Calendar vs Cycle Battery Degradation

  • Marc Gaulier
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19 Sep 2019 16:39 #7667 by Marc Gaulier
Calendar vs Cycle Battery Degradation was created by Marc Gaulier
I was hoping to get a better understanding of how the SAM integrates the two types of battery degradation, cyclic and calendar. Is the overall degradation taken to be the maximum degradation level between the two, or is there some weight applied to each? I have run simulations with standalone batteries where only cycle degradation seems to be a factor (typically in cases with regular daily cycling and high DoD). Conversely, I have run simulations with almost no regular cycling and the degradation there seems to primarily follow the calendar degradation regime. There are in-between cases with low-DoD daily cycling where the overall degradation appears to switch between a linear (cyclic) and exponential (calendar) degradation regime (see attached screenshot). I would like to understand better how the model determines which type of degradation to apply at a given timestep.

Thank you,

Marc

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  • Paul Gilman
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24 Sep 2019 17:23 #7672 by Paul Gilman
Replied by Paul Gilman on topic Calendar vs Cycle Battery Degradation
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.

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