Energy bars tested.

  • 22. July 2015
  • E-Mobility
  • Photos: Daimler
  • Text: Walter Wuttke

On the edge of the Swabian Jura in southern Germany, Accumotive develops energy storage units for Daimler. The formulas being worked on in this modern-day alchemist’s kitchen are for the electric cars of the future.

In Nabern, a small, nondescript town of less than 2000 inhabitants, there is nothing to suggest that Daimler subsidiary Accumotive is developing power cells here for the electric cars of the future. Around 200 technicians and engineers work in this modern-day alchemists’ laboratory on innovative methods and materials for storing energy.


There’s no doubt about it, this is not Silicon Valley, but rather the somewhat austere foothills of the central Swabian Jura. But there is a tradition of experimenting and inventing here. And so the Accumotive think tank and development centre is perhaps not quite so out of place as it might first appear.



A large part of the development work is devoted to the cell: “the core element of the battery and where the energy is stored,” in the words of Hartung Wilstermann, executive director of Accumotive. The individual cells, aggregated into modules, form the actual energy storage unit. To make batteries more powerful, Wilstermann focuses his efforts on the cell: “I try to pack more energy into it, make it more compact and ultimately cheaper to produce.”


The development work is being driven forward along these lines – using new materials such as lithium-sulphur, but also, as Wilstermann says, “through the existing lithium-ion technology where there is still some scope for improvement”. Here, however, the lithium plays more of a minor role. “By changing the structure of the cell and the way it’s integrated into the battery, the energy density can be significantly increased.” And although that might sound simple enough, it’s anything but.

Mercedes-Benz: Mr. Hartung Wilstermann

Hartung Wilstermann, CEO Accumotive


Wilstermann’s describes the scenario as follows: “We’re competing in a decathlon here. As well as the energy density we have to consider how safe the battery is and how long it lasts. It’s obviously no use to anyone if we develop a fantastic battery that then fails after just a few charging cycles.” And then, of course, there are the costs of producing the battery in the first place. After all, there’s no point in making a top-spec power cell if it means no one can afford it.

The effects of the weather also have to be considered. Batteries that can only perform to their optimum at room temperature won’t be much use in everyday traffic. “At the end of the day we need to find a compromise that doesn’t overly emphasise any one characteristic,” says Wilstermann, describing the challenge of squaring the circle.

' We're competing in a decathlon here. '



If, say, you substantially raise the nickel content, the cell’s energy density will increase. But that has its drawbacks in terms of safety. “The cell then reacts more dramatically when something happens,” says Wilstermann, summing up the consequences of such a step. The appropriate precautionary measures start with getting the arrangement and cooling of the cell right and end with the specially designed housing.

It’s all a question of fine-tuning. Making improvements at one point can potentially be to the detriment of something else, meaning you have to adjust again. Once again it sounds straightforward, but once again it is complex – particularly if the cells, as in the Accumotive battery, are packed tightly together in order to achieve a higher energy density than competitors’ batteries. This is a more complex approach if you also want to guarantee safety under all conditions.


There will be no such thing as a universal battery in the future either. Unlike conventional drive systems, whose technically identical engines can be transplanted into various models with relative ease, Wilstermann develops “made-to-measure batteries for the different Daimler vehicles. For example, we use cells that are geared towards performance in the hybrid models and cells geared towards capacity in the electric vehicles. The batteries