J7-C USB Tester Data Logger

Recently bought a J7-C USB tester to measure charging voltage/current.

It also features Bluetooth interface to connect to Android apk or PC app for data logging.

But I'm a little squirmish about side-loading application from a dubious mediafile folder.

Luckily, I've found some discussion regarding the Bluetooth protocol on Arduino forum USB Digital Tester (BT) Protocol

After a little experiment and reverse engineering, I've found that my device seems to output the data in a slightly different format. Once we open the Bluetooth serial port, it will send a 36-byte packet every 1 second. Example packet received:

ff 55 01 3 00 01 f4 00 0 34 00 0f fa 00 0 8 52 00 09 00 0a 00 20 00 03 2c 18 3c 0c 80 00 00 03 20 00 9e

Comparing the the info from the Arduino forum, it seems the packet format is still mostly similar but slightly offset as below:

Offset	Data Byte	Multibyte hex	Value	Data Type
0	ff
1	55
2	01
3	03
4	00	0001f4	5.00	voltage
5	01
6	f4
7	00	000034	0.52	current
8	00
9	34
10	00	000ffa	4090	mAh
11	0f
12	fa
13	00	00000852	21.30	energy, Wh
14	00
15	08
16	52
17	00	0009	0.09	D+ voltage
18	09
19	00	000a	0.1	D- voltage
20	0a
21	00	0020	32	CPU temperature (celcius)
22	20
23	00	00	03:44:44	duration day:hh:mm:ss
24	03	03
25	2c	2c
26	18	18
27	3c
28	0c
29	80
30	00
31	00
32	03
33	20
34	00
35	9e

It does not seems to provide power value but we can easily calculate power P = V x I

So, I've written a simple python script to read the data and save it as CSV file so that it can be processed by other apps such as Excel or LibreOffice.

Here's the steps:

1. Connect to the bluetooth serial port.
2. Run python3 j7-c_usb_tester.py --csv out.csv <bluetooth_port>
   • for Linux:
     • /dev/rfcomm0 for legacy Linux bluetooth rfcomm device
     • bluetooth MAC address for example: D6:E6:53:AA:BB:CC
   • COM port on Windows.
3. Press Ctrl-C to exit after you are done logging the data.

Example data

I've recorded the measurements when charging my OnePlus phone with the 65W SuperVOOC charger in supervooc_2022-09-03_131744.csv.

Using LibreOffice to open the CSV file, I've plotted the charging curve as below:

Web Interface

With --web=<port>option, it will create a nicer web dashboard interface, but will require python bottle framework. For example:

python3 j7-c_usb_tester.py --relative --web=8080 <bluetooth_port>

Note: On Linux with Bluetooth MAC addresses, UV's managed Python doesn't support socket.AF_BLUETOOTH. Use your system's Python installation instead.

$ uv sync --no-managed-python
Using CPython 3.12.3 interpreter at: /usr/bin/python3
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Resolved 3 packages in 7ms
Prepared 2 packages in 158ms
Installed 2 packages in 5ms
 + bottle==0.13.4
 + pyserial==3.5

$ uv run ./j7-c_usb_tester.py --relative --web=8080 D6:E6:53:AA:BB:CC
Web dashboard: http://localhost:8888/