Tell us about yourself!
Putting recycled 18650 cells in series and charging them without current balancing circuit WILL result in fire and/or explosion.
Homemade DIY Power Bank using 18650 battery
How would you guys handle undervoltage protection? this device doesn't have it, it will basically draw power from the batteries untill they are at 0.9 volts, at which point the batteries will be dead and unstable.
I've found out your circuit actually has the undervoltage protection, but that is because u use an upgraded version of the tp4056 with protection module instead of the cheapest one!
Variable Lab Power Supply
How to build a 24V Power Supply from 2 ATX PSU
Make your own breadboard
The 10 Cent Mini-Survival Knife...v.2.x.....