Energy storage battery systems (ESS) are playing an increasingly important role as the global demand for sustainable energy and grid stability grows. Whether they are used for grid-scale energy storage, commercial and industrial applications, or residential solar packages, understanding the key technical terminology of energy storage batteries is fundamental to communicating effectively, evaluating performance, and making informed decisions.
However, the jargon in the energy storage field is vast and sometimes daunting. The purpose of this article is to provide you with a comprehensive and easy-to-understand guide that explains the core technical vocabulary in the field of energy storage batteries to help you get a better grasp of this critical technology.
Basic Concepts and Electrical Units
Understanding energy storage batteries begins with some basic electrical concepts and units.
Voltage (V)
Explanation: Voltage is a physical quantity that measures the ability of an electric field force to do work. Simply put, it is the ‘potential difference’ that drives the flow of electricity. The voltage of a battery determines the ‘thrust’ it can provide.
Related to energy storage: The total voltage of a battery system is usually the sum of the voltages of multiple cells in series. Different applications (e.g., low-voltage home systems or high-voltage C&I systems) require batteries of different voltage ratings.
Current (A)
Explanation: Current is the rate of directional movement of electric charge, the ‘flow’ of electricity. The unit is the ampere (A).
Relevance to Energy Storage: The process of charging and discharging a battery is the flow of current. The amount of current flow determines the amount of power a battery can produce at a given time.
Power (Power, W or kW/MW)
Explanation: Power is the rate at which energy is converted or transferred. It is equal to voltage multiplied by current (P = V × I). The unit is the watt (W), commonly used in energy storage systems as kilowatts (kW) or megawatts (MW).
Related to energy storage: The power capability of a battery system determines how fast it can supply or absorb electrical energy. For example, applications for frequency regulation require high power capability.
Energy (Energy, Wh or kWh/MWh)
Explanation: Energy is the ability of a system to do work. It is the product of power and time (E = P × t). The unit is the watt-hour (Wh), and kilowatt-hours (kWh) or megawatt-hours (MWh) are commonly used in energy storage systems.
Related to energy storage: Energy capacity is a measure of the total amount of electrical energy a battery can store. This determines how long the system can continue to supply power.
Key Battery Performance and Characterisation Terms
These terms directly reflect the performance metrics of energy storage batteries.
Capacity (Ah)
Explanation: Capacity is the total amount of charge that a battery can release under certain conditions, and is measured in ampere-hours (Ah). It usually refers to the rated capacity of a battery.
Related to energy storage: Capacity is closely related to the energy capacity of the battery and is the basis for calculating energy capacity (Energy Capacity ≈ Capacity × Average Voltage).
Energy Capacity (kWh)
Explanation: The total amount of energy that a battery can store and release, usually expressed in kilowatt-hours (kWh) or megawatt-hours (MWh). It is a key measure of the size of an energy storage system.
Related to Energy Storage: Determines the length of time a system can power a load, or how much renewable energy can be stored.
Power Capacity (kW or MW)
Explanation: The maximum power output that a battery system can provide or the maximum power input that it can absorb at any given moment, expressed in kilowatts (kW) or megawatts (MW).
Related to energy storage: Determines how much power support a system can provide for a short period of time, e.g. to cope with instantaneous high loads or grid fluctuations.
Energy Density (Wh/kg or Wh/L)
Explanation: Measures the amount of energy a battery can store per unit mass (Wh/kg) or per unit volume (Wh/L).
Relevance to energy storage: Important for applications where space or weight is limited, such as electric vehicles or compact energy storage systems. Higher energy density means more energy can be stored in the same volume or weight.
Power Density (W/kg or W/L)
Explanation: Measures the maximum power a battery can deliver per unit mass (W/kg) or per unit volume (W/L).
Relevant to energy storage: Important for applications that require fast charging and discharging, such as frequency regulation or starting power.
C-rate
Explanation: C-rate represents the rate at which a battery charges and discharges as a multiple of its total capacity. 1C means the battery will be fully charged or discharged in 1 hour; 0.5C means in 2 hours; 2C means in 0.5 hours.
Relevant to energy storage: C-rate is a key metric for assessing a battery’s ability to charge and discharge quickly. Different applications require different C-rate performance. High C-rate discharges typically result in a slight decrease in capacity and an increase in heat generation.
