The lithium ion polymer battery that I used for this project does not properly hold a charge anymore.
I figured an 18650 is a good substitute since I have lots lying around. The only problem is that it's way way bigger than a polymer battery. I could not fit it inside the case so the solution I came up with is to put the battery outside.
It's now uglier than ever. And I love it.
Also, I learned form the ESPBoy project that you could change the resistor on the TP4056 charging module to change the charging current used for the battery.
This meant that if I wanted to ensure a longer battery life I need to make sure that I choose the correct resistor value. It is recommended that the charging current is half or less of the battery capacity.
So I used a 2.2k resistor since my 18650 battery has a 1200 mAH capacity. Not sure if I did it correctly but it seems to be working. We will see just how well soon.