I would like to charge some batteries (maybe 1 lipo?) from either a 12V solar panel (about 300mA) or 2 peltiers (also about 200-300mA) in series (about 4V, but it’s really unstable and voltage changes every second) and get stable 5V for USB charging from that battery or battery pack. What is the best approach? Can you please recommend me some circuits, buck/boost IC or anything?
look for a mppt with buck boost built in. You would connect the solar panels + peltiers to the mppt, then have the output charge an intermediate battery (to allow for stable output to charge from 5v) with a 5v charger off of the battery.