Battery glossary

Plain definitions of the lithium-ion battery terms that come up most in degradation, safety, testing and modelling. Each one links to a question you can ask next.

C-rate: Current expressed relative to the cell's capacity. 1C fully charges or discharges the cell in about one hour, 2C in about 30 minutes, and C/10 in about ten hours.
Ask: “How does fast charging degrade lithium-ion cells?”
Calendar aging: Degradation that happens while a cell sits unused. It is driven mainly by temperature and storage state of charge, with SEI growth as the usual main mechanism.
Ask: “How do temperature and state of charge affect calendar aging?”
Capacity fade: The gradual loss of the charge a cell can store, usually given as a percentage of its starting capacity. Many applications call a cell end-of-life when it falls to 70–80%.
Ask: “What are the main causes of capacity fade?”
Cell expansion: The change in a cell's thickness as lithium moves in and out of the electrode materials (reversible), plus slow permanent swelling as it ages. Measuring the expansion or the resulting force can reveal state of charge and degradation.
Ask: “How does cell expansion help state estimation?”
Cycle aging: Degradation caused by charging and discharging. It depends on current, depth of discharge, voltage window and temperature, and adds to calendar aging.
Ask: “How does depth of discharge affect cycle life?”
DC internal resistance (DCIR): The resistance of a cell estimated from the voltage change when a current pulse is applied. It sets how much power the cell can deliver and how much heat it generates, and it rises as the cell ages.
Ask: “How does internal resistance relate to heat generation?”
Doyle–Fuller–Newman model (DFN): The standard physics-based model of a lithium-ion cell, also called the pseudo-two-dimensional (P2D) model. It describes lithium transport in the electrolyte and in electrode particles, coupled through reactions at their surfaces.
Ask: “When do you need a physics-based model instead of an equivalent circuit?”
Equivalent circuit model (ECM): A model that represents a cell as a voltage source plus resistors and resistor–capacitor pairs, fitted to test data. It is fast and widely used in battery management systems, but it doesn't describe the chemistry inside the cell.
Ask: “When do you need a physics-based model instead of an equivalent circuit?”
Formation: The first, carefully controlled charge and discharge cycles a cell goes through at the factory. They build the initial SEI layer, and the conditions used affect the cell's capacity and lifetime.
Ask: “How do formation conditions affect cell life?”
Hybrid pulse power characterization (HPPC): A standard test that applies charge and discharge current pulses at a series of states of charge, to measure resistance and power capability across the cell's range.
Ask: “What can pulse tests reveal about cell degradation?”
Knee point: The point in a cell's life where capacity fade suddenly accelerates after a long, roughly linear phase. It is often linked to lithium plating or to one electrode running out of usable capacity, and predicting it early is a major goal of battery research.
Ask: “What causes knees in lithium-ion aging?”
Lithium plating: Metallic lithium depositing on the surface of the graphite negative electrode instead of inserting into it. It is most likely when charging fast, at low temperature or at high state of charge. It wastes lithium, and in the worst case can grow into dendrites that short the cell.
Ask: “How can you detect lithium plating early?”
Lithium–sulfur battery (Li–S): A battery chemistry with a sulfur positive electrode and a lithium metal negative electrode. It offers high theoretical energy density, but faces challenges including the polysulfide shuttle and short cycle life.
Ask: “What limits lithium–sulfur battery performance?”
Loss of active material (LAM): Degradation in which part of an electrode can no longer store lithium, through particle cracking, loss of electrical contact, or dissolution. It can affect the positive or the negative electrode, and the two are often tracked separately.
Ask: “What causes loss of active material in the electrodes?”
Loss of lithium inventory (LLI): Degradation in which cyclable lithium gets locked away, for example in SEI growth or plated lithium, so less of it can shuttle between the electrodes. It reduces capacity even if both electrodes are still intact.
Ask: “How do you tell loss of lithium inventory apart from loss of active material?”
Open-circuit voltage (OCV): The voltage of a cell at rest, with no current flowing and after it has relaxed. The OCV curve against state of charge is the basis of most state estimation and degradation diagnostics.
Ask: “How is the open-circuit voltage curve used to diagnose degradation?”
Parallel cell imbalance: Uneven sharing of current between cells connected in parallel, caused by differences in their resistance, capacity or temperature. Over time it can make some cells age faster than others.
Ask: “What happens when cells are connected in parallel?”
Power fade: The loss of a cell's ability to deliver or accept high power, caused mainly by rising internal resistance. A cell can keep most of its capacity and still lose much of its power.
Ask: “Why does internal resistance increase as cells age?”
Second life: Reusing batteries, usually from electric vehicles, in less demanding applications such as stationary storage once they no longer meet the original requirements. Screening which cells are safe and worth reusing is a key step.
Ask: “How do you screen used cells for second-life use?”
Single particle model (SPM): A simplified physics-based model that represents each electrode as a single particle. It is much faster than the DFN model, and accurate at low to moderate currents.
Ask: “How accurate are simplified physics-based battery models?”
Solid electrolyte interphase (SEI): A thin film that forms on the negative electrode when the electrolyte reacts at low potentials, mostly during the first charges (formation). It protects the electrolyte from further reaction, but it keeps growing slowly over the cell's life, using up lithium and raising resistance.
Ask: “How does SEI growth drive capacity fade?”
State of charge (SOC): How full a cell is, from 0% (empty) to 100% (full), relative to its current capacity. It can't be measured directly, so battery management systems estimate it from voltage, current and models.
Ask: “How does cell expansion help state of charge estimation?”
State of health (SOH): A measure of how much a cell has aged compared with when it was new, usually based on remaining capacity, resistance, or both.
Ask: “How can you estimate electrode state of health in a working cell?”
Tab cooling: Cooling a cell through its electrical tabs rather than its large surfaces. It creates smaller temperature differences between the layers inside the cell, which can reduce uneven aging.
Ask: “Why does tab cooling beat surface cooling?”
Thermal gradient: A temperature difference across a cell or pack. Gradients make different parts of a cell age at different rates, and they can speed up overall degradation even when the average temperature looks fine.
Ask: “How do thermal gradients and cooling affect cell degradation?”
Thermal runaway: A self-heating chain reaction in which rising temperature speeds up exothermic reactions inside the cell, which raise the temperature further. It can end in venting, fire or explosion, and can spread from cell to cell in a pack.
Ask: “How can you detect venting or thermal runaway early?”
Venting: The release of gas from a cell when internal pressure builds up, through a designed vent or by the case rupturing. It is often an early warning sign before thermal runaway.
Ask: “What signals come before a cell vents?”