I'm building a simple battery backup box for a 5V IP camera. I've got a few options for battery, sealed lead battery or a mix of portable LIPO style "USB" power bricks. I've seen boards built for security systems that supply power to the system via 110 to 12V DC while keeping a sealed lead battery charged then without interrupting power switch over to the battery when the power goes out. These boards are $70-$100.
The USB battery pack bricks are nice because they regulate down to 5V built in. The one I'm testing now has a 19V power supply for charging and so far it will charge the battery while the camera pulls power from the USB connection. My concern in this configuration is that The battery will be in a constant state of use and charge while the camera pulls power.
I'm guessing the impact of constant use and charge will shorten battery life versus keeping the battery charged without use. So my goal is to find a way to connect two 5V sources, one is the factory 5V camera adapter, the other being the battery source and have it switch to the battery source when the DC adapter looses power without interrupting power to the camera. The battery has a built in charge regulator so while the camera runs off of the 5V from the DC adapter the battery will stay charged.