Hi Marc,
SAM reports a "battery capacity percent for lifetime" value in each time step of the simulation. This value shows how the battery capacity degrades over time and is restored when the batteries are replaced.
There are two options for modeling battery capacity degradation on the Battery Storage input page: The cycle degradation model reduces the available capacity as the number of charge and discharge cycles increases. This is the default option and is always enabled. (You can effectively disable it by modifying the Capacity vs Depth-of-discharge table so that the capacity is always 100%.) The calendar degradation model reduces available capacity as the age of the battery increases, and offers an option for lithium-ion batteries that accounts for the effect of temperature on battery life.
When you enable the calendar degradation model, for each time step of the simulation, both the cycle and calendar models apply, and SAM chooses the minimum "battery capacity percent for lifetime" value so that worst-case degradation estimate dominates.
If you are interested, you can see where that happens in the C++ source code on GitHub.com -- it's in the runLifeTimeModels() function around Line 788:
github.com/NREL/ssc/blob/develop/shared/lib_battery.cpp. That function is called in
github.com/NREL/ssc/blob/develop/ssc/cmod_battery.cpp.
Best regards,
Paul.