What are the Technical Specifications of Battery Energy Storage Systems (BESS)?

Definition

Key figures for battery storage systems provide important information about the technical properties of Battery Energy Storage Systems (BESS). They allow for the comparison of different models and offer important clues for potential utilisation and marketing options. Investors can use them to estimate potential returns.

Power Capacity

The capacity of a battery is the amount of usable energy it can store. This is the energy that a battery can release after it has been stored. Capacity is typically measured in watt-hours (Wh), unit prefixes like kilo (1 kWh = 1000 Wh) or mega (1 MWh = 1,000,000 Wh) are added according to the scale.

Power Capability

The capability of a battery is the rate at which it can release stored energy. As with capacity, the respective maximum is specified. The common unit of measurement is watts (W), again, with unit prefixes like kilo (1 kW = 1000 W) or mega (1 MW = 1,000,000 W).

C-Rate

The C-rate indicates the time it takes to fully charge or discharge a battery. To calculate the C-rate, the capability is divided by the capacity. For example, if a fully charged battery with a capacity of 100 kWh is discharged at 50 kW, the process takes two hours, and the C-rate is 0.5C or C/2.

As a specification of a battery, the C-rate usually indicates the maximum C-rate, meaning that the higher this key figure, the faster the battery can be charged and discharged. However, charging and discharging at maximum power can reduce the battery's service life. Choosing a below-maximum C-rate can protect the battery cells.

The maximum C-rate largely depends on the technology used. Lithium-ion batteries typically can provide higher C-rates than lead-acid batteries. Redox flow batteries can be constructed with very low and very high C rates.

A low C-rate tends to be more important in mobility than in BESS used for load shifting, for example, from day to night.

Energy (conversion) efficiency

Usually, this key figure indicates the percentage of usable energy still available in the desired form after one or more conversion steps. Unless otherwise stated, for batteries this always refers to the electrical efficiency. (In the case of combined heat and power plants, the thermal efficiency is also relevant in addition to the electrical efficiency).

For example, if a lithium-ion battery has an energy efficiency of 96 % it can provide 960 watt-hours of electricity for every kilowatt-hour of electricity absorbed. This is also referred to as round-trip efficiency. Whether a BESS achieves its optimum efficiency depends, among others, on the Battery Management System (BMS).

Energy conversion efficiency and overall energy efficiency differ for many technical devices. For example, the energy efficiency of an electric car depends not only on the conversion efficiency of its technical components (battery, motor, etc.) but also on factors such as drag coefficient, tyres and driving style. Self-discharge (see below) can reduce the energy efficiency of a battery. An oversized BESS whose capacity and performance are rarely or never fully utilised is inefficient in several respects.  

Round-trip efficiency

A distinction is also made between energy conversion efficiency and round-trip efficiency. Energy conversion efficiency refers to the efficiency of each step, such as current conversion processes. Round-trip efficiency, on the other hand, represents the percentage of energy taken from the grid that is fed back into the grid after storage.

Service life

According to a common industry standard, a BESS is considered to have reached the end of its service life when its actual charging capacity falls below 80% of the original nominal capacity. The degradation of a BESS depends on two main factors:

Cycle life: Cyclical ageing indicates how often a electicity storage system can be expected to be fully charged and discharged before it reaches the end of its service life.

Calendar life: Calendar ageing refers to degradation that occurs independently of use, for example, due to corrosion of individual components. Environmental conditions like temperature and humidity can significantly influence the calendar service life.

Self-discharge rate

Charged batteries lose energy over time, even when they are not used. The self-discharge rate measures the percentage of energy lost within a certain period (usually 1 month) and under certain conditions (usually 20 degrees Celsius). Factors such as temperature and charge level can influence the self-discharge rate, but it mainly depends on the technology: Lithium-ion batteries, for instance, have a lower self-discharge rate compared to lead-acid batteries. A low self-discharge rate ensures higher round-trip efficiency.

Temperature range 

The optimum operating temperature for most BESS is around 20 degrees Celsius. However, they tolerate temperatures between 5 and 30 degrees Celsius. Some technologies are more tolerant of temperature variations than others. Depending on the climate, this factor can be crucial for the right choice.

Voltage range

This figure refers to the voltage a battery can be charged and discharged with safely. The voltage range of an accumulator largely depends on the storage technology and the power electronics.

Energy density

There are two types of energy density: The volumetric energy density indicates the ratio of storage capacity to the volume of the battery; so possible measures are kilowatt-hours per litre (kWh/L) or megawatt-hours per cubic metre (MWh/m³). The gravimetric energy density indicates the capacity in relation to the weight, for example in kilowatt-hours per kilogramme (kWh/kg). 

Both key figures are often of secondary importance for stationary batteries. However, they can play a role in domestic BESS or BESS large storage systems in urban areas if space is limited and the statics of buildings are to be taken into account.

In contrast, energy density is crucial in e-mobility. The low gravimetric energy density of available battery technologies, compared to fossil fuels, has so far prevented air transport from being electrified.

Power density

As with energy density, the power density of BESS can also be relevant. It can be expressed accordingly in kilowatts per litre (kW/L) or kilowatts per kilogram (kW/kg).