A very popular charging module for li-ion and li-po batteries, this module can be had for cheap and is available almost everywhere.
This module charges and monitors the charging of one 18650 cell very well, and I have not had any problems with it yet.
It is intended for USB charging, and I have retained this functionality, since this is a charger first and solar harvester second. But, if we were to check the input range of the TP8056 in the datasheet, it can take up to 8 volts as input. This is great, since our panel, at maximum tilt outputs around 8 volts.
Now, in a proper device, this would be a controversial design choice, because devices should not be run at the maximum allowable voltage, and some leeway should be created by derating parts. (Eg: you have a 20 volt rail, you should use a 35 volt capacitor etc.)
However, after testing, the chip seems to behave fine at 8 volts or even a bit above. It seems the derating has already been baked in, or this chip is more resilient than it should be.
This is great, since we can hook it up directly to the solar panel, without any other regulation or anything else.
⚠️ WARNING ⚠️
THIS IS CERTAINLY A HACK. I AM NOT LIABLE FOR ANY CONSEQUENCES THAT ARISE FROM USING DEVICES ABOVE THEIR RATED VOLTAGE LEVEL.
This should not work, but it does. And in my testing, it has not created any problems. If you want to improve on this, I would reccomend a swich mode power regulator, or a specialised solar harvesting chip/module.
The two modules are soldered in parralel, so both get the same voltage, regardless of power source (USB or solar), but USB charging seems to be way faster, since it can pull more current than the solar panel can output.
Some modules also include a DW-01 chip with the relevant mosfets attached, that acts like a overcharge/overdischarge protection module, and also overcurent/short prevention. This project can certainly do without it because it is only meant to charge the cells, but I have future plans and might use this as a solar sensor platform, and discharge control/protection is very welcome.