Specs: BP-TCA-12/2510 SN battery, rated at 4V 4400mAh 17.6 Wh

Battery charge percentage, from 100% (4 VDC) to 0% (3 VDC), over time. Here we are powering a simple Raspberry Pi Zero W. We start at 100% battery (4V) at around midnight (00:00) and we get to 0% battery (3V) after 24 hours.

It’s noteworthy that the last stable/flat battery level is at 40% battery, after which time it starts discharging faster:

This means that on a UPS uninterrupted power supply, you should set your Raspbery Pi to shut down when the battery percentage reaches around 40%, for this specific battery.