The API for card.voltage mentions
Provides the current V+ voltage level on the Notecard, and provides information about historical voltage trends. When used with the mode argument, configures voltage thresholds based on how the device is powered.
But there is no V+ pin on Notecard. After some hunting, I saw V+ is on Notecarrier F which is routed, together with VBAT and VUSB to VMAIN and then to VMODEM_P, so it looks like VMODEM_P voltage is what is driving the voltage-sensing.
The voltage-variable modes have a usb setting (like usb:4.6 for lipo mode). Does the state of the VUSB pin (3.3-5v) play a part in setting the usb voltage category independently of the voltage on VMODEM_P?