The reason this Uninterrupted Power Supply (UPS) was created was because I wasn't satisfied with the things available on the market that would just lower the incoming voltage to charge the battery and immediately boost it to power the pi. This way efficiencies of around ~80% can be achieved. Don't get me wrong this is not terrible but I wanted to do better, which is why this design incorporates a power MUX.

For those who don't know a power MUX is just like a regular MUX but instead of signal lines it switches sources of power. My schematic is shown below:

Here the R7 and D1 create a reference voltage that is around 2.4V this is used to set the minimum desired voltage before switching over to the DC-DC converter that constantly generates 5.1V from the battery. USB input voltage (+5V) is divided by 2 using a voltage divider formed by R2 and R3 and fed in to the non invering input of the op amp thus when the voltage is above 2 * 2.4 = 4.8V the USB input is directly sent to the pi. Whenever the voltage drops below 4.8V it switches over to the uninterrupted supply.

The schematic for which can be seen below:

It is a TPS61088 with it's standard reference schematic and a lot of output and input capacitance to make sure that suddenly connected loads by the power MUX are handled correctly. There are also 2x 3.5A polyfuses on the input line to protect the IC and the output in case of a short circuit. The EN pin is connected to a switch (for turning off the DC-DC converter) between ground and VSYS which is a power line that the battery management IC creates for internal use.

The schematic for which is below:

This design is also largely based on the reference schematic, both Type C and Micro USB can be used for the input on this unit, as well as an external battery that is connected through J6.

This design works pretty well however there are a few issues that are better documented on the github. These will be fixed in the next revision.

Both versions have some flaws, the project is not fully finished yet, I would recommend only ordering it if you intend to develop it further/fix the errors that matter to you.