[SOLVED] Red light flashes next to barrel-port

[xyzzy]

Unfortunately, the red LED starts flashing after about half an hour of the compilation described in the previous post, with the laptop in upside-down position.  However, it's pretty safe to assume that the root cause is heat soak from the SoC.

Could it be battery temperature?  The STAT led will blink if the battery temp sensor is out of range.

It's not clear to me if the STAT led will blink on thermal shutdown, from §9.3.20:

As added level of protection, the charger converter turns off and self-protects whenever the junction temperature exceeds the TSHUT threshold of 150°C

It sounds like maybe it just turns off, rather than blink.  It does limit charge current at 120°.  Perhaps that causes blinking?

Also, I don't think the SoC can get to 150.  It should shutdown well before that.  If the SoC is at 90°, there's no way to can raise the temperature anywhere else higher than that.

The MOSFETs for the charger are internal to the BQ24171, and the power supply design of the PBP forces the BQ24171 to regulate the 5V input down to the battery 4.2V even when the battery is fully charged.  A more sophisticate design would allow the BQ24171 to bypass the battery when fully charged and directly connect the DC input.

This means the QB24171 is dissipating not insignificant amounts of power itself and might be hitting its thermal limit without help from the SoC.