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Battery Encyclopedia

Everything you want to know about batteries from A to Z, curated by TWAICE experts.

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Anode, cathode, state of health, depth of discharge, end of life - read about the most important battery terms and definitions.

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Battery Cell

Battery Cell Chemistries

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Battery Modeling

Battery Terminology

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Aging model

Battery Modeling

An aging model uses mathematical descriptions of relevant processes that predicts the degradation of lithium-ion batteries over time. It captures the effects of various factors, such as temperature, state of charge, and cycling patterns, on battery life and performance. Aging models typically consider capacity fade and increased internal resistance as primary indicators of battery aging. By predicting the battery's state of health (SoH) and estimating its remaining useful life, aging models help optimize battery management strategies, plan for maintenance or replacement, and improve the overall performance and reliability of battery-powered systems.

Anode

Battery Cell

Electrode on which oxidation occurs – releases electrons on discharge. Usually made from graphite and binder material. Battery science and industry agreed to call it the “negative” electrode, even though oxidation and reduction process changes from discharge to charge.

BMS (battery management system)

Battery Components

A BMS is an electronic system that monitors and manages the performance of a battery pack. It protects the battery from operating outside its safe voltage, temperature, and current limits, ensuring optimal performance and longevity. A BMS also provides critical information on the battery's state of charge, state of health, and other performance parameters.

Battery cell

Battery Cell

Smallest individual electro-chemical unit that provides a certain amount of energy which depends on size, chemistry, and usage.

Battery modeling

Battery Modeling

Battery modeling involves the creation of mathematical representations of lithium-ion batteries using fundamental descriptions from physics, chemistry, and thermodynamics to predict their performance, behavior, and degradation. These models help engineers in various ways, among others are the enhanced design of battery management systems, optimize charging algorithms, and improve overall battery performance.

Battery module

Battery Components

Modules combine 'n' number of cells to one bigger package where n is greater than 1. Usually, modules are the smallest unit of a battery pack that can be replaced during maintenance.

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Battery pack

Battery Components

Battery packs combine 'n' number of modules where n is greater than 1.

Battery swelling

Battery Terminology

Batteries are swelling up when Li ions are moving back and forth between cathode and anode. The swelling is dependent on the electrode chemistries used. Some electrode materials such as silicon expand by more than 300% during the intake of Li. Additionally, battery swelling can occur when gas accumulates inside a lithium-ion battery, causing it to expand. This is especially obvious for pouch cells. The gas generation can result from various factors, including overcharging, high temperatures, and manufacturing defects. Swollen batteries pose a safety risk and should be replaced promptly.

Battery system

Battery Components

Battery systems combine 'n' number of packs where n is greater than or equal to 1.

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C-rate

Battery Terminology

The C-rate is a measure of a battery's charge or discharge rate relative to its capacity. A higher C-rate indicates faster charging or discharging. Batteries with high C-rates can deliver more power but may experience reduced cycle life and increased heat generation.

Capacity

Battery Terminology

Capacity of a cell, electrochemically speaking, is the amount of lithium ions that can be exchanged between the cathode and the anode between the upper cut-off voltage and the lower cut-off voltage. Theoretical values usually differ from the practicably achievable ones, since only a part of the lithium stored in the electrodes is available for the electrochemical reactions. In practice, the capacity is calculated by the integration of the current over time. Additional complexity arises from the fact that cathode and anode potentials change with temperature and hence influence the upper and lower cut-off voltage criteria. Thus, to define the capacity, both the current rate and temperature information are needed. Further, it must be specified if the capacity is measured during charge or during discharge.

Cell manufacturers usually provide a rated capacity value for their cells: Rated capacity means the capacity during discharge, usually measured in ampere-hours, of a cell as measured under predefined specifications. Usually, the cell manufacturer provides information to determine the rated capacity such as temperature, applied current and cut-off voltage. The influence of different parameters on the measurable capacity makes it challenging to compare tests from different parties with the same battery cell. Only if every detail is specified and agreed on, a direct comparison is possible. Therefore, there is not one true capacity, but just various ways to determine the available capacity under certain conditions.

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Cathode

Battery Cell

Electrode in a battery cell in which reduction takes place, meaning the acceptance of electrons. Usually made from metal oxides, an electrically conductive powder and binder material. Battery science and industry agreed to call it the “positive” electrode.

Cycle

Battery Terminology

A cycle is defined as the moment when the cell returns to the starting point after undergoing a charge and discharge process that involved both the upper and lower cut-off voltage limits as defined by the operation. Since except during cell testing (especially cell aging testing), a cycle is rarely seen, the driving profile is often characterized using equivalent full cycles:

Equivalent full cycle (EFC): It is used to classify any cycle or any charge or discharge in terms of the charge throughput of a full cycle. For example, a cycle between 50 and 100% State of Charge (SoC) (so starting at 50% SoC, charging to 100% and then discharging to 50% SoC again) is equivalent to 0.5 EFC

Half cycle: The real driving profile can be broken up into a series of half cycles based on different algorithms such as the rain flow algorithm, etc. Classifications for half cycles can be quite specific such as each time the current drops to zero, a half cycle is finished. Or if the SoC signal changes direction.

