A friend and I set out on a quest to design a UPS for the RPi that really worked for higher power applications.
Most designs, even the ones I have listed on this Forum myself, suffer from a lack of true output power for the more power hungry RPi Models, like the Model 3, or when there are enough devices feeding off the USB port bus.
You will be hard pressed to find UPS designs, even commercial ones, that are able to supply and sustain more than 1 Amp to the Rpi. I purchased a couple, and all of them ended up in the "salvage for parts" bin. They were useless. For all those that will chime in and claim otherwise. Provide the proof as we did.
I designed several UPS incarnations myself over the past years, and currently use the last version every day on several of my classic Model B's. I build three, and used them for my 24x7 file server, for a data logger & web server and also while I'm doing designs and tests on my desk. It helps to protect my work and the SD card, of course.
The version I'm currently using is described here : viewtopic.php?f=37&t=145954
However, as with so many others, it is limited in the amount of power it can supply. As I said, most of them max out at about 1 Amp. As an example, I can only use mine on a very lightly tasked Model 3.
There are a few major challenges you need to overcome. One is the booster design, another is the handling of the current and then there is the heat factor. Consider that at a minimal 3V cell voltage, the booster needs to get and handle upwards of 5A from the cell to be able to supply 2.5A at 4.8V to the RPi.
My mouse-pal friend Bud Bennet (BudBennett on this Forum), an ex analog chip designer from Linear Technologies and I (no qualifications to speak of), set out to design a solution. In the process, we actually designed a few, for different applications, and stumbled several times before we got it right.
Here is the blog with some of the "gory" details but also with the complete design information :
https://hackaday.io/project/25116-singl ... spberry-pi
This design is not for the beginner, nor is it a kit. Everything is there, but if you don't have more than average smd solder skills, beware.