I actually have soldered the battery to the board. See results for one of the boards:
13:08:10.364 → PIN_VBAT=3654 P0_31=3693
13:08:10.882 → PIN_VBAT=3635 P0_31=3683
13:08:11.398 → PIN_VBAT=4012 P0_31=4060
13:08:11.898 → PIN_VBAT=4021 P0_31=4089
13:08:12.381 → PIN_VBAT=3654 P0_31=3683
13:08:12.881 → PIN_VBAT=3644 P0_31=3673
13:08:13.381 → PIN_VBAT=3644 P0_31=4050
13:08:13.893 → PIN_VBAT=4050 P0_31=4050
13:08:14.398 → PIN_VBAT=4031 P0_31=4070
13:08:14.898 → PIN_VBAT=3644 P0_31=3673
13:08:15.398 → PIN_VBAT=3635 P0_31=3673
13:08:15.881 → PIN_VBAT=4012 P0_31=4089
13:08:16.377 → PIN_VBAT=4041 P0_31=4079
13:08:16.881 → PIN_VBAT=4031 P0_31=4089
13:08:17.395 → PIN_VBAT=3635 P0_31=3664
13:08:17.864 → PIN_VBAT=3635 P0_31=3683