Understanding your calendar aging result
Why a battery ages even when it's never used
Calendar aging comes from slow chemical side reactions inside the cell that happen continuously, not just during charge and discharge cycles, a mechanism NREL's energy storage research program has documented in detail across chemistries. This is why a battery sitting in storage or used only occasionally for backup still loses some capacity over time, even without heavy cycling.
Why state of charge matters more than most people expect
A cell held at a high state of charge keeps its electrodes in a more chemically reactive state, which accelerates the same side reactions that drive calendar aging. Storing at roughly 50% charge instead of 100% can cut the calendar aging rate roughly in half, one of the simplest adjustments available without changing hardware, and it's exactly the setting the seasonal cabin battery storage over winter tool walks through for a system that sits idle for months at a time.
How storage temperature compounds with state of charge
Temperature and state of charge don't just add together, they multiply, so a battery stored both hot and full ages considerably faster than either factor would suggest alone, an interaction NREL's lithium-ion lifetime modeling work factors directly into its capacity fade projections. The temperature derate calculator covers the separate, more immediate effect temperature has on capacity output rather than long-term aging.
What to check next
Once you know your calendar aging rate, the end of life threshold calculator lets you set a custom cutoff instead of the standard 80% mark, and the degradation vs usage pattern calculator shows how this compares across different real-world use cases. For a rough replacement year rather than just an annual rate, the battery lifespan estimator carries the number the rest of the way.