An application note (PDF) from MAXIM.
The typical specification for lithium coin-cell batteries has been to provide a 10-year battery lifetime in the absence of system power. End users should evaluate the anticipated lifetime in their specific application, especially for those that exceed typical commercial environments or that need to last more than 10 years. This article gives the reader an overview of the major factors affecting the lifetime of an IC that can be powered by either the system power or a lithium battery for a backup supply.