Charging a 18650 battery directly with a solar panel and a voltage regulator?

I know a TP4056 would be the best solution, but I don't have it and I need to finnish this project.
I was thinking if I reduce the solar panels 5V-ish voltage to about 4.15V with a voltage regulator circuit plus the needed diode then can I charge my 18650 Li-Ion cell?

I'm told that if you continuously charge an 18650 at 4.2V, it will eventually swell up, then catch fire or explode. That's why Li-ion and LIPO chargers terminate charging when the battery is charged. I don't know if that's also true of 4.15V, but would just remind you that charging lithium batteries is one of the few things in electronics not involving mains power that can still mess up your life if you do it wrong.

In addition to the terminal voltage mentioned by @ShermanP you need to limit the current to < 500 ma for safe charging of the cell.

There are boards that are designed to properly charge the 18650 batteries available for less maybe a buck or so available on eBay. They worked very well for me. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!

You not only have to keep the max charge current/voltage within spec, you need to dissconnect the battery from the load when it gets below its minimum output voltage.

Does your project involve a micro-controller, you may be able to write some code to dissconnect your PV cell when the battery approaches 4.2V.

Tom... :slight_smile: