Battery Industry Basics: A Glossary for Battery Terms and Terminology

A battery cell is the smallest energy-storing unit of a battery. A battery cell comes in various physical forms, from a small AA cell that you might find in a TV remote to large-format prismatic cells typically used in energy storage systems.

A battery pack is a collection of battery cells packaged into an application-specific format. These can be as small as a single cell or as large as thousands of cells arranged in series and parallel configurations, along with any associated electronics and mechanical components.

A battery model is a digital representation of a battery. The more accurate a model is, the more useful it is. The insights from a precise model can help you build and operate safer, longer-lasting, more cost-effective, and more reliable battery packs. Battery models are also known as Digital Twins.

A Battery Management System (BMS) is a piece of hardware that measures the voltage, current, and temperature of each cell in the battery system. The BMS performs basic safety functions to keep battery cells within rated operating conditions. BMS are often paired with generic software algorithms to predict state of charge (SoC) and state of health (SoH). They are often inaccurate in any specific application and not guaranteed for accuracy.

Battery chemistry is the combination of chemicals from which the battery cathode, anode, and electrolyte are constructed. Battery chemistry radically impacts battery characteristics and performance. Standard cathode chemistries include Nickel Manganese Cobalt (NMC), Lithium Iron Phosphate (LFP), and Lithium Cobalt Oxide (LCO).

Battery runtime is the amount of time a battery can provide useful energy to an application. This varies unintuitively with the rate of discharge and other factors, such as temperature. For example, increasing the discharge rate can reduce the runtime faster than expected. A battery discharged at double its standard discharge rate will deliver less than half its standard runtime. As batteries degrade, runtime is reduced, and factors such as discharge rate and temperature have an increasingly significant impact.

Battery State of Charge is the charge left in a battery, usually represented as a percentage from 0 to 100%. While often thought of as a "fuel gauge" for a battery, this measurement does poorly predict remaining energy, as changes to several external factors can influence a battery's usable energy.

Battery time-to-empty is a Zitara Live LookAhead simulation of how much longer a battery can provide useful energy to an application. This is similar to State of Charge but considers far more external factors to accurately predict how much longer a battery can keep being useful before needing to be charged again. Conversely, Time-to-Full is also used to predict when charging will be complete.

Battery distance-to-empty is similar to time-to-empty with the addition of external factors for vehicle applications where distance is more meaningful than time.

Battery State of Health quantitatively represents how much a battery has degraded since manufacture. SoH can vary significantly across cells within a single asset. Degradation affects how batteries perform under all conditions. Many manufacturers oversimplify and only warranty the availability of a percentage of the original battery capacity under idealized conditions, but the true state of health is much more complex.

Battery Observability is about the limit of what is possible to know about a physical battery system and has two key dimensions. The first is the quantity and quality of sensors that collect data on measurable quantities like voltage, current, and temperature. A better, often more expensive, sensor array leads to better observability. The second dimension is the ability to estimate quantities that cannot be measured, like SoC and SoH. The quality of both the algorithms and sensors has a significant impact on the accuracy of the estimate. Observability can be improved with better software, better sensors, or both.

Batteries in parallel are when the cells are electrically wired together with all positive terminals connected, and all negative terminals connected. The resulting capacity is the sum of all parallel cells, and the voltage remains the same as for a single cell. Multiple small battery cells wired in parallel are functionally equivalent to a single larger battery cell.

Batteries in series are when the cells are electrically wired together in a chain where each positive terminal is connected to the next cell’s negative terminal. The resulting capacity remains the same, and the voltages are added together. Multiple small battery cells wired in series are functionally equivalent to a higher voltage battery of the same capacity. The weakest battery in a series string limits the performance of the entire string.

A battery module is a useful subdivision of a battery pack for engineering or manufacturing purposes.

A battery pack configuration is a description of how the battery cells are configured for a battery pack. Often expressed as a number in front of P (Parallel) and S (Series). Examples are 4P12S, 18P100S, 1P2S, etc.

