Raspberry-USB-UPS

Simple DC UPS circuit for every kind of device!

You must read me!

This circuit is far from being perfect, but it works like a charm with my Raspberry Pi 3B! Here are some notes about the circuit:

You must attach a dissipator to the LM317 because when the Pb battery is discharged, it will limit the current by dissipating power through himself.
I also used a little fan because I chose a very little dissipator for space reasons.
You can set the charging voltage as usual (classical LM317 formulas).
You can set the maximum charging current by the formula R4/Vbe, where the Vbe of Q1 is typically between 0.6V and 0.7V.
You can use every type of DC converter at the output (Buck, Boost ecce cc...) to fix the voltage, it's up to your needs; you can also check my THT-BuckConverter.
The output max current is given by the two diodes: you can choose for D2 every Schottky diode you like with a high max current and power dissipation and whatever you want for D1 (just make sure it won't burn out due to excessive power dissipation).