Other Parts Discussed in Thread: BQSTUDIO, EV2400
Tool/software:
Hello,
I’m using a bq40z80 in a 6‑S Li‑ion pack design and cannot get the gauge to wake/communicate over SMBus. My schematic is attached.
System Snapshot
| Measurement / Observation | Value |
|---|---|
| PACK+ | 24 V (battery stack) |
| PBI | ~1.8 V at power‑up, decays to 0 V within seconds |
| CHG & DSG Gate | ~12 V (FETs are permanently ON) |
| SMBus (ESP32 host) | No ACK, no communication |
| PRES pin | Pulled to GND |
| Wake‑Up attempt | Momentarily shorted PACK+ to BAT+, no change |
| Removed parts | Q8, R11, D2 physically removed |
| Extras | SCL/SDA have 4.7 kΩ pull‑ups to 3.3 V on ESP32 side |
What puzzles me is that CHG and DSG gates sit at 12 V (FETs stay ON all the time) while the gauge never responds on SMBus. The 1.8 V on PBI collapses to zero after a few seconds, as if the device re‑enters SHUTDOWN. Cell voltages and wiring look correct, and I see no shorts on the board.
Questions
-
Is the rapid discharge of PBI normal, or does it indicate the device never leaves SHUTDOWN?
-
Do the permanent 12 V gate levels on CHG/DSG imply the gauge is actually awake even though SMBus is silent?
-
How can high CHG/DSG gates coexist with zero ACK on SMBus?
-
Any recommended wake‑up tests besides briefly applying BAT+ to PACK+?
-
Could removing Q8, R11 and D2 explain this behaviour?
Any insights would be greatly appreciated.
Thank you!
NOTE: I am not using EV2400 or bqStudio