Battery cell balance refers to the differences in state of charge of the series cells in a battery pack. The amount of imbalance is the highest cell’s state of charge (SoC) minus the lowest cell’s SoC, which represents the amount of charge that is unavailable. A pack is in balance if all cells have identical SoC. Over time, even in packs that begin with all cells at an identical SoC, the cells in a battery pack can get out of balance due to manufacturing differences, pack design, and external environmental factors. Balancing the cells of a battery pack is the process of bringing all cells to the same SoC, allowing you to access all the energy in every cell.

Open Circuit Voltage (OCV) is a measurement of a battery cell’s voltage at a known state of charge when at resting equilibrium. The OCV curve is the mapping of the OCV from 0-100% SoC. A simple but imprecise method of estimating State of Charge is to use the manufacturer-defined relationship between voltage and charge level to look up the SoC based on a measured OCV. OCV can also be used to improve other charge-tracking methods like Coulomb Counting. Both techniques can be unreliable in many cell chemistries, especially Lithium Iron Phosphate (LFP), where the OCV relationship to charge is flat and there are significant hysteresis effects. It also fails to take into account other factors such as age, temperature, and time since the cell has last been used.

Coulomb counting is a simple method of measuring a battery's state of charge by integrating how much current flows into or out of a battery. Coulomb Counting error compounds due to cell capacity changes and current sensing noise, which accumulate over time.

Battery cycle life is often confused with overall battery longevity, and cycle life represents the number of times a battery can be charged and discharged before it is no longer capable of serving its application. Cycle life is highly dependent on operating conditions, such as temperature, charge/discharge rates, and depth-of-cycle. Overall battery longevity is determined by a combination of cycle aging and calendar aging effects.

Battery calendar aging is the effects of time on battery health. Batteries degrade over time, even when they are not cycled, especially while under high temperature and/or high voltage conditions. Overall battery longevity is determined by a combination of cycle aging and calendar aging effects.

Battery State of Energy (SoE) is an estimate of the remaining usable energy in a battery system and a capability of Zitara Live LookAhead algorithms. Unlike SoC, which only addresses current over time, SoE accounts for the voltage at which the current will be supplied and represents power over time.

Battery State of Power (SoP) is a measure of the maximum possible charge or discharge power that a battery system can support over a fixed time interval. SoP estimation is a capability of Zitara Live LookAhead algorithms that is more precise than a static maximum charge or discharge rate.

C-rate is the ratio of electrical current to charge capacity of a battery. For example, a battery with 10Ah of charge capacity supplying 5A is operating at a C rate of (5/10) = 0.5C.

P-rate is the ratio of electrical power to the energy capacity of a battery. For example, a battery with 100Wh of energy capacity supplying 75W is operating at a P rate of (75/100) = 0.75P.

Hysteresis is a phenomenon where an output lags behind its input when the system changes direction. In battery systems, this is most commonly seen after charging when the open circuit voltage is different compared to the OCV after discharging to the same SoC. It can significantly impact the accuracy of an SoC estimation algorithm and, therefore, the entire system's performance.

Anode is the negative electrode of a battery. Typically manufactured from graphite, though silicon doping is becoming common. Lithium ions move to the anode when charging and return to the cathode during discharge.

Cathode is the positive electrode of a battery. Popular chemistries use nickel and cobalt to achieve high energy density and specific energy. Lithium iron phosphate (LFP) is becoming common as a lower-cost alternative in energy storage systems (ESS) and mass-market electric vehicles. Lithium ions leave the cathode when charging and return during discharge.

A battery LookAhead is a prediction of the state of a battery in the future. Enabled by the model-based algorithms found in Zitara Live, LookAheads are a configurable prediction of the state of a battery in response to the anticipated electrical and thermal loads. LookAheads answer critical operational questions such as voltage response, available energy, heat generation, and power capability.

The Safe Operating Area (SOA) of a battery is a set of electrical and thermal limits defined by the manufacturer of a battery. The battery must be kept within these limits to guarantee safe operation. The SOA of a battery typically consists of at least three dimensions: temperature, voltage, and current.