CTEK CS ONE Gen 2 — ESPHome BLE Gateway

An ESPHome configuration that creates a BLE gateway for the CTEK CS ONE Gen 2 battery charger, exposing charging data as sensors in Home Assistant.

This is believed to be the first open-source integration for the CTEK CS ONE Gen 2. The device launched in January 2026 and uses BLE 4.2 Secure Connections with MITM-protected bonding, which required reverse engineering the GATT protocol via Android HCI snoop logs and Bluetooth LE Explorer.

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Hardware Required

• CTEK CS ONE Gen 2 battery charger
• ESP32 board with BLE support (see notes below)
• USB-C power supply for the ESP32

ESP32 Board Requirements

Any ESP32 board with BLE support should work. The minimum requirements are:

• ESP32 with BLE 4.2 or later
• Sufficient flash for ESPHome firmware (~2MB minimum, 4MB recommended)
• WiFi support

This configuration was developed and tested on a Waveshare ESP32-C3-Zero-M. If you use a different board, note that:

• ESP32-C3, S2, S3, C6 variants require framework: esp-idf and may need other minor adjustments
• Original ESP32 (Xtensa) can use framework: arduino which may require changes to the BLE lambda syntax and LED platform
• The LED configuration assumes a WS2812 RGB LED — if your board has a different LED or none at all, remove or adjust the light section

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Features

The following sensors are exposed to Home Assistant:

Sensor	Unit	Notes
Battery Voltage	V	Sampled voltage at battery terminals
Charger Voltage Out	V	Output voltage from charger
Charge Current	A	Current being delivered
Charger Temperature	°C	Internal transformer temperature
Battery SOC	%	State of charge estimate
Estimated Battery Capacity	Ah	CTEK's auto-detected capacity
Voltage Setpoint	V	Target charge voltage
Current Setpoint	A	Target charge current
Remaining Time	h	Estimated time to full charge
Total Charge Time	s	Cumulative charge time
Charge Status	-	Raw status value (decoding in progress)
Detected Battery Chemistry	-	Raw chemistry value (decoding in progress)
Available Programs	text	e.g. APTO, RECOND, SUPPLY
Firmware Version	text	Charger firmware version string

Controls:

• CTEK BLE Connection switch — enable/disable BLE connection
• Status LED — WS2812 RGB LED (green = connected, red = disconnected)

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How It Works

The ESP32 acts as an authenticated BLE GATT gateway:

1. Scans for the CTEK CS ONE via esp32_ble_tracker
2. Connects and requests MITM-encrypted bonding via esp_ble_set_encryption
3. Responds to the passkey request automatically using the configured PIN
4. Polls GATT characteristics from service f00a0001-d91a-4679-9261-542c9bc01626
5. Exposes all values as Home Assistant sensors via the ESPHome API

The BLE connection is exclusive — the CTEK app on your phone cannot connect while the ESP32 is connected. Close the app to allow the ESP32 to connect, or disable the CTEK BLE Connection switch in HA temporarily.

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Installation

Prerequisites

• ESPHome installed (CLI or dashboard)
• Home Assistant with ESPHome integration
• Your CTEK CS ONE Gen 2 PIN (shown during initial app pairing)
• Your CTEK's Bluetooth MAC address (visible in the CTEK app or a BLE scanner)

Setup

1. Clone this repository:

   git clone https://github.com/zeg064/CTEK-CS-One---ESP-BLE-Gateway
   cd CTEK-CS-One---ESP-BLE-Gateway

2. Create your secrets.yaml (not included in this repository):

   wifi_ssid: "YourWiFiSSID"
   wifi_password: "YourWiFiPassword"
   api_encryption_key: "YourAPIEncryptionKeyHere"
   ota_password: "YourOTAPasswordHere"
   fallback_password: "YourFallbackPasswordHere"
   ctek_mac: "XX:XX:XX:XX:XX:XX"
   ctek_passkey: "000000"

3. Generate an API encryption key if you don't have one:

   esphome generate-secret

4. For the first flash, put your ESP32 into bootloader mode (typically hold BOOT while connecting USB), then compile:

   esphome compile ctek_cs1_esphome.yaml

   Flash firmware.factory.bin via https://web.esphome.io

5. For subsequent updates, use OTA:

   esphome run ctek_cs1_esphome.yaml

First Connection & Bonding

On first boot the ESP32 will attempt to bond with the CTEK. The passkey exchange happens automatically using the PIN in secrets.yaml. The bond is stored in the ESP32's flash and subsequent connections don't require re-pairing.

If bonding fails, power cycle the CTEK charger and restart the ESP32.

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Adding to Home Assistant

Once flashed and running, the device will be auto-discovered by HA via the ESPHome integration. Accept the device and it will appear under Settings → Devices & Services → ESPHome.

All sensors are grouped under a single CTEK CS ONE device.

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Known Issues & Unknowns

• Charge Status reads a raw numeric value — the meaning of individual bits is not yet decoded. Capturing values at different charge states (bulk, absorption, float, supply mode) would help decode this.
• Charger State characteristic (f00a0037) is not always found during service discovery — under investigation.
• Detected Battery Chemistry returns a raw number — mapping to chemistry types (AGM, GEL, lithium etc.) not yet confirmed.
• Estimated Battery Capacity may not match actual battery capacity until the CTEK has completed several charge cycles in APTO mode.
• The CTEK app and this gateway cannot be connected simultaneously.

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Reverse Engineering Notes

The integration was developed by:

1. Capturing BLE HCI snoop logs from an Android device using adb bugreport
2. Using Bluetooth LE Explorer on Windows to enumerate all GATT services, characteristics and UUIDs with human-readable descriptions
3. Cross-referencing characteristic handles from the HCI logs with the UUID map from Bluetooth LE Explorer to identify sensor data
4. Iterating on ESPHome config to handle BLE 4.2 Secure Connections authentication

All data characteristics are big-endian uint16 values unless otherwise noted. The 4-byte Total Charge Time is big-endian uint32.

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License

MIT License — see LICENSE for details.

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Compatibility

This configuration was developed and tested on the CTEK CS ONE Gen 2 (with WiFi).

The CTEK CS ONE Gen 1 (Bluetooth only) may also work as if uses the same GATT service structure, however this is untested. If you test this with a Gen 1 device please open an issue or PR with your findings.

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Contributing

PRs welcome, particularly for:

• Decoding the Charge Status bitfield
• Identifying the Charger State characteristic
• Mapping Detected Battery Chemistry values
• Testing with different battery types and charge modes
• Testing on different ESP32 board variants