Until recently I did run several Pi Zero[W]s from 1S lipo (nominal 3.7V, fully loaded 4.2V) by directly connecting to Pi GND/5V pins.
This stopped to work (steady reboots, directly when LCD gets initialized) for my first mobile Pi ZeroW. It seems that Pi CPU, Pi Wifi, 320x240 SPI LCD and wireless keyboard USB dongle were just too much for a 1S 600mAh 25C lipo. I don't understand that because the lipo should be able to deliver 3.7V at 25*0.6=15A?!?!?
I replaced the 1S lipo with a 3S 1000mAh 25C lipo and now have 12.6V fully loaded.
And use LM2596 step-down converter with voltage LCD to step down to 5V:
https://www.aliexpress.com/wholesale?Se ... wn+display
There is a switch on the step-down module that decides whether input or output voltage gets displayed.
Output voltage is interesting for calibrating the module to deliver 5V, input voltage to see when lipo needs to be recharged: