ESPHome Tethercell

An ESPHome integration for controlling and monitoring Tethercell batteries over Bluetooth Low Energy (BLE).

The project was created to give unused Tethercell batteries a second life by integrating them into an ESPHome and Home Assistant environment. An ESP32 with BLE support connects directly to the Tethercell, handles authentication, controls its internal FET, and reads the battery voltage.

The integration is provided as a reusable ESPHome package, making it possible to control one or multiple Tethercells from a single ESP32.

Features

• 🔵 Bluetooth Low Energy communication
• 🔐 Tethercell PIN authentication
• 🔌 Remote control of the internal FET
• 🔋 Battery voltage monitoring
• 🏠 Native ESPHome integration
• 🏡 Home Assistant integration through ESPHome
• 🔁 Support for multiple Tethercells from a single ESP32
• 📦 Reusable ESPHome package

How It Works

The Tethercell is a battery adapter that can remotely control the power output of a connected battery.

This project uses an ESP32 as a BLE client. The ESP32 connects to the Tethercell using its Bluetooth MAC address and authenticates using the configured Tethercell PIN.

Once connected and authorized, the ESP32 can communicate with the Tethercell's BLE service.

The current implementation provides two main functions:

1. FET control – turn the Tethercell's battery output on or off.
2. Battery voltage monitoring – read and expose the current battery voltage.

The resulting entities are automatically available through ESPHome and can therefore be used in Home Assistant dashboards and automations.

Supported Functions

FET Control

The Tethercell's internal FET can be controlled using an ESPHome switch.

ON

Enables the Tethercell's battery output.

OFF

Disables the Tethercell's battery output.

This makes it possible to use a Tethercell as a remotely controlled power source and to integrate it into Home Assistant automations.

Battery Voltage

The Tethercell battery voltage is exposed as an ESPHome sensor.

The voltage is read over BLE and converted to volts using the same conversion formula used by the original Tethercell Node.js implementation.

The sensor currently updates every 5 minutes.

Example:

Tethercell 1 Battery voltage: 1.42 V

Requirements

You need:

• An ESP32 with Bluetooth Low Energy support
• One or more Tethercell devices
• The Bluetooth MAC address of each Tethercell
• The PIN of each Tethercell
• ESPHome
• Home Assistant (optional, but recommended)

The ESP32 must be within Bluetooth range of the Tethercell.

ESPHome Configuration

The integration is designed to be included as an ESPHome package.

First, enable the ESP32 BLE tracker:

esp32_ble_tracker:

Then include the Tethercell package:

packages:
  tethercell1:
    url: https://github.com/w00dst0ck/esphome-tethercell
    files:
      - path: esphome_tethercell_generic.yaml
        vars:
          tethercell_number: 1
          tethercell_mac: 88:33:14:49:XX:XX
          tethercell_pin: 00000000
    ref: main
    refresh: 1d

Replace the following values with the information from your Tethercell:

Parameter	Description
tethercell_number	A unique number used to identify the Tethercell
tethercell_mac	The Bluetooth MAC address of the Tethercell
tethercell_pin	The PIN used to authenticate with the Tethercell

Multiple Tethercells

Multiple Tethercells can be controlled by including the package multiple times.

Each instance needs its own unique tethercell_number, MAC address, and PIN.

Example:

esp32_ble_tracker:

packages:
  tethercell1:
    url: https://github.com/w00dst0ck/esphome-tethercell
    files:
      - path: esphome_tethercell_generic.yaml
        vars:
          tethercell_number: 1
          tethercell_mac: 88:33:14:49:XX:XX
          tethercell_pin: 00000000
    ref: main
    refresh: 1d

  tethercell2:
    url: https://github.com/w00dst0ck/esphome-tethercell
    files:
      - path: esphome_tethercell_generic.yaml
        vars:
          tethercell_number: 2
          tethercell_mac: 88:33:14:49:XX:XX
          tethercell_pin: 00000000
    ref: main
    refresh: 1d

Home Assistant

When the ESPHome device is connected to Home Assistant, each configured Tethercell exposes its entities automatically.

For each Tethercell, the current implementation provides:

Entity	Type	Description
Tethercell X FET	Switch	Turns the battery output on or off
Tethercell X Battery voltage	Sensor	Reports the current battery voltage

This allows Tethercells to be used directly in Home Assistant dashboards, scripts, and automations.

