Keeping Voltage: a second life for batteries

An EQC battery can be on the road for eight years or 160,000 kilometers without a significant loss in capacity. And then? What happens with the still functioning batteries after their time in a vehicle? The development team of Mercedes-Benz Energy established in 2016 has come up with numerous answers.

There are two ways to get to Kamenz. From Dresden, the route runs through Leppersdorf, Pulsnitz and Gelenau straight into the center of the tranquil Saxon county seat. The show bakery Kahreoperates a café here.Visitors from out of town are likely to order Lessing Cakes: the small biscuit cakes with cherry-vanilla cream filling are an homage to the famous poet of the German Enlightenment who spent his childhood in Kamenz.

The road from Berlin, which German Chancellor Angela Merkel presumably also chose when she visited Kamenz in May 2017, leads directly to the northern part of the city: to the industrial park where Daimler subsidiary Accumotive opened one of the world's largest and most modern battery factories two years ago. This is where – amongst others – the electric heart of the EQC is manufactured.

Steven Kinzel works as a system manager at Mercedes-Benz Energy.

"We are working on the future of mobility," says Steven Kinzel, "on electric cars becoming an alternative to combustion engine cars for more and more people." The electrical engineer worked at Accumotive until 2017 before moving to Mercedes-Benz Energy (MBE), a wholly owned subsidiary of Daimler. The first mission: to develop a concept for a large-scale storage unit for the new generation of vehicle batteries – tog ether with old and new colleagues.

Greater stability in the power grid

Jens Liebold works as a specialist in technical sales at Mercedes-Benz Energy.

Different types of storage unit solutions are becoming increasingly important with the energy revolution. Because they help with balancing out the volatile generation of electricity from renewable energies and with keeping the voltage in the grids constant – a basic prerequisite for keeping the grid frequency stable at 50 Hz. "The energy industry refers to this as the so-called operating reserve," explains Jens Liebold, a technical sales specialist of MBE. "An operating reserve of about 40 megawatts, can cover almost ten percent of the operating reserve required in Germany. This is what our six installed large-scale storage units already provide."

The Industry is also interested in battery storage units from Kamenz. That is because vehicle batteries are able to optimize the load management and thus also the costs wherever energy-intensive processes are running. Jens Liebold: "Industrial customers pay a service price that remains constantthroughout the year, and is based onthe maximum power requested. Storage units lower the consumption peaks in this regard by supplying power at the right time." The investment in a stationary storage unit thus quickly pays off for many companies.

Breakthrough with smart batteries

The team turned to used second-generation batteries from electric smart models for one of the first storage units. It came online in Lünen in 2016. "The batteries had an approval of four years before we took them back and built the world's largest second-use storage unit with them." The team interconnected 1000 batteries to a total capacity of 13 megawatt hours. This amount of energy corresponds to the daily electricity demand of 2,300 households. Jens Liebold recalls the challenging groundwork: "We ventured into absolutely new territory back then. There was nothing comparable that we could have used for orientation. It was quite exciting when the storage unit was finally sitting there and worked."

The team had already tested various solutions up to that point. "The EV battery needs an insane amount of information in order to function reliably in a large-scale storage unit. Of course, it was not originally designed to work outside the vehicle and then to communicate with other batteries," says Steven Kinzel. Thus, the first challenge was to develop a reliable battery control system for the operation of EV batteries connected in parallel in the storage unit. In addition, the batteries had to be certified in accordance with the industry standard for the use as stationary storage systems.Fire safety aspects were just as important as a low susceptibility to errors: As a back-up solution in the grid, the systems must be 100 percent fail-safe. "Mercedes-Benz Energy storage units are currently the pioneers in this regard. They run continuously for 365 days a year and we are very proud of that," says Jens Liebold.

The joint success, but also the obstacles overcome, the time pressure and the at times high workload, has welded the young team together. People at MBE know each other, they go to lunch in the cafeteria together or have a barbecue party at a colleague's house. "We are proud of what we do," relates Matthias Knödler who is responsible for the marketing of MBE. "It motivates us to work in a field that contributes to the success of the energy revolution and of electric mobility."

