News & Views

14th May 2021

Battery Storage: What do you need to know?

Continuing from the last article published in the NECA News Dec2017 edition, let’s talk battery storage. In the last issue we discussed Key Battery Terminology and the all-important relevant Australian Standards. Now let’s discuss what we need to know to design a system.

The basic considerations when first consulting with an end-user would include, clarifying the requirements, expectations, budget, Australian Standards and installation requirements. Continuing the consultation, discuss a site assessment, an energy & power assessment and other general requirements, to complete the initial scope.

By achieving the above, you’ve defined the basic requirements for the system including, energy requirements, power demand requirements, battery chemistry/type, operating temperature (based on installation requirements), enclosure – style, type & rating, approximate weight (mounting options) and warranty expectations.

Now, let’s determine the battery system with reference to the output requirements. There are many battery systems out there, and many more on its way to Australia. Unfortunately, there is no simple answer to which system to choose. Why? It all relies on the client’s energy and power requirements.

The following will define the key features of a BESS – Battery Energy Storage System and assist you in designing an accurate system for your client.



Total Battery Capacity

A battery’s total capacity is measured by how much energy (chemical potential energy) a battery can store and deliver at a rated voltage. The total battery capacity is the maximum amount of energy stored within the battery unit. The most common measurement of a battery’s capacity is ampere-hours (Ah). However, when discussing battery storage (for home or work), it is not uncommon to read the rating as kilo-watt hours (kWh). The kWh rating is easily recognisable and comparable when discussing energy bills and/or energy generation.

Though the Battery Capacity defines the total kWh’s of the system, we need more information to determine the output – Usable Capacity. The total capacity may not determine the output, as the total size and deliverable energy may be different.


Usable Capacity

In many types of batteries, the full energy stored within a battery cannot be completely extracted or withdrawn (i.e. discharged), without causing irreversible damage to the battery. Though many systems state the total capacity, the actual energy that is available for consumption, is simply defined as Usable Capacity. The portion of usable capacity is mainly determined by the Depth of Discharge (DoD).

Understanding the usable capacity or calculating the usable capacity (derived from the DoD), will give you a true indication of the available kWh output and assist in designing a BESS more accurately. When reading a product datasheet/specification, make sure the Usable Capacity or Depth of Discharge is stated, as these are very important features when selecting the right solution for you or your client.



Depth of Discharge

The Depth of Discharge (DoD) describes the energy that has been discharged from a battery at any given time. The Depth of Discharge is measured as a percentage of the total battery’s capacity from full capacity (i.e. 100% full). If a battery is 100% full, then discharges 15%, the instantaneous reading for DoD would be 15%, leaving 85% capacity within the battery.

Manufacturers also use the DoD to define the regular discharge percentage of a battery. Knowing this, you can safely discharge a battery to this percentage and recharge it without significantly reducing the life. Discharging batteries beyond the recommended DoD, can significantly damage or reduce the battery’s expected lifespan.

The larger the percentage of DoD, the more capacity a battery can discharge before recharge. However, knowing the DoD only, does not determine the lifespan of the battery - it only determines the maximum available kWh’s before recharge on a regular basis. To determine the lifespan, the manufacturer will define the battery cycle life at a specific depth of discharge. A manufacturer may reference more than one set of values to determine the lifespan of the battery. For example, 80% DoD at 2,500 cycles; 50% DoD at 4,000 cycles; 20% DoD at 8,000 cycles. The given value(s) from the manufacturer will assist in selecting the right battery and lifespan for your application.

A typical lithium-ion battery will have a DoD of 80% - 95%, where as a typical lead-acid battery will have a DoD of 40% - 60%. Comparing this to flow and LifePO4 (lithium iron phosphate) batteries, these batteries can have up to 100% DoD without damaging or reducing the battery’s lifespan.

As an example, if a 10kWh Battery Storage system had a DoD of 100%, then we can conclude that this system has 10kWh of available energy for use and it is safe to completely discharge the battery 100% on a regular basis.

As another example, if a 10kWh system had a DoD of 80%, then the maximum available energy to regularly discharge is 8kWh. The DoD is defined to regularly discharge the battery to 2kWh. i.e. 80% of usable capacity, while having 20% in reserve.



Battery Cycle Life

Battery Cycle Life is the number of complete discharge/charge cycles that a battery can support over its estimated life. To extend the life of a system (in other words, gain more cycles), review the manufacturer’s cycle life and DoD specifications. As mentioned, if a manufacturer references multiple Cycle-DoD combinations, a lower DoD percentage will most likely have a greater number of cycles; hence increased lifespan.



System Efficiency or Round-Trip Efficiency

When storing energy within the Battery Storage system, there may be losses due to heat, environmental factors and other internal inefficiencies. The energy absorbed and released during operation can be stated as a ratio of the input vs. output, round-trip efficiency or system efficiency; all stating the same value as a percentage. The efficiency describes how much energy is lost in a round-trip from the time the battery is charged and discharged.

Though many other efficiencies can be measured within the battery system including Battery Charge Efficiency, Battery Discharge Efficiency, Energy Efficiency – charging vs. discharging under constant current, manufacturers tend to generally reference Round-Trip Efficiency. 

As an example, if 10kWh of energy was generated and stored within a battery system, yet only 9.5kWh could be discharged, then the system efficiency is said to be 95% efficient.



State of Charge

The State of Charge (SoC) describes the level of energy stored within the battery and defined as a percentage from 0% - 100%; where 0% is empty and 100% is full. Simply put, if you wanted to know how much energy you have in your system at any given time, with reference to the maximum stored, then you would read the State of Charge. i.e. 100%, 75%, 60%, etc.



Maximum Continuous Output

As the heading implies, this is the maximum constant output the battery system can discharge at a given time. It is measured in kW (power output), not kWh (energy output), as the output is not defined over time. When designing a system, the user will define the length of time they want the system to run for. Your role is to measure the essential loads are size the system accordingly.

As an example, if the user wanted a constant output of 5kW for 1 hour, you could offer a system with a Maximum Continuous Output of 5kW with battery storage system equal or greater than 5kWh.



What do these features mean for your BESS?

Let’s put a system together using the above key features;

Client requirements:

2.5kW power demand, 4-hour runtime when the power fails, 10-year expected lifespan

From the above, we can determine the Usage Capacity of 10kWh (min.) is required with a Battery Cycle Life of 3,650 (based on 1 cycle per day).

From this, we can determine the discharge requirement from the BESS is 10kWh.

Using the manufacturer’s Total Capacity, DoD, Battery Cycle Life and Max. Continuous Output information, we choose a system that has 15kWh of total capacity, 2.5kW max. output, DoD of 70% at 4,000 cycles.

Using this information, with the client’s other requirements, enables us to choose a system accurately, assisting in providing the best cost-effective solution.