Li-ion Batteries, Scales of Storage and Key Terminologies

The scale of Li-ion batteries usage ranges from toys and cell phones to electric vehicles and utility scale power systems up to 100 MW or larger. Traditionally this technology was only used for short-time applications, such as frequency regulation or renewables-firming, but in current times these batteries are increasingly used in longer duration (2- to 6-hour) applications.

Configurability and Chemistries in Li ion Battery

Lithium batteries can be easily configured into a variety of string sizes and battery racks to create a wide range of voltages, power ratings, or energy increments. As per the application they can be designed in ranges from a few kilowatts with a few minutes of storage, until up to multi-megawatt solutions with hours of storage that can be retrieved at a utility substation or a wind farm.
 Various chemistries are available for li-ion batteries, making them attractive to utilities, especially for applications that require output duration of 4 hours or less. The most widely used chemistry is the Lithium nickel manganese cobalt (NMC) for stationary applications. NMC chemistries demonstrate balanced performance characteristics in terms of energy, power, cost, and cycle life. However, lithium-iron-phosphate (LFP) batteries have become increasingly prevalent driven by higher cobalt prices.

Li ion Battery and Grid Scale Storage

Lithium-ion batteries have become the most widely used battery technology for grid-scale energy storage requiring megawatts of power for hours at a time. They have been deployed in a wide range of energy-storage applications, ranging from energy-type batteries of a few kilowatt-hours in residential systems with rooftop photovoltaic arrays to multi-megawatt containerized batteries for the provision of grid ancillary services. For storage durations of 30 minutes to three hours, lithium batteries are currently the most cost-effective solution, and have the best energy density compared to the alternatives. For longer durations, lithium may or may not be the most cost-effective choice depending on the application, particularly when considering lifetime costs.

Three categorizations of Li ion storage scales are shown in table below.

Key Storage Metrics And Terminologies in Li ion Battery Storage Systems

There are a few key technical parameters that are used to characterize a battery storage technology and they apply to lithium ion storage as well .One of the key terms to be noted here is the Energy/ Storage capacity of a battery, which will be elaborated later in the section “What is Energy/Storage Capacity”.

Storage Terms and Metrics

Rated Power Capacity is the total possible instantaneous discharge capability (in kilowatts [kW] or megawatts [MW]) of the battery storage system, or the maximum rate of discharge that the battery storage system can achieve, starting from a fully charged state.

Energy Capacity also called storage capacity is the maximum amount of stored energy (in kilowatt-hours [kWh] or megawatt-hours [MWh])

Storage Duration is the amount of time storage can discharge at its power capacity before depleting its energy capacity. For example, a battery with 1 MW of power capacity and 4 MWh of usable energy capacity will have storage duration of 4 hours.

Cycle life/lifetime is the amount of time or cycles a battery storage system can provide regular charging and discharging before failure or significant degradation

What is Energy/Storage Capacity?

Storage Capacity

Capacity essentially means how much energy maximum can be stored in the system. For example, if a battery is fully charged, how many watt-hours are put in there? Storage capacity is typically measured in units of energy: kilowatt-hours (kWh) and megawatt-hours (MWh).

What is a Megawatt-hour?

A Megawatt-hour (MWh) is the unit used to describe the amount of energy a battery can store. Sometimes the capacity of storage is specified in units of power (watt/MW and its multiples) and time (hours). Take, for instance, a 240 MWh lithium-ion battery with a maximum capacity of 60 MW. The storage in battery can be analogized to a lake storing water shown in Fig.1 where lake water can be released to create electricity. With 240 MWh storage, assuming a 60 MW battery system with 4 hours of storage, so, 60 MW means that the system can generate electricity at the maximum power of 60 MW for 4 hours straight. That also means that the total amount of energy stored in the system is: 60 MW x 4 hours = 240 MWh
But it can also provide less power if needed. For example, if the load only requires 30 MW, the system can supply it for 8 hours. The total amount of stored energy is the same, but it is used more slowly: 30 MW x 8 hours = 240 MWh. Fig. 2 represents the different permutations of wattage and storage hours graphically. Basically, power and time ratings are very explanatory as they also define at what maximum rate this energy can be potentially used. As shown in the lake analogy in Fig.1, one can run the battery at max power for 4 hours or half power for 8 hours.

Figure 1: Analogizing battery storage to a lake (source: www.energy.gov)

Figure 2: Power and storage duration permutations (source: www.energy.gov)

To conclude, one can get a lot of power in a short time or less power over a longer time. That is why a storage system is referred to by both the capacity and the storage time (e.g., a 60 MW battery with 4 hours of storage) or by the MWh size (e.g., 240 MWh).