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Depth of discharge (DoD)

Battery Terminology

Depth of Discharge (DoD) is the difference between the upper and lower State of Charge (SoC) bounds of a cycle. It is sometimes referred to as cycle depth. Thus, cycling for example, between 10% and 80% SoC results in a DoD of 70% DOD. It is usually expressed as a percentage.

Electric model

Battery Modeling

An electric model is used to replicate the behavior of a physical battery cell. Often, the electric model uses components from electric circuits to mimic the electric response from batteries, such as resistors, capacitors or inductors. By leveraging an electric model, the electric response of a battery can be predicted based on a predefined input profile. An accurate electric model needs to take effects such as the hysteresis of batteries and also temperature-dependencies of model parameters into account.

Electrodes

Battery Cell

Thin coatings on either aluminum or copper foil made out of a mix of materials within which the electrochemical reactions take place.

Electrolyte

Battery Cell

The component in the battery which allows charged particles to travel from one electrode to another and blocks the flow of electrons. In conventional batteries, the electrolyte is liquid. In solid-state batteries, the electrolyte is a solid.

End of life (EoL)

Battery Terminology

End of Life criteria is when the cell is retired from its (first) application, usually a State of Health (SoH) of 80% or an increase of the ohmic resistance up to 200% is used for automotive applications. After automotive applications, there might be a second life in stationary applications with different EoL criteria (e.g. 50% SoH) possible.

A remaining capacity of 80% might seem as like an arbitrary choice for retiring the cell from its primary application but it might have its origins in rapid cell degradation going beyond this state of health of the cell. Above 80% of the remaining capacity, the capacity fading and resistance increase is generally observed in a quasi-linear way. After the80% to 70% crossing, capacity fading and resistance increased behave in a more non-linear way, which makes longer term forecasts more challenging., the While the above characteristic parameters have been defined for cells, they are also widely used for higher levels such as for modules, systems and batteries.

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Energy

Battery Terminology

Energy is the total amount of work a lithium-ion battery can perform, usually measured in watt-hours (Wh). It is a product of the battery’s voltage and capacity, determining the duration for which a battery can power a device.

Energy density

Battery Terminology

Energy density is the amount of energy a battery can store per unit volume (volumetric energy density) or weight (gravimetric energy density). Higher energy density batteries can store more energy in a smaller, lighter package, making them desirable for applications such as electric vehicles and portable electronics.

Fast charging

Battery Terminology

Fast charging refers to charging a battery at a higher current or voltage than standard charging rates, reducing the time required to reach a full charge. While fast charging can be convenient, it may generate more heat and stress the battery, potentially affecting its lifespan. There is no defined threshold for classification of fast charging. Some applications are deemed fast charging if full charging is performed within 30 mins, while others only call it fast charging if a full charge is done within 10 to 12 mins.

Internal resistance

Battery Terminology

Internal resistance is a measure for the required voltage which needs to be applied to ensure that a certain current flows. It can cause energy losses in the form of heat and reduce the battery's overall efficiency. Factors such as temperature, age, and state of charge can affect a battery's internal resistance, but also the overall battery design and the materials used.

LCO (Lithium Cobalt Oxide)

Battery Cell Chemistries

LCO is a widely used lithium-ion battery cathode chemistry known for its high energy density and good cycle life. It’s predominantly used in portable electronic devices such as smartphones, laptops, and cameras. It was the initial cell chemistry in the 1990s when Sony commercialized the Li-ion battery.

LFP (Lithium Iron Phosphate)

Battery Cell Chemistries

LFP is a lithium-ion battery cathode chemistry offering high thermal stability, long cycle life, and excellent safety features. It’s commonly used in electric vehicles, grid storage systems, and industrial applications.

LMO (Lithium Manganese Oxide)

Battery Cell Chemistries

LMO is a lithium-ion battery cathode chemistry that provides high power output and good thermal stability. It’s used in power tools, electric bikes, and some electric vehicles.

LTO (Lithium Titanate)

Battery Cell Chemistries

LTO is a lithium-ion battery anode chemistry known for its extremely fast charging capabilities, long cycle life, and high safety. It’s used in applications that require rapid charging and discharging, such as electric buses and grid storage.

NCA (Lithium Nickel Cobalt Aluminum Oxide)

Battery Cell Chemistries

NCA is a high-performance lithium-ion battery cathode chemistry known for its high energy density, long cycle life, and fast charging capabilities. It’s commonly used in electric vehicles, such as Tesla models, and portable electronics.

NMC (Lithium Nickel Manganese Cobalt Oxide)

Battery Cell Chemistries

NMC is a popular lithium-ion battery cathode chemistry, offering a high energy density, good thermal stability, and relatively low cost. It’s widely used in electric vehicles, portable electronics, and grid storage applications. First NMC cathodes contained the same amount of nickel (Ni), manganese (Mn) and cobalt (Co) and were called NMC111 or NMC333. Recent developments increased the amount of Ni and reduced the Mn and Co content leading to relations such as 8 portions of Ni to 1 portion of Mn and Co, also called NMC811.

Nominal voltage

Battery Terminology

Nominal voltage is the average voltage at which a battery operates during its discharge cycle. It is a key parameter for determining the battery's compatibility with devices and applications. For lithium-ion batteries, the nominal voltage typically ranges between 3.3V and 3.8V, depending on the cell chemistry.

Power

Battery Terminology

Power refers to the rate at which energy is supplied or consumed by a lithium-ion battery, measured in watts (W). A battery with higher power can deliver more energy in a shorter period, enabling faster charging and discharging rates.

Self-discharge

Battery Terminology

Self-discharge is the loss of stored energy in a battery when not in use. All batteries exhibit self-discharge to some extent, but lithium-ion batteries generally have a lower self-discharge rate compared to other storage technologies. Minimizing self-discharge can help prolong battery life and maintain optimal performance.

Separator

Battery Cell

The separator is a critical component in lithium-ion batteries, providing a physical barrier between the anode and cathode to prevent short circuits while allowing lithium ions to passthrough. Separators are typically made from porous materials like polyethylene or polypropylene.

Solid-state batteries

Battery Cell Chemistries

Solid-state batteries are an emerging technology that replaces the liquid electrolyte and the separator in conventional lithium-ion batteries with a solid electrolyte. They offer increased energy density, enhanced safety due to reduced risk of thermal runaway, and improved cycle life. However, solid-state batteries face challenges related to manufacturing, scalability, and cost. In the ideal case, the use of solid-state electrolytes enables the use of anode-free cell configurations, meaning there is no anode available during the manufacturing process, but it is created each discharge process in the form of a thin metallic lithium layer on the anode current collector.

State of charge (SoC)

Battery Terminology

In simple terms, the State of Charge (SoC) provides information on how much charge is still available in the battery. In an ideal case, the SoC can be determined by measuring the charge drawn from and pushed back into the battery. A challenge arises since measurements are not fully precise, meaning the measurement of charge out and charge in might be different, just because sensors are not capable enough to count every change of charge. Also, not every charge which is inserted into the battery will be available later on to leave the battery again, since some processes inside consume charge via side reactions. Hence, the SoC needs to be determined by other means, not only by measuring charge in and out, but also by checking the resulting voltage.

The voltage measured outside at the battery is the difference between the cathode and anode potential. The cathode and anode potential is determined by the amount of lithium ions stored within the material. Therefore, one way would be to use a look-up table to check which voltage level corresponds to the amount of lithium ions stored in the battery electrodes, which then can be used to determine the SoC. However, the voltage itself is also affected by temperature and age of the battery. Other effects such as polarizations make the SoC determination only by voltage readings challenging as well.

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State of health (SoH)

Battery Terminology

It is generally defined as the ratio of the current capacity to the initial capacity. SoH is generally defined with capacities (SOHc). However, SoH can also be defined in terms of resistance increase (SOHr) or it can be based on the available energy compared to the initial energy (SOHe). It is also seen that SoH is sometimes scaled between 0 and 1, where 1 is a new cell and 0 is when the cell reaches the End of Life criteria (e.g. 80% remaining capacity).

SoH is a challenging KPI. Firstly, there is often no attention paid to the right framing of the SoH. Since the value is a result based on the division of two values, we need to ensure that both values are actually comparable. That means, the available capacity needs to be determined under the same conditions as the initial capacity. Concretely that means, that i.e. voltage or State of Charge limits need to be the same. Otherwise, the available and initial capacity represent different states and are not comparable.

Secondly, the available capacity, energy or resistance needs to be determined during the everyday operation, which follows dynamic and uncontrolled patterns. Additionally, temperature and the operation history influence the available capacity, energy and resistance but their influence must be compensated to determine the actual available state. Therefore, sophisticated models and algorithms are needed to ensure accurate results even with noisy and uncontrolled field data.

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Thermal model

Battery Modeling

A thermal model mimics the thermal response of a battery cell to an applied load. The thermal behavior is the response due to the reversible and irreversible generation of heat. Irreversible heat generation is driven by effects such as Joule heating and internal, chemical side reactions. Reversible heat is created by the entropy changes of the respective materials during electrochemical reactions.

Thermal runaway

Battery Terminology

Thermal runaway is a dangerous condition in which a battery's temperature rapidly increases due to an uncontrollable exothermic reaction (that means reactions create heat, which trigger further reactions which themselves, create even more heat), leading to the release of toxic gases, fire, or explosion. Factors leading and contributing to thermal runaway include internal short circuits, overcharging, excessive heat, and mechanical damage.

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