I've seen applications (Nintendo game cartridges, for example) that uses a coin cell battery for backup power to some RAM-chips (for savegames). When the ordinary 5V is gone, the battery takes over and lets the IC retain its contents.
However, won't the CR2032 (3V) take damage when connecting it to a power rail that contain almost its double voltage? Or is there some overvoltage protection within the coin cell that shuts it down when external voltage is applied?
Or how does it work?