5V Solar Charge Controller JEITA Li-ion Battery Charger | Dual USB Output, 5V/3.3V Output, 18650 Solar Charger Board, Compatible with Arduino, Raspberry Pi, STM32

Why use a battery bank? You already have the 18650 batteries, just sort of "make one" with the addition of a charging/output module. That's all those battery banks really are. If you're using a solar panel with a USB out, you can use this: https://www.amazon.com/HiLetgo-Lithium-Charging-Protection-Functions/dp/B07PKND8KG If you're using solar panels where you need to wire it up, get a board that has the proper voltage specs for your panel. As an example, if it's a 5V panel, you can use this: https://www.amazon.com/WatangTech-Solar-Manager-Compatible-Raspberry/dp/B0FQP915R9