Still wondering about battery monitoring in a Raspberry Pi? Yes, I’m still wondering about that. Sure, there is that VBAT pin or whatever on the Broadcom BCM microcontroller that the Raspberry Pi uses, and purportedly that is for the microcontroller to measure the battery level, but for now, let’s assume that software access and control to that pin is off limits on the Raspberry Pi. If so, your only next best option is to set up your own circuit and software on the GPIO pins. Use a power management integrated circuit (PMIC) is another option.
20181119/DuckDuckGo raspberry pi battery voltage monitoring
20181119/https://github.com/kmcallister/rpi-battery-monitor
20181119/https://raspi.tv/2013/controlled-shutdown-duration-test-of-pi-model-a-with-2-cell-lipo
UPDATE: VBAT looks to be a voltage supply pin, not a measurement pin. Also, it is only directly accessible on the Raspberry Pi compute module.
20181119/DuckDuckGo raspberry pi VBAT
20181119/https://www.raspberrypi.org/forums/viewtopic.php?p=707418