The fraction or percentage of a rechargeable electrochemical cell's nominal capacity that has been removed (delivered) during a specified discharge interval, calculated as discharged capacity divided by nameplate capacity for the cycle or interval considered.

Principle

Depth of discharge (DoD) and state of charge (SoC) are complementary metrics related by SoC = 1 − DoD (when both are expressed as fractions of the same nominal capacity); DoD for a cycle must be reported with the nominal capacity and test conditions because capacity varies with rate, temperature and ageing.

Demonstration

Illustrative calculation: a battery with nominal capacity 100 Ah is discharged by 30 Ah in a defined cycle → DoD = 30 Ah / 100 Ah = 30%. If the cell's usable capacity has faded to 90 Ah with age and that is not accounted for, the operational DoD relative to current capacity would be ≈33%—showing the need to specify reference capacity.

Misapplication

Using DoD interchangeably with remaining runtime or treating nameplate capacity as invariant. The error is ignoring that available capacity depends on C-rate, temperature and degradation; quoting DoD without stating the reference capacity and conditions misrepresents delivered energy.

Consequence

DoD is a primary operational parameter for battery management: higher DoD per cycle increases usable energy per cycle but typically accelerates cycle ageing; specifying DoD informs lifecycle planning, warranty terms and system sizing decisions.

Reversal

For some chemistries and specific operating regimes, the relationship between DoD and calendar/cycle life is non-monotonic or overridden by other degradation modes (e.g., calendar ageing, temperature-driven side reactions), so reducing DoD alone may not proportionally extend service life.

Boundary

Clearly within: a defined discharge interval of a rechargeable electrochemical cell or battery pack where discharged ampere-hours relative to the stated nominal capacity are computed. Boundary case: partial charge-discharge sequences within complex duty cycles require cycle-definition conventions to compute meaningful DoD. Clearly outside: single-use primary cells (no repeated cycles) or thermal energy storage where 'depth' is defined differently.

Semantic Tension

Immediate usable energy versus longevity: operating at higher DoD yields more energy per cycle but trades off against accelerated degradation and reduced cycle life, forcing system designers to balance runtime against lifecycle cost.

Synthesis

DoD is an operationally defined ratio that requires an explicit reference capacity and conditions; its value is meaningful only when paired with test or field conditions because it directly mediates the tradeoff between delivered energy and battery longevity.