Section 3 - Performance and Capacity

Section 3 · Performance & Degradation

Performance and Degradation Dynamics

A lithium-ion cell is a moving target: its deliverable capacity depends on the load and temperature of the instant, and its total capacity erodes through two independent aging processes that can be slowed but not stopped.

Specifying a pack means separating two things the cell blurs together — the capacity it temporarily withholds and the capacity it permanently loses. The first is a question of load and temperature and fully recovers; the second is irreversible, and it runs on two clocks that advance whether or not the cell is in use.

Fig. 3.0 — Two clocks, one inventory. Calendar aging (time, SoC, temperature) and cyclic aging (cycle count, depth of discharge) both drain the cell’s finite cyclable-lithium inventory and raise its internal resistance, converging on capacity fade. End-of-life is conventionally 70–80% of original capacity.

The central distinction

Temporary losses (cold, high load) are recoverable — the charge is still there, just electrically inaccessible. Permanent fade (SEI growth, plating, cracking) is irreversible — cyclable lithium is gone. Misreading one for the other is the most common error in field diagnosis.

What this section covers

The topics work from metric to mechanism. Capacity and energy delivery define how charge is measured and why high discharge rates reduce deliverable capacity. Lifecycle management separates the two aging clocks and the design levers — depth of discharge, SoC windows — that slow them. Operational stress factors show how the five stressors compound, and develop the delayed, catastrophic failure of over-discharge: copper dissolution.

Topics in this section

• 3.9  Capacity metrics and energy delivery
• 3.10  Lifecycle management and degradation mitigation
• 3.11  Operational stress factors & copper dissolution

Battery Shortcut · Li-ion Cell EssentialsSection 3 of 7