One of the questions we’ve been frequently asked at SMA Solar Technology is how compatibility with the existing variety of batteries in the market can be put into practice.
Let’s start with some brief historical background. Batteries have always been one of the main focus topics in the SMA world. In the 1980s, the first off-grid systems supplying power to whole islands were commissioned. A battery inverter would act as the heart, charging and discharging a large-scale lead acid battery and controlling the operation as well as stability of the overall system. One important aspect was – and still is – to understand the battery’s needs under various operational conditions, and then find an algorithm to meet them while optimising the system’s performance.
More than 20 years’ experience has qualified us to understand the influences of specific applications on the sizing and performance of the battery. As the development and diversity of battery technologies gained momentum, it became obvious that there is no “perfect” technology for any application and each conceivable requirement. As a system solution provider, it was more than obvious that it is mandatory to support a variety of technologies to meet the customer’s demand.
Which battery technology for which application?
About two or three years ago, it looked pretty much like the tasks appropriate to various different battery technologies had been set, plain and clear.
The upcoming technologies were divided into two basic categories: the right solution for high power solutions was Lithium, which was very expensive but allowed to provide enormous amounts of power in short time periods, while for high energy solutions, Sodium based technologies were chosen, promising the lowest costs for stored energy, but with some drawbacks in terms of dynamics.
Nickel based technologies looked like they might be reduced to niche applications - which still looks to be the case - and flow technologies promised to be a solution for long term energy storage. lead acid batteries, even while being flawed with the stigma of being a more or less “vintage” technology, were the right choice for cost-driven applications with basic requirements regarding cycle life and efficiency. Examples include like UPS (uninterruptible power supply, or back-up power), or for off-grid systems, where the installed capacity was merely influenced by the “survival time” after an emergency, typically the loss of generation. A battery designed to provide power for two or three days after no generation is available, will have to withstand daily cycling of less than 10% of its capacity while generation is available.
From a general perspective, all of this is true. The major game changer was the competitive situation on the market. Whereas sodium battery suppliers have not been facing strong competition, and actually, there are only three or four of them owning the IP to provide solutions, the lithium world has had to face the fight for the pole position in the e-mobility projects of large OEMs.
Resulting in huge investments into R&D as well as production capacity, lithium suppliers succeeded in bringing down the costs in a way that changes the game completely. Comparing current or recent tender results for large scale energy projects, lithium seems to beat sodium even if the costs per kWh are compared. And in residential markets, the first lithium battery systems provide a better pricing than lead acid systems, if the costs per “usable” kWh are taken into account – a result of the fact that lithium batteries allow lower depths of discharge without a reduction of the expected lifetime.
Comparing the figures on the German residential energy storage system (ESS) market, within two short years, from a small share of lithium installations that was not really challenging the leading position of lead acid battery based systems, has shifted, dominating this market with more than 80% of lithium installations today. Supporting both technologies was – and is – key to maintain a leading position in this market segment.
Understanding that each technology has a preferred way of operation (some of them prefer being fully charged frequently, others don’t etc.), and accepting that high performing technologies require a more “intelligent” battery management system (BMS) due to safety reasons, we decided more than 5 years ago to develop a proprietary but open universal interface to connect our inverters to different battery technologies.
The use of lead acid batteries was mandatory from the very beginning – available all over the world, comparably cost effective, no need for a specific BMS. We developed and from then, continuously improved our specific, configurable set of algorithms for the optimised operation of lead acid batteries, so at least the majority of today’s available lead acid batteries can be connected to an SMA inverter. In the last few years, we’ve been working closely together with battery manufacturers to define a set of parameters for their specific cells/batteries, allow a long service life and outstanding performance.
Upcoming technologies, starting with lithium as the new state of the art residential battery technology, require a matching BMS not only for diagnostics but also for safety reasons. These management systems set up a communication link with the inverter and define both the current limits and preferred set points within the operational range. The inverters will adapt these values into the operational strategy to match the demands of the application with the effective capability of battery, influenced by environmental values like temperature etc. Both communication protocol and safety requirements are publicly available, to allow battery manufacturers as well as system integrators to implement this link into the BMS feature set.
But compatibility is not only a question of the communication link. First of all, the voltage has to match – batteries are a combination of battery modules, consisting of battery cells, so the battery manager can chose a system voltage in a certain range. We discussed the system voltage in our last PV Tech Storage Guest Blog contribution, so we will not elaborate on this in detail. System integration has an impact on a variety of other functions, such as the awakening procedure after a system shutdown. Connecting power electronics to a battery means directly connecting the discharged capacitors of a DC link to a fully charged battery – typically causing significant inrush currents.
If you ever tried to mount a fully charged starter battery to your car, you might have an idea of this effect. If the inrush currents would put the “health” of the battery at hazard, a pre-charge strategy has to be implemented, either on the inverter or on the battery side.
Testing – ensuring customer benefit
In order to enable the customer to select the “best” battery currently available, we collaborate with the mayor players in the battery world. Collaboration in this context dies not only consist of the implementation of a communication link. In our test-center, we perform a defined set of integration tests, starting with the communication interface, then investigating the matching of the overall system safety concept, and finally analyzing the system’s behavior under different conditions, defined by the application. As a result of these tests, we set up suitable recommendations for the optimised operation of the system – to ensure the maximum benefit for the customer.
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