That is, assume the following:
- You don't want to spend a lot of money (that is,more than about $20). Note that if you were willing to spend more than that, you'd just buy a regular, "mains" supply UPS, and plug your RPI's power supply into that. You want somethng at the Pi end (i.e., a battery of some sort).
- You, of course, want a battery that can be charged and be powering the Pi at the same time.
- You don't need very much off time - assume mostly just momentary power glitches. At most, say, 2 minutes of power outage is all that's needed.
- You don't need any fancy "shutdown the Pi if the power goes out" kind of circuitry/functionality. If the power does go out for "too long" and the Pi crashes, that's not the end of the world.
- You want something "off the shelf", not a custom (soldering involved) design/job.