How does a battery storage system work?

Battery storage is gaining in importance

Storing electricity is becoming increasingly important. Particularly in connection with sustainable energy generation with photovoltaic systems, battery storage systems are increasingly in focus in order to be able to use the electricity generated even more efficiently. 
However, 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 do an inverter, the BMS and a transformer have to do with it? 
Many people might think of transformers as alien car robots. Although these components are not quite as cool as transformers, it is still exciting to find out how the various components of a battery storage system work together to store and deliver energy intelligently.

How does a battery work?

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

Second-Life Batterie Module

If the two poles are connected, an electrical voltage is created, i.e. a gradient, which attempts to balance out the electron saturation of the two poles. This causes electrons to flow from the negative pole to the positive pole. 
If a load is connected between the negative and positive terminals, it can perform a certain amount of work, depending on the current, voltage and therefore the power of the battery. This can be a light bulb, for example, which starts to light up. 
In a lithium-ion battery, one of the most common types of battery, this is achieved by having a cathode as the positive pole and an anode as the negative pole. The charge carriers that move between the cathode and anode are lithium ions, which can move between the two poles through a so-called electrolyte. This electrolyte is interrupted by a separator which only allows ions to pass through. 
If a voltage is applied to this battery, a surplus of negatively charged electrons is created at the anode, while electrons are removed 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 to the graphite as a neutral particle. The battery is thus 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 between the two poles to release the stored energy. 
This is how a battery cell works. These battery cells are then connected to form modules and these modules in turn are connected to form the entire 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?

Sinus konventionell

A battery can only supply direct current. This results from a direct voltage between the positive and negative poles. Thanks to Nikola Tesla, however, the electricity 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 aim of every 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 attached to the wall or placed next to the battery pack. In a conventional inverter, the voltage is converted by switching the battery voltage on and off for different lengths of time in order to generate the desired sinusoidal AC voltage on average. This process is called ‘Pulse Width Modulation’ (PWM) and causes high switching losses, as the switching frequency is often several kHz. If you want to find out more about PWM, you can find a good explanation on Wikipedia. 

The output voltage of conventional inverters is often blurred and subject to interference. You can think of it like the TV picture on an old tube TV, which flickers and wobbles. A mains filter must therefore always be connected downstream to smooth the output voltage and make it grid-compliant. 
But how does the inverter know when to charge and discharge? This is where the battery and energy management system come in, which we explain in the next section.

By the way

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

How do BMS and EMS work?

The BMS, i.e. the Battery Management System, monitors and controls a battery. It monitors the state of charge of the battery, whether a battery is overcharged or deeply discharged, and collects, processes and forwards important parameters for battery operation. Without a functioning BMS to monitor a battery in use, it can be damaged very quickly and cannot be used adequately. It would be impossible to determine the condition of a battery. 
This would be comparable to a cell phone without a charge indicator, which breaks at a charge level of 0% and at a charge level of 100%. It would be impossible to use such a cell phone without damaging it sooner or later, as you could only use empirical values and not facts to determine the charging cycles and the battery would inevitably be overcharged or over-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 system.

What was a transformer again?

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


All these components – battery, inverter, BMS – are necessary to operate a battery storage system. 
In summary, the interaction looks like this. The energy management system specifies when and how much the storage system should be discharged. This signal is sent to the inverter, the muscle of the battery storage system. The batteries then release or absorb their energy and the BMS monitors the status of the battery cells so that they are always operated in the optimum range.

STABL Energy Testing Rack

Efficiency is definitely important when selecting and designing an electricity storage system, as it determines how much energy and money the storage system saves you. You can find out how STABL Energy raises the efficiency of electricity storage systems to a new level by downloading our white paper or contacting us directly via the info -mail. 

Are you interested in a STABL storage system?

Get in touch with us! We look forward to hearing from you and answering your questions.

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