Capacity Fade: How Battery Capacity Drops Over Cycles
Capacity fade is the gradual reduction in a battery's Ah capacity over charge-discharge cycles. Capacity fade models fit this decline to a curve to predict remaining life. The knee point marks where decline accelerates.
Capacity is the total amount of electric charge a battery can deliver at a specific voltage, from a fully charged state to fully discharged. It is measured in Ampere-hours (Ah). Capacity fade is the reduction of this value over repeated charge-discharge cycles.
Why Capacity Fades
Each charge-discharge cycle causes incremental chemical and physical changes inside the cell (electrolyte decomposition, lithium plating, active material loss). These changes are largely irreversible. Over hundreds or thousands of cycles the cumulative effect is a measurable drop in Ah capacity.
Temperature accelerates the process. The Arrhenius equation describes this: the rate of chemical reactions inside the cell increases exponentially with temperature. A battery that runs consistently hot will reach end-of-life significantly faster than one kept at optimal temperature.
Capacity Fade Models
Capacity fade models are mathematical equations fitted to measured capacity data across many cycles. The goal is to predict how quickly the battery is declining and when it will reach an unusable state (typically defined as 80% of original capacity for EV applications).
The model takes cycle number (and sometimes temperature, current, and depth of discharge) as inputs and outputs an expected remaining capacity. Fitting the model to real data from the battery in operation gives a prediction specific to that cell’s actual usage history rather than a generic estimate.
The Knee Point
The capacity fade curve follows a characteristic shape: a long, relatively flat decline followed by a sudden sharp drop. The inflection point where the slope steepens significantly is called the knee point.
Before the knee point, capacity loss is gradual and predictable. After it, degradation accelerates rapidly. The battery loses usable capacity much faster than it did in earlier cycles. Reaching the knee point is effectively the end of useful battery life for most applications.
SOH monitoring and capacity fade modelling both aim to detect the approach of the knee point early enough to take action, whether that means replacing the battery, reducing charge rate, or narrowing the operating window.