Demystifying the World of Battery Storage

Written By: The Field Team
Posted 02 Aug 2023
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When most of us think of batteries, we probably think of the double A or triple A batteries that we use in household gadgets – like the TV remote control or our bedside alarm clock. Although the principles of how they work aren’t that different, in terms of size and complexity the batteries we use are a long way from those that go in the back of the Duracell bunny.

The world of energy storage systems has its fair share of technical jargon, so in this two-part series, Fielders explain some of the basic principles of how our sites work and some of the terms you may come across.

How do batteries work?

Put in the simplest possible terms, our batteries charge up when the proportion of renewable energy being generated is high, and discharge when that generation is low. So if the sun isn’t shining as much or the wind isn’t blowing, batteries can help fill the gap to avoid having to switch on a gas or coal-fired power plant. This means that more batteries are good news for everyone because they reduce the amount of energy produced from fossil fuels, helping us on our way towards net zero.

What is a battery energy storage system?

It makes sense to start at the very beginning, with what exactly a battery energy storage system is. Picture a giant rechargeable battery that stores electricity for future use, connected to a number of other components that make the batteries work how we need them to, such as the switchgear and the transformers (on which, more later).

What are the key terms?

State of charge

All batteries have a state of charge, which refers to the amount of energy stored in them at any one time as a share of its maximum capacity. Just like the bar on your mobile phone, it’s usually shown as a percentage, with 100% representing a fully charged battery and 0% indicating an empty or discharged battery.

State of health

Over time, batteries can experience degradation, a term used to describe a reduction in a battery’s capacity or performance. This is similar to how the rechargeable batteries in our laptops and phones lose capacity over time. A battery’s state of health indicates how much capacity it has remaining compared to how much it started with. It is affected by many factors but especially the number of charge/discharge cycles it has completed.

Round Trip Efficiency

Some energy is lost when charging up and discharging a battery. This will depend on factors such as heat loss and electrical resistance within the battery, the Balance of Plant equipment and any power used to keep the battery cool while it charges. Round-trip efficiency is a measure of how much of the stored energy can be retrieved from a battery, and how much is lost during the charge/discharge cycle. Lithium-ion batteries have one of the highest round-trip efficiencies of all storage technologies (around 80 - 90%) which together with sharp falls in cost explains why they have become so widely used.

The Balance of Plant

As well as the batteries themselves, a battery energy storage system includes various supporting components that are collectively called “the balance of plant”. This typically includes components such as transformers, power conversion systems, switchgears, cabling, and other elements. This equipment all ensures that the storage system is run safely, efficiently and reliably.

Point of Connection

For any battery to work successfully and contribute energy it will need a point of connection to the grid. This is simply the place where we plug the battery into the wider network.


This is the collection of circuit breakers, fuses and switches that protect, control and isolate electrical equipment, so that if there is a surge in power, the equipment is kept safe.


These change the operating voltage between one part of a system and another. Generation equipment usually produces electricity at low voltage but the distribution or transmission system requires a higher voltage for maximum efficiency. Transformers act as the bridge between these systems, turning voltage levels up and down.

Power Conversion System

Another crucial part of a battery energy storage system is the power conversion system or PCS. This system manages the flow of electricity between the batteries and the power grid. Most importantly, it converts what’s called the direct current (DC) stored in the batteries to alternating current (AC), which is the form of current used in the public electricity system and the type of current we use in our homes.


Also known as the charge-capacity rate, this describes the charging or discharging speed of a battery relative to its capacity. If you think of the battery’s energy capacity as the amount of water in a bucket, the C-rate tells us how fast we can fill or empty that bucket. So a battery with a C-rate of 1 could fully charge or discharge its capacity in 1 hour. Conversely a battery with a C-rate of 0.5 would require 2 hours for a full charge or discharge.


Feeling demystified?

Battery energy storage systems are game-changers in the transition to renewable energy, but also relatively new to the renewable energy space. We’ve only just begun to scratch the surface on energy storage systems, so stay tuned for the next instalment of the series: a deep-dive into how these battery storage systems actually power up the UK.