Robot batteries take a lot of beating. We don't treat them well at all; especially when they are discharged beyond what is reasonable. Our batteries are not labeled "deep cycle" yet we often treat them as such in practice and competition. So let's make sure we retire old batteries before they surprise us and die at the wrong time.
We're on a schedule where we keep batteries 4 years. So for 2017 (2016-7 school year) we retired batteries bought in FRC 2013.
All batteries should be labeled with the year of acquisition.
Retired batteries can still be used for other purposes such as lighting in the outside container, test-bed use, etc.
When they finally totally die, they need to be properly recycled (they contain lead).