So it turns out that a decisive factor in the whole thing is good placement of the solar panels over the course of the day. Over the weekend they only lay in front of the window on the table and so got only so-so light. Result: even though I had switched the phone off completely overnight instead of just leaving it on standby, and so had less to charge, there is one little light fewer on the power bank, only second Advent left. Unfortunately it is still raining and is supposed to stay that way for the next few days. Now the question is whether it is better to charge outside (cold but brighter) or inside (warm but darker).
The battery charges worse in the cold; exact figures are hard to come by. From [1] you can take a rule of thumb for lithium-ion batteries in e-bikes, possibly comparable. It says you gain 12% at 20 degrees Celsius compared with 5 degrees. The light outside seems to me to be more than 12% more, but I haven't measured it yet. So out onto the balcony with the bowl.
After a few hours outside, still second Advent. But perhaps a little closer to the third. It's time for the sun. The sun is asked to shine on the gizmo, please.
[1] https://www.stuttgarter-nachrichten.de/inhalt.lithium-ionen-akku-warum-akkus-kaelte-hassen.38cd5ef8-0c27-473c-8eb1-5c8a50c2b8ee.html (archived copy; in German)