How does a battery storage system work?

July 15, 2021

Battery storage is gaining in importance

The storage of electricity is becoming increasingly important. Especially in connection with sustainable energy generation with photovoltaic systems, battery storage is increasingly in focus in order to be able to use generated electricity even more efficiently.

But not everyone can imagine how a battery storage system actually works.

It seems obvious that a battery is installed in a battery storage system - but how does it actually work and what else is installed?

Why can't I just connect the battery directly to the photovoltaic system? And what does an inverter, the BMS and a transformer have to do with it?

Many people might think of transformers as alien car robots. These components are not quite as cool as transformers - but it is still exciting to find out how the various components of a battery storage system work together to intelligently store and deliver energy.

How does a battery work?

In principle, every battery works in the same way: There are two terminals which have an opposite charge due to different saturation with negatively charged electrons. At the so-called negative terminal there is an excess of electrons, which is why it is negatively charged, whereas at the positive terminal there is a shortage of electrons, which is why it is positively charged.

If the two terminals are connected, an electrical voltage is created, in other words a gradient that attempts to balance out the electron saturation of the two terminals. Thus electrons flow from the negative terminal to the positive terminal.

If a consumer is connected between the negative and positive terminal, it can perform a certain amount of work, depending on the current, voltage and thus the power of the battery. This can be, for example, a light bulb that starts to light up.

In a lithium-ion battery, one of the most common forms of battery, this is done by having a cathode as the positive terminal and an anode as the negative terminal facing each other. The charge carriers that move between the cathode and anode are lithium ions, which are able to move between the two terminals through what is called an electrolyte. This electrolyte is interrupted by a separator, which only allows ions to pass.

If a voltage is applied to this battery, an excess of negatively charged electrons is created at the anode, whereas electrons are withdrawn from the cathode. This produces positively charged lithium ions, which migrate through the separator to the anode, where they reconnect with an excess electron and then attach themselves to the graphite as a neutral particle. Thus the battery is charged.

When discharging, this process takes place in exactly the opposite way: lithium ions migrate through the electrolyte to the cathode and the electrons migrate through a connection of the two poles, thus releasing the stored energy.

This is how a battery cell works. These battery cells are then interconnected to form modules and these modules in turn are interconnected to form the whole battery pack. This battery pack is then connected to the inverter, the second central component of a battery storage system.

How does an inverter work?

A battery in itself can only deliver direct current. This results from a DC voltage between the positive and negative terminals. However, thanks to Nikola Tesla, the power grid is based on alternating current or alternating voltage, i.e. a current that changes direction at a certain frequency (50 Hz in Europe).

The goal of any inverter is therefore to generate a grid-compliant sinusoidal voltage from the battery's DC voltage and vice versa. Conventional inverters are refrigerator-sized boxes that are mounted on the wall or placed next to the battery pack. In a conventional inverter, voltage conversion is done by switching the battery's voltage on and off for varying lengths of time to generate, on average, the desired sinusoidal AC voltage. This process is called pulse width modulation (PWM) and causes high switching losses because the switching frequency is often several kHz. If you want to find out more about PWM, Wikipedia has a good explanation.

The output voltage of conventional inverters is often fuzzy and subject to interference. You can imagine it like the TV picture of an old tube TV, which flickers and wobbles. So a mains filter still has to be connected downstream to smooth the output voltage and make it conform to the mains.

But how does the inverter know when to charge and discharge? The battery and energy management systems, which we will explain in the next section, help it here.

By the way

In contrast to normal inverters, with STABL Energy the voltage conversion is done by innovatively connecting the battery modules one after the other in series. To return to the television analogy: Conventional inverters are the flickering tube TVs and STABL Energy is the new HD standard. The output voltage is of higher quality, the mains filter can be lower and thus power storage systems with STABL Energy are more efficient and cheaper to operate! You can also find details on our technology page or in our Whitepaper.

How do BMS and EMS work?

The BMS, or battery management system, monitors and controls a battery. It monitors the state of charge of the battery, whether a battery is overcharged or deep-discharged, and collects, processes and transmits important parameters for the operation of the battery. Without a functioning BMS to monitor a battery in use, it can very quickly become damaged and not be used adequately. It would be impossible to determine what condition a battery is in.

This would be comparable to a cell phone without a charge indicator, which breaks down at a charge level of 0 % and at a charge level of 100 %. Such a cell phone would be impossible to use without damaging it sooner or later, since one could only use experience values and not facts to determine the charge cycles, and thus inevitably the battery would be overcharged or deep discharged at some point.

The BMS communicates this data to the EMS, the energy management system. The EMS evaluates this data and then determines when, with what current and for how long the inverter should charge and discharge the storage.

What was a transformer again?

Transformers are optional components in a battery storage system. If the voltage delivered by the battery storage system does not correspond to that required by a consumer, it may be necessary to transform the voltage, i.e. convert it to a desired voltage. This is done by applying a voltage to a coil with a certain number of turns, which is connected by a common magnetic core to another coil with a different number of turns. This allows the voltage and current to be adjusted to supply an appropriate load. Transformers are typically only installed in megawatt storage systems and are rather irrelevant for home storage/commercial storage applications.

Summary

All these components - battery, inverter, BMS - are necessary to operate a battery storage system.

In a nutshell, the interaction looks like this. The energy management system specifies when and how much to discharge the electricity storage system. This signal goes to the inverter, the muscle of the battery storage system. The batteries then release or absorb their energy, and the BMS monitors the state of the battery cells to ensure they are always operating in the optimal range.

Efficiency is definitely important in the selection and design of the electricity storage system, as it determines how much energy and money the storage system will save you. You can find out how STABL Energy takes electricity storage efficiency to a new level by downloading our Whitepaper or contacting us directly via the info email.