,
"cell3": 3.303,
"cell4": 3.304,
"cell5": 3.304,
"cellDiff": 0.002,
"tempCell": 29.5,
"tempMosfet": 28.2
}Controls battery LED indicators (if supported).
Resets battery error codes. Use with caution.
Based on testing, these error codes have been observed:
| Code | Meaning |
|---|---|
| 4 | Battery fault/lockout (over-discharge protection) |
| 6 | Normal operation |
Note: Official Makita error code documentation is not publicly available.
The firmware reads two temperature sensors:
- Cell Temperature: Thermistor near battery cells
- MOSFET Temperature: Thermistor near power MOSFETs
Readings are typically 5-10°C above ambient due to:
- Sensor location inside insulated battery enclosure
- Quiescent current from BMS circuitry
This project is a port of the excellent Open Battery Information by Martin Jansson. The original project provides a Python GUI application and documents the Makita battery communication protocol.
Additional resources:
- OBI Build Video - Excellent tutorial showing the original Arduino build
- 3D Printed Enclosure - Compact case design on MakerWorld
MIT License - see LICENSE file.
Same license as the original Open Battery Information project.