State of Charge (SOC)
Explanation: Indicates the percentage (%) of a battery’s total capacity that is currently remaining.
Related to energy storage: Similar to a car’s fuel gauge, it indicates how long the battery will last or how long it needs to be charged.
Depth of Discharge (DOD)
Explanation: Indicates the percentage (%) of the total capacity of a battery that is released during a discharge. For example, if you go from 100% SOC to 20% SOC, DOD is 80%.
Relevance to Energy Storage: DOD has a significant impact on the cycle life of a battery, and shallow discharging and charging (low DOD) is usually beneficial to prolonging battery life.
State of Health (SOH)
Explanation: Indicates the percentage of current battery performance (e.g. capacity, internal resistance) relative to that of a brand new battery, reflecting the degree of aging and degradation of the battery. Typically, a SOH of less than 80% is considered to be at end of life.
Relevance to Energy Storage: SOH is a key indicator for assessing the remaining life and performance of a battery system.
Battery Life and Decay Terminology
Understanding the life limits of batteries is key to economic evaluation and system design.
Cycle Life
Explanation: The number of complete charge/discharge cycles that a battery can withstand under specific conditions (e.g., specific DOD, temperature, C-rate) until its capacity drops to a percentage of its initial capacity (usually 80%).
Relevant to energy storage: This is an important metric for evaluating the life of a battery in frequent use scenarios (e.g., grid-tuning, daily cycling). Higher cycle life means a more durable battery
Calendar Life
Explanation: The total life of a battery from the time it is manufactured, even if it is not used, it will age naturally over time. It is affected by temperature, storage SOC, and other factors.
Relevance to Energy Storage: For backup power or infrequent use applications, calendar life may be a more important metric than cycle life.
Degradation
Explanation: The process by which a battery’s performance (e.g., capacity, power) decreases irreversibly during cycling and over time.
Relevance to energy storage: All batteries undergo degradation. Controlling temperature, optimising charging and discharging strategies and using advanced BMS can slow down the decline.
Capacity Fade / Power Fade
Explanation: This refers specifically to the reduction of the maximum available capacity and the reduction of the maximum available power of a battery, respectively.
Relevance to Energy Storage: These two are the main forms of battery degradation, directly affecting the system’s energy storage capacity and response time.
Terminology for technical components and system components
An energy storage system is not just about the battery itself, but also about the key supporting components.
Cell
Explanation: The most basic building block of a battery, which stores and releases energy through electrochemical reactions. Examples include lithium iron phosphate (LFP) cells and lithium ternary (NMC) cells.
Related to energy storage: The performance and safety of a battery system depends largely on the cell technology used.
Module
Explanation: Combination of several cells connected in series and/or in parallel, usually with a preliminary mechanical structure and connection interfaces.
Relevant to energy storage: Modules are the basic units for building battery packs, facilitating large-scale production and assembly.
Battery Pack
Explanation: A complete battery cell consisting of multiple modules, a battery management system (BMS), a thermal management system, electrical connections, mechanical structures and safety devices.
Relevance to energy storage: The battery pack is the core component of the energy storage system and is the unit that is delivered and installed directly.
Battery Management System (BMS)
Explanation: The ‘brain’ of the battery system. It is responsible for monitoring the battery’s voltage, current, temperature, SOC, SOH, etc., protecting it from overcharging, over-discharging, over-temperature, etc., performing cell balancing, and communicating with external systems.
Relevant to energy storage: The BMS is critical to ensuring the safety, performance optimisation and maximisation of the life of the battery system and is at the heart of any reliable energy storage system.
(Internal linking suggestion: link to your website’s page on BMS technology or product benefits)
Power Conversion System (PCS) / Inverter
Explanation: Converts direct current (DC) from a battery to alternating current (AC) to supply power to the grid or loads, and vice versa (from AC to DC to charge a battery).
Related to Energy Storage: The PCS is the bridge between the battery and the grid/load, and its efficiency and control strategy directly affect the overall performance of the system.
Balance of Plant (BOP)
Explanation: Refers to all supporting equipment and systems other than the battery pack and PCS, including thermal management systems (cooling/heating), fire protection systems, security systems, control systems, containers or cabinets, power distribution units, etc.