Success factor: material efficiency

The successful premiere in Lünen validated the newly developed business model: the economic value of lithium-ion batteries doubles if they are used and marketed for another ten years in a large-scale storage unit after having been used in a vehicle . At the same time, the extended use phase improves the life-cycle assessment of the batteries, because the valuable raw materials remain in the cycle. "Disposal is always the worst alternative. We want to sharpen the awareness for this throughout the corporate group and among our customers," says Jens Liebold. To this end, the team from Kamenz works closely together with colleagues in Mannheim from remanufacturing, the reconditioning of vehicle batteries. "Batteries that no longer meet the requirements for the road are not suitable for the used parts market. On the other hand, minor capacity losses are not so important for our storage units, which is why these batteries come to us." In return, Kamenz supplies batteries that are stored as spare parts in the stationary storage units.

Wellness for the battery

The development work at MBE does not stop in the meantime. The team has just finished the last detail work on the latest battery management system, which now must pass its first endurance test. The goal is to launch a standardized solution that allows installing different types of EV batteries with as few modifications as possible – a kind of plug-and-play system for large-scale storage units. Not only does this lower the effort for the development team, it also has another practical effect: as a living spare parts warehouses, stationary storage units store the electric batteries produced by Accumotive until they are needed for installation in the vehicle. In the future, EQC, smart and plug-in-hybrid batteries of different generations would be used in a single large-scale storage unit string by string. The periodic charging and discharging of the batteries has the effect of live-cell therapy: it ensures that the sensitive cells do not lose any of their original charging capacity even after months.

While the production-ready storage unit solution is completed, the next areas of application are already emerging: If the remaining six nuclear power plants in Germany are dismantled by 2022 and a large proportion of coal-fired power plants are dismantled by 2030, energy suppliers will face new challenges in terms of grid stability. "We are currently examining together with our development partners from the energy industry how storage units must be designed in order to meet these requirements," relates Jens Liebold.

Direct current for the carbon-neutral factory

Lithium-ion batteries could also play a key role in the vision pursued by Daimler to make production in all European plants carbon-neutral by 2022. The reason for this is that many industrial customers need direct current (also: DC) for powering electric motors, electronic control units, computers and LED lights, for example. Consequently, the alternating current (AC) from the grid is in part converted in a multi-step process, during which energy is lost. A DC energy supply would save at least one conversion step and would thus be significantly more efficient. "This is particularly true where plants are supplied from a local photovoltaic system anyway. In the research project DC Industry , together with Daimler AG and various partners we are currently examining how such a local DC grid can be stabilized with the help of a battery storage unit," explains Steven Kinzel. Daimler is working on industrial direct current concepts together with 20 companies from industry, four research institutes and the German Electrical and Electronic Manufacturers' Association (ZVEI). One of these concepts is tested on the premises of the corporate TecFactory since October 2018. "Here, we are at the forefront of development, too. A DC supply in the production environment has not been realized so far."

The innovative storage unit concepts from Kamenz, but also the energy revolution, are expanding the spectrum of possible applications. In future, it will be about balancing out the increased demand for electricity at certain times created by the growing fleet of electric vehicles. "Energy grids are often not designed for megawatts of power suddenly being demanded at central points in a large city for the rapid charging of electric vehicles," says Steven Kinzel. Battery storage units in the vicinity of the charging stations could bolster the grid for such schemes. The picture is similar for public transit: electric buses, for example, must be supplied with large amounts of energy in order to spend enough time on the road despite the high charging capacity. Storage units at the bus depots could provide this charging capacity while balancing out expensive load peaks.

"Last but not least, IoT applications and cloud technologies offer the possibility to operate thousands of widely distributed storage units in an overall system," says Jens Liebold with a view to the future development tasks. There is still a lot to do until then. The team in Kamenz feels well prepared for it. "We are a small location where every employee witnesses a lot and wants to makea difference together," says Matthias Knödler. "That is a crucial advantage for the tasks that lie ahead of us."

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