For example, a Tethercell could be switched on only when another condition is met, or its battery voltage could be monitored to determine when the battery needs to be replaced.

Bluetooth Communication

The ESPHome implementation communicates directly with the Tethercell using its BLE service.

The following UUIDs are currently defined by the integration:

UUID	Purpose
5EC0FFF0-3CF2-A682-E211-2AF96EFDF667	Tethercell service
5EC0FFF1-3CF2-A682-E211-2AF96EFDF667	Family
5EC0FFF2-3CF2-A682-E211-2AF96EFDF667	FET state
5EC0FFF3-3CF2-A682-E211-2AF96EFDF667	Battery voltage
5EC0FFF4-3CF2-A682-E211-2AF96EFDF667	Timers
5EC0FFF5-3CF2-A682-E211-2AF96EFDF667	Timer access index
5EC0FFF6-3CF2-A682-E211-2AF96EFDF667	Most recent battery voltage index
5EC0FFF7-3CF2-A682-E211-2AF96EFDF667	Battery voltage history
5EC0FFF8-3CF2-A682-E211-2AF96EFDF667	Password
5EC0FFF9-3CF2-A682-E211-2AF96EFDF667	Device name
5EC0FFFA-3CF2-A682-E211-2AF96EFDF667	UTC time
5EC0FFFB-3CF2-A682-E211-2AF96EFDF667	Advertising period
5EC0FFFC-3CF2-A682-E211-2AF96EFDF667	Authorization

Not all of these characteristics are currently exposed as ESPHome entities. They are included in the configuration because they are part of the Tethercell BLE interface and may be useful for future development.

Authentication

The Tethercell requires authentication before its service can be used.

The configured PIN is sent to the authorization characteristic when the ESP32 connects to the Tethercell. Authentication is also performed again before changing the FET state.

The PIN is configured as part of the ESPHome package:

vars:
  tethercell_pin: 00000000

Do not commit your personal Tethercell PINs to a public repository.

If you are sharing your ESPHome configuration, replace the actual PIN with a placeholder.

Project Structure

The repository currently contains the reusable ESPHome configuration:

esphome-tethercell/
├── esphome_tethercell_generic.yaml
├── README.md
└── LICENSE

The esphome_tethercell_generic.yaml file contains the BLE client configuration, authentication logic, battery voltage sensor, and FET switch.

Credits & Attribution

This project builds on the excellent groundwork provided by Sandeep Mistry in the original node-tethercell project.

The original project was a Node.js library for communicating with the Tethercell and provided important information about the Tethercell's Bluetooth Low Energy interface. It implements functionality for discovering and connecting to Tethercells, authenticating with a PIN, controlling the FET, and reading the battery voltage.

The BLE service and characteristic UUIDs used by this ESPHome integration are based on the UUID definitions from the original node-tethercell project. The battery voltage conversion and FET control behavior also follow the protocol information implemented there.

Many thanks to Sandeep Mistry for reverse-engineering and documenting the Tethercell BLE interface and for making this work available to the community.

Original project:

https://github.com/sandeepmistry/node-tethercell

The original node-tethercell project is released under the MIT License.

This ESPHome implementation is a separate project and is not affiliated with, sponsored by, or endorsed by Sandeep Mistry.

Project Status

This project is currently focused on the basic functionality required to integrate Tethercells into ESPHome and Home Assistant.

Currently implemented

• BLE connection
• PIN authentication
• FET ON/OFF control
• Battery voltage monitoring
• Multiple Tethercell support
• ESPHome package configuration

Potential future improvements

The Tethercell exposes additional BLE characteristics that are not yet implemented as ESPHome entities.

Possible future improvements include:

• Reading and displaying the current FET state
• Device name support
• Timer support
• Battery voltage history
• Additional Tethercell configuration options
• Improved connection and error handling
• Further investigation of the Tethercell BLE protocol

Contributions, testing, and additional protocol information are welcome.

Disclaimer

This project is provided as-is and is intended for experimentation, reuse, and integration with ESPHome.

The authors of this project are not responsible for damage resulting from the use or misuse of this software or hardware.

License

This project is released under the MIT License.

See LICENSE for the full license text.

Links

• This project: https://github.com/w00dst0ck/esphome-tethercell
• Original node-tethercell project: https://github.com/sandeepmistry/node-tethercell
• ESPHome: https://esphome.io/
• Home Assistant: https://www.home-assistant.io/

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Made to give old Tethercells a new life with ESPHome and Home Assistant.