Related to Energy Storage: BOP ensures that the battery system operates in a safe and stable environment and is a necessary part of building a complete energy storage system.
Energy Storage System (ESS) / Battery Energy Storage System (BESS)
Explanation: Refers to a complete system integrating all necessary components such as battery packs, PCS, BMS and BOP, etc. BESS specifically refers to a system using batteries as the energy storage medium.
Related to Energy Storage: This is the final delivery and deployment of an energy storage solution.
Operational and Application Scenario Terms
These terms describe the function of an energy storage system in a practical application.
Charging/Discharging
Explanation: Charging is the storage of electrical energy in a battery; discharging is the release of electrical energy from a battery.
Related to energy storage: the basic operation of an energy storage system.
Round-Trip Efficiency (RTE)
Explanation: A key measure of the efficiency of an energy storage system. It is the ratio (usually expressed as a percentage) of the total energy withdrawn from the battery to the total energy input to the system to store that energy. Efficiency losses occur primarily during the charge/discharge process and during PCS conversion.
Related to energy storage: Higher RTE means less energy loss, improving system economics.
Peak Shaving / Load Leveling
Explanation:
Peak Shaving: The use of energy storage systems to discharge power during peak load hours on the grid, reducing the amount of power purchased from the grid and thus reducing peak loads and electricity costs.
Load Leveling: The use of cheap electricity to charge storage systems at low load times (when electricity prices are low) and discharge them at peak times.
Related to energy storage: This is one of the most common applications of energy storage systems on the commercial, industrial and grid side, designed to reduce the cost of electricity or to smooth load profiles.
Frequency Regulation
Explanation: Grids need to maintain a stable operating frequency (e.g. 50Hz in China). Frequency falls when the supply is less than the use of electricity and rises when the supply is more than the use of electricity. Energy storage systems can help stabilise the grid frequency by absorbing or injecting power through rapid charging and discharging.
Related to energy storage: Battery storage is well suited to provide grid frequency regulation because of its fast response time.
Arbitrage
Explanation: An operation that takes advantage of differences in electricity prices at different times of the day. Charge at times when the price of electricity is low and discharge at times when the price of electricity is high, thereby earning the difference in price.
Related to Energy Storage: This is a profit model for energy storage systems in the electricity market.
Conclusion
Understanding the key technical terminology of energy storage batteries is a gateway into the field. From basic electrical units to complex system integration and application models, each term represents an important aspect of energy storage technology.
Hopefully, with the explanations in this article, you will gain a clearer understanding of energy storage batteries so that you can better evaluate and select the right energy storage solution for your needs.
Frequently Asked Questions (FAQ)
What is the difference between energy density and power density?
Answer: Energy density measures the total amount of energy that can be stored per unit of volume or weight (focusing on the duration of discharge time); power density measures the maximum amount of power that can be delivered per unit of volume or weight (focusing on the rate of discharge). Simply put, energy density determines how long it will last, and power density determines how ‘explosive’ it can be.
Why are cycle life and calendar life important?
Answer: Cycle life measures the life of a battery under frequent use, which is suitable for high-intensity operation scenarios, while calendar life measures the life of a battery that naturally ages over time, which is suitable for standby or infrequent use scenarios. Together, they determine the total battery life.
What are the main functions of a BMS?
Answer: The main functions of a BMS include monitoring the battery status (voltage, current, temperature, SOC, SOH), safety protection (overcharge, overdischarge, over-temperature, short-circuit, etc.), cell balancing, and communicating with external systems. It is the core of ensuring the safe and efficient operation of the battery system.
What is C-rate? What does it do?
Answer: C-rate represents the multiple of charge and discharge current relative to the battery capacity. It is used to measure the rate at which a battery is charged and discharged and affects the actual capacity, efficiency, heat generation and life of the battery.
Are peak shaving and tariff arbitrage the same thing?
Answer: They are both modes of operation that utilise energy storage systems to charge and discharge at different times. Peak shaving is more focused on lowering the load and cost of electricity for customers during specific high-demand periods, or smoothing the load curve of the grid, whereas tariff arbitrage is more direct and makes use of the difference in tariffs between different time periods to buy and sell electricity for profit. The purpose and focus are slightly different.
Post time: May-20-2025