Howto connect to BMS with BLE (working sample in gatttool) #1863

RonGroen · 2025-11-22T22:28:43Z

RonGroen
Nov 22, 2025

I want to connect with python to my BMS.
Have been debugging with wireshark and now have a working sample in gatttool

(timeout 10) gatttool -b 53:0a:f5:3b:48:ff --char-write-req -a 0x000c -n aa21002100 --listen >>output2100.txt

result in Wireshark (to BMS)
0000   02 02 0e 0c 00 08 00 04 00 12 0c 00 aa 21 00 21
0010   00
response
Notification handle = 0x000e value: aa 21 1a e0 33 00 00 74 fa ff ff 57 64 b2 f9 03 00 e0 93 04 
Notification handle = 0x000e value: 00 04 00 15 1a 1a 1a 16 1a 31 09

Which I can relate to the data I want to see.
I already made a lot of test python/bleak programs but none of them sends the correct data to the BMS
So the BMS does not respond correctly.

the last thing I tried was:

await client.write_gatt_char(b'\x00\x0c', b'\xaa\x21\x00\z21\x00',response=True)

but that does not produce the right data to the BMS

Any help will be appreciated.

dlech · 2025-11-23T00:34:18Z

dlech
Nov 23, 2025
Collaborator

Bleak accesses characteristics by UUID rather than by handle. So the first argument to client.write_gatt_char() is wrong. You can use the service_explorer.py example to figure out what the UUID is for each characteristic.

0 replies

RonGroen · 2025-11-24T22:16:46Z

RonGroen
Nov 24, 2025
Author

Thank you:
await client.write_gatt_char("00000002-0000-1000-8000-00805f9b34fb", b'\xAA\x23\x00\x23\x00',response=True)
did the trick, I could see with WireShark.
But how do read the response in Python?
I read trough the documentation and looked at the examples names, but can't find it...

1 reply

dlech Nov 24, 2025
Collaborator

The response is just success or error. Python will raise and exception if there is an error. If no exception was raised, then it was successful.

You are probably asking about notifications though, which would be the start_notify() method.

RonGroen · 2025-11-24T22:54:53Z

RonGroen
Nov 24, 2025
Author

Thank you again:
with start_notify() I checked the function description and the examples/enable_notifications.py
So I came up with:

def notification_handler(characteristic: BleakGATTCharacteristic, data: bytearray):
    print(f"response using callback function: {data}")

and

	CMD_UUID = "00000002-0000-1000-8000-00805f9b34fb"
	await client.start_notify(CMD_UUID, notification_handler)
	await client.write_gatt_char(CMD_UUID, b'\xAA\x23\x00\x23\x00',response=True) # this send data to device
	await client.stop_notify()

I get:

    await client.start_notify(CMD_UUID, notification_handler)
  File "/usr/local/lib/python3.10/dist-packages/bleak/__init__.py", line 799, in start_notify
    await self._backend.start_notify(
  File "/usr/local/lib/python3.10/dist-packages/bleak/backends/bluezdbus/client.py", line 930, in start_notify
    assert_reply(reply)
  File "/usr/local/lib/python3.10/dist-packages/bleak/backends/bluezdbus/utils.py", line 27, in assert_reply
    raise BleakDBusError(reply.error_name, reply.body)
bleak.exc.BleakDBusError: [org.bluez.Error.NotSupported] Operation is not supported

Can you see what I am doing wrong?

1 reply

dlech Nov 24, 2025
Collaborator

Is the notification on a different characteristic? Again, this is where the service_explorer.py example comes in handy.

RonGroen · 2025-11-24T23:14:40Z

RonGroen
Nov 24, 2025
Author

I don't understand I am afraid
service_explorer.py said:
thank you! service_explorer.py reports:

2025-11-24 21:32:37,332 __main__ INFO: starting scan...
2025-11-24 21:32:37,453 __main__ INFO: connecting to device...
2025-11-24 21:32:39,761 __main__ INFO: connected to HS48FF (53:0A:F5:3B:48:FF)
2025-11-24 21:32:39,761 __main__ INFO: [Service] 00000001-0000-1000-8000-00805f9b34fb (Handle: 10): SDP
2025-11-24 21:32:39,761 __main__ INFO:   [Characteristic] 00000003-0000-1000-8000-00805f9b34fb (Handle: 13): RFCOMM (notify)
2025-11-24 21:32:40,157 __main__ ERROR:     [Descriptor] 00002902-0000-1000-8000-00805f9b34fb (Handle: 15): Client Characteristic Configuration, Error: [org.bluez.Error.Failed] Operation failed with ATT error: 0x80 (Unknown code)
2025-11-24 21:32:40,157 __main__ INFO:   [Characteristic] 00000002-0000-1000-8000-00805f9b34fb (Handle: 11): Vendor specific (write)
2025-11-24 21:32:40,157 __main__ INFO: [Service] 00001801-0000-1000-8000-00805f9b34fb (Handle: 6): Generic Attribute Profile
2025-11-24 21:32:40,246 __main__ INFO:   [Characteristic] 00002a05-0000-1000-8000-00805f9b34fb (Handle: 7): Service Changed (read,indicate), Value: bytearray(b'\x01\x00\xff\xff')
2025-11-24 21:32:40,428 __main__ INFO:     [Descriptor] 00002902-0000-1000-8000-00805f9b34fb (Handle: 9): Client Characteristic Configuration, Value: bytearray(b'\x02\x00')
2025-11-24 21:32:40,428 __main__ INFO: disconnecting...

How should be my start_notify and stop_notify lines?

1 reply

dlech Nov 25, 2025
Collaborator

[Characteristic] 00000003-0000-1000-8000-00805f9b34fb (Handle: 13): RFCOMM (notify)

This line gives the UUID of the characteristic that has the notify property. So this is the UUID you need to use with start_notify().

[Descriptor] 00002902-0000-1000-8000-00805f9b34fb (Handle: 15): Client Characteristic Configuration, Error: [org.bluez.Error.Failed] Operation failed with ATT error: 0x80 (Unknown code)

This line should not have an error. It probably means the device vendor messed up and didn't implement the descriptor correctly. Hopefully writing to the descriptor works even if reading doesn't. Otherwise you likely won't be able to use it with BlueZ because it doesn't follow the Bluetooth specifications on how it should work.

RonGroen · 2025-11-25T07:43:40Z

RonGroen
Nov 25, 2025
Author

you were right:
await client.start_notify("00002902-0000-1000-8000-00805f9b34fb", notification_handler)
gave
bleak.exc.BleakCharacteristicNotFoundError: Characteristic 00002902-0000-1000-8000-00805f9b34fb was not found!
So bleak can´t help me to read data from my BMS. Any suggestions to solve it in an other way?
remember: gatttool -b 53:0a:f5:3b:48:ff --char-write-req -a 0x000c -n aa21002100 --listen just works fine.

Thank you for all your Help!

1 reply

dlech Nov 25, 2025
Collaborator

Try it with 00000003-0000-1000-8000-00805f9b34fb.

00002902-0000-1000-8000-00805f9b34fb is the UUID of the descriptor, not the characteristic, so that will fail for sure.

RonGroen · 2025-11-27T08:38:31Z

RonGroen
Nov 27, 2025
Author

YES!!
That did the job :-)
Thank you so much!!
I think I can make the rest of the program from here.
Final lines:

def notification_handler(characteristic: BleakGATTCharacteristic, data: bytearray):
    print(f"response using callback function: {data}")    

async def run():
	adres="53:0A:F5:3B:48:FF"
	client = BleakClient(str(adres))
	await client.connect()
	CMD_UUID =  "00000003-0000-1000-8000-00805f9b34fb"
	CMD_UUID2 = "00000002-0000-1000-8000-00805f9b34fb"
	await client.start_notify(CMD_UUID, notification_handler)
	await client.write_gatt_char(CMD_UUID2, b'\xaa\x23\x00\x23\x00',response=True) # this send data to device
	await client.stop_notify(CMD_UUID)

if __name__ == "__main__":
    asyncio.run(run())

1 reply

dlech Nov 28, 2025
Collaborator

This program stops notifications immediately after the write, so may miss some notifications.

If you want to disconnect after receiving the notification, you can use some combination of asyncio.Event, asyncio.Queue and asyncio.timeout.

Or if you want to stay connected "forever", you can simply await asyncio.Event().wait().

RonGroen · 2025-11-27T21:42:02Z

RonGroen
Nov 27, 2025
Author

Unfortunately I responded to fast. the program response with the start_notify but with the wrong data... As I said before Looking at Whireshark the BMS send out the correct data, but is does not reach Python.... I tried all the other UUID from the service_explorer.py but the all end up with an error.

1 reply

dlech Nov 28, 2025
Collaborator

What is the difference between the "right" data and the "wrong" data?

RonGroen · 2025-12-01T19:58:38Z

RonGroen
Dec 1, 2025
Author

As soon as I have acces to the BMS, I will let you know.

0 replies

RonGroen · 2026-01-01T20:45:36Z

RonGroen
Jan 1, 2026
Author

For an other BMS, with Home Assistant close by, I used https://github.com/patman15/BMS_BLE-HA
It run immediately and will support my other BMS as well.
Thank you for your time, My problem is solved :-)

0 replies

Howto connect to BMS with BLE (working sample in gatttool) #1863

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RonGroen Nov 22, 2025

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