Why doesn't work a external Battery on RPI Pico

Question

I have connected an SGP30 sensor to my Pico and display the measurement results on a small OLED display that is also connected to the Pico. As long as I operate the whole thing via USB with the RPI, it works great (a MicroPython program runs independently on the Pico). Now I wanted to connect a 4.5 V external battery voltage to VSYS, but unfortunately that doesn't work. Is 4.5V too little for this configuration. How should I switch it to operate the Pico with sensor independently, without USB? I would be very grateful for some tips.