DIY Li-ion based Uninterruptible Power Supply

Important: I don't recommend adding a capacitor at the DC input side of the circuit, it doesn't really do anything and might end up causing unexpected issues. I also ran into some problems with the input boost converter that was responsible for keeping the batteries fully charged. It would overheat and cause issues with the circuit. Might be something specific to the XL6009 or it might just be an issue with my module or the cheap protection board. I have now gotten rid of it alongside the input capacitor. With a 12V input, the 3S battery pack wont fully charge but it should still work fine enough. And the raspberry pi doesn't function super well when powered with the MP1584, I believe the buck converter is behind the issues, so ideally use a better one if you decide to build this UPS.