XIAO nRF52840 Sense - Charging consistently stalls at 3.7V, never reaches 4.2V

Products & Technology XIAO
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Hi all,

I’m having a persistent charging issue with my XIAO nRF52840 Sense and would appreciate any insight.

Setup:

  • Board: XIAO nRF52840 Sense
  • Battery: 2-pin LiPo (JST 1.25mm connector, no TS wire)
  • Charging via USB-C, tested with both a PC port and a standalone USB wall adapter

Symptom:
The battery voltage consistently stalls around 3.7V and never progresses further toward the expected 4.2V full-charge voltage. The charge indicator (via P0.17 CHARGE_LED pin) shows “charging” for a while, then switches to “not charging” and stays there - the voltage does not continue climbing.

If I unplug the USB cable and wait at least ~10 seconds before reconnecting, charging resumes briefly, and the voltage ticks up a small amount - but it reliably stalls again at the same ~3.7V point. Repeating this cycle many times has not gotten the battery past 3.7V.

What I’ve already ruled out:

  • No heat from the board or battery during/after charging attempts
  • Verified continuity and voltage on all relevant pins (VCC/GND to battery connector) with a multimeter - all normal, 3.3V present as expected
  • Tested with a completely bare-minimum sketch (empty loop(), no BLE, no sensor polling) to rule out firmware/current-draw interference - same stall behavior occurs
  • Confirmed via datasheet that the charge IC is the BQ25101 (250mA linear charger, 4.2V charge voltage, 3-phase precharge/CC/CV charging)
  • Reconnecting the cable (power-cycling VBUS) does trigger a brief resumption of charging, consistent with the datasheet’s note that cycling input power resets the CHG pin state - but the underlying stall recurs almost immediately at the same voltage

Question:
Has anyone else observed charging reliably stalling at a specific voltage (not just slow tapering near 4.2V, but a hard stop well before it)? Is this a known behavior/limitation of the BQ25101 on this board, or does it point to something specific I should check (e.g., is a TS thermistor required even though the board ships supporting 2-pin batteries by default)?

Any guidance would be greatly appreciated. Thanks!