Traditional UPS devices convert back and forth to AC and there is a huge loss because of that. I have the UPS that @dJOS made which is a awesome piece of tech. It is slick and with just two LiFEPo4 batteries i get over 12 hours of backup. Do i really need that much no, but It makes it allot easier.
I think this is about the voltage you charge the battery to. In generally from a perspective of keeping the battery healthy for as long as possible it is generally not optimal to charge a battery to it's max voltage. I believe with @dJOS setup his ups he set it to charge to a voltage slightly below the max charge value to improve battery life.
If you are taking this on you may want to reach out to @dJOS and see where he left off. I think at one point there was talk about that solution being enhanced with a micro controller so it could talk to the hub directly.
This is the UPS that resulted from that USB-C battery backup i mention above
