At sonnen we are independent of any particular battery manufacturer and not fixed on a particular battery chemistry. This enables us to offer our customers the best available battery chemistry on the market and in our opinion this is currently lithium-iron-phosphate technology.
Lithium-ion battery technology is well known to most people for its application in mobile phones or laptops. However, there are a wide array of types of lithium-ion batteries which have varying characteristics and perform very differently in various applications. This is actually useful because each application has different requirements of what it needs the battery to do. For example, a mobile phone battery is required to perform differently in different conditions to a battery for an electric car or one that is installed in a home storage system.
At sonnen we have always used lithium-iron phosphate battery technology – also known under its abbreviations LiFePO4 or LFP. This means that one of the two battery electrodes consists of lithium iron phosphate. In most batteries used in mobile phones, laptops or electric cars, this electrode consists of a lithium-cobalt mixture such as nickel-manganese cobalt (NMC) or nickel-cobalt aluminum (NCA).
Four good reasons for lithium-iron-phosphate.
The battery units installed in the sonnenBatterie actually consist of hundreds of individual battery cells which are packed together in a battery module – 224 battery cells make up a 2 kWh battery module. The battery modules are then interconnected and can be sized in order to supply a household with stored solar energy over many hours.
In selecting the right battery technology, we look at 4 criteria which currently exclude all other batteries that we have tested.
This is an issue where in our view there can be no compromise as the sonnenBatterie is installed in our customers’ homes. the sonnenBatterie incorporate various safety features to protect against possible faults; we also ensure that we are in line with all appropriate international and national industry standards. However, this is in itself not enough for us and this is why we do not use battery technology with a very high energy density, such as those typically used in electric cars or mobile phones, such as NMC or NCA.
To understand how the batteries react in the worst (but very unlikely case) of an internal short circuit, we also carry out a nail test in our battery laboratory. Through this test we drive a nail into a battery cell to create an internal short circuit. Only when a battery is punctured and does not explode or set fire would we consider it as an option for the sonnenBatterie.
With the NMC or NCA cells tested in this way, the punctured cells ignite, burn and reach very high temperatures of over 700°C. This in turn melts the separator around the battery cell enabling it to spread to the other cells causing a chain reaction. What follows is an inextinguishable fire, as the oxygen contained in the battery material continues to burn even if the cells are submersed in water.
In our tests, even fully charged LFP battery cells do not have the same response as seen with the NMC or NCA cells. LFP cells do not ignite or reach the critical temperatures which can cause the separator to melt. This naturally becomes a very clear factor in favour of LFP batteries.
Longevity and performance.
A battery storage system must perform reliably for many years for it to be a truly sustainable and economically viable proposition for homeowners. Here too, the choice of battery technology is of crucial importance.
In simple terms, a battery loses a bit of its original capacity with each charge and discharge; in effect, it can store less and less energy over time. This degradation process is minimal and continues for years until it reaches a level commonly called ‘end of life’ which can often come quite suddenly with rapid deterioration in performance. Most people have experienced this with their mobile phone, which after 2 years can barely provide for longer conversations even after completing a full charge.
The time it takes to reach this point differs considerably with each battery technology. Compared to a home battery storage system such as the sonnenBatterie, the mobile phone battery is very short-lived and usually reaches only 300-500 charging cycles. We have also tested NMC battery cells, which are often used in competitor battery storage products as well as electric cars, which achieve significantly less charging cycles. However, when used in an electric car this is not so important as just 1,000 charge cycles would provide good value when – with a range per charge of perhaps 300 km – this would correspond to a service life of 300,000 km. Many diesel or petrol vehicles would struggle to achieve this level of performance.
However, when charging a battery with solar power, 1,000 charge cycles just is not enough. In the UK (and Germany), around 250 charging cycles are required each year to enable optimum solar self-consumption. With just 1,000 cycles the battery would have to be replaced after just four years
From sonnen’s perspective a battery should last for 15-20 years to give households a long-term return on investment. For us having a high number of charge cycles is vital to be able to deliver on other applications such as using the sonnenBatterie for charging from the grid on off peak tariffs, generating revenues through grid services, or working in conjunction with micro-CHP.
The LFP batteries of the sonnenBatterie can be charged and discharged more than 10,000 times and still have 70% of their output capacity – this is market-leading performance. Even after 15,000 cycles, batteries will still have over 60% of their capacity. In this way, lithium-iron phosphate provides us with a solution which allows our customers to use their system for various applications for the long term.
Lithium-iron phosphate technology has been around for more than 15 years and has proven itself in applications such as buses and even in submarines. Since the company was founded in 2010, sonnen has exclusively used lithium-iron phosphate and has since sold over 25,000 sonnenBatterie systems. One of the principal suppliers of our battery modules is Sony, which launched the first commercial lithium-ion battery in 1991 and has since developed its own lithium-iron phosphate technology. Sony offers a best in class, environmentally friendly battery module with state-of-the-art design and highest quality mass production. What’s more, the strong international brand gives customers even greater trust and confidence that they are investing in a quality product.
Lithium-iron phosphate is the only battery material which, in its chemical composition, also occurs as a natural mineral. Remember that a battery consists of two electrodes: one of them is graphite while the other is a nickel-cobalt mixture or even lithium-iron phosphate. In our batteries there are neither cobalt nor nickel, both of which are regarded as toxic heavy metals. The mining of cobalt has also been a cause for concern. Whilst cobalt used in Europe is often imported from Canada or Australia, a large part of the production is also located in the Congo, where the raw material is mined under questionable circumstances.
In summary, it is only when a battery technology fulfills all these four features that we use it in our sonnenBatterie systems. In the future new battery technologies will be developed and become commercially viable – offering improved performance, longevity and safety at a lower cost. However, for now only lithium-iron-phosphate hits the mark to be included in a sonnenBatterie.