Battery terms, in plain language.

A unit of electric charge: one ampere flowing for one hour. Battery capacity is usually rated in ampere-hours, which measure charge, not energy.

The rule that a rate constant is proportional to exp(−Ea/RT), so a thermally activated reaction speeds up steeply with temperature. Near room temperature many degradation reactions roughly double their rate for every +10 °C.

Current divided by nominal capacity, in reciprocal hours. 1C on a 20 Ah cell is 20 A; 2C is 40 A. C-rate lets you compare loads on cells of different sizes.

Degradation that happens with time even when a cell is not cycled — in storage, for example. It is driven mainly by side reactions such as SEI growth and accelerates with temperature.

Compressing a dried electrode coating between rollers to set its thickness, density and porosity. It is one of the electrode-making steps in cell production.

The loss of charge a cell can store and deliver, for example through lithium locked into the SEI or active material that has cracked out of service.

The thin metal foil that carries electrons between an electrode and the external circuit — usually copper on the negative side and aluminium on the positive side.

The terminal-voltage limit that ends a discharge, or caps a charge — in constant-current, constant-voltage (CC-CV) charging the cell is then held at that voltage until the current falls to a cutoff. A real discharge ends at the lower cutoff, which is why actual runtime can differ from the ideal duration.

A tree-like metal deposit that can grow from the negative electrode through the separator, raising self-discharge or causing an internal short circuit.

How much of a cell’s capacity is used in a cycle. Deep cycles cause more mechanical stress in the active materials than shallow ones.

The proportionality constant D in Fick’s first law, in m²/s. It sets how fast a species spreads for a given concentration gradient.

The ion-conducting medium between the electrodes. In a liquid-electrolyte cell it fills the pores of both electrodes and the separator, so it is not a separate structural layer.

J = −D dc/dx: diffusive flux is proportional to the concentration gradient and points down it.

The first controlled charge and discharge cycles of a newly assembled cell, during which the SEI forms. It is part of cell production, before aging and grading.

t = 1 / C-rate: how long a constant current would take to transfer the full nominal capacity. A definition, not a prediction of real runtime.

An increase in a cell’s internal resistance as it ages. It costs voltage under load and therefore power, even while the stored charge is still there.

Lithium inserting into, and leaving, the host structure of an electrode material — between graphite layers, for example — without destroying it.

Lithium metal depositing on the negative-electrode surface instead of intercalating into it. It is promoted by charging at low temperature or high current.

The charge a cell is rated to deliver under specified test conditions — current, temperature, rest and voltage limits — usually in ampere-hours.

The formation of a surface film that slows further reaction. In lithium-ion cells, the SEI passivates the graphite negative electrode.

An electrode made of active-material particles, conductive additive and binder with electrolyte-filled pores between them. Its large internal surface is where the reaction happens.

The slow loss of stored charge while a cell is not in use. An unexpected rise can signal a leakage path or a soft internal short.

A porous, electronically insulating film between the electrodes. Electrons cannot cross it; ions move through the electrolyte in its pores.

The mixture of active material, conductive carbon, binder and solvent that is coated onto a current-collector foil to make an electrode.

A solid ion conductor that replaces the liquid electrolyte in an all-solid-state cell. Oxide, sulfide, halide and polymer families each need a different production route.

A film of electrolyte reaction products on the negative-electrode particles — graphite, and also silicon or lithium metal. It protects the electrode from further reaction but grows slowly for the whole life of the cell, consuming cyclable lithium.

How full a cell is, as a fraction of its available capacity. State of charge, like temperature, affects how fast a stored cell ages.

A summary of how far a cell has aged, often expressed through remaining capacity or increased resistance compared with when it was new.