In the next iteration, I would like to integrate a solar panel into the PionEar sensor to avoid the need for charging. I did some research for suitable low-power solar charger modules with MPTT functionality. I discovered AMELION from @Jasper Sikken. In combination with IXYS SLMD960H09L solar panel, it seemed to be a good starting point. AMELION module is a great solution for low-power applications. It can still provide some current even in the indoor environment. I could measure the following charging currents:

Indoor: 20 to 50 uA

Outdoor (cloudy/shadow): 2 to 5 mA

Outdoor sunlight: 25 to 30 mA

These numbers seem to be promising. I think that behind the car's front window, there should be plenty of light to keep the battery charged - unless you park in the garage.