Here's an EBAY link to the new version of a 18650 Lithium Battery Charging Board with a better battery protection. This charges Li-Ion batteries (in parallel) as is (without the solar panel) using a micro USB cable.

18650 Micro USB Lithium Battery TP4056 Charger Module

If you look at the Micro USB socket at the top of the board, it has -ve and +ve 5V terminals either side of this socket. This is the point where we connect the solar panel, you need to ensure the solar panel delivers 5V at this point. Depending on the solar panels max output you can use diodes in series (cheapest), or a 5V buck step down converter (ideal), or a LM7805 voltage regulator.

Always use blocking diodes when working with Solar Panels!

I use 4 batteries recovered from discarded laptop battery packs, when I run the fishing lights at night, 36 LEDS, it lasts all night (10hours)

my Solar Powerbank with Underwater LED fishing lights

hope this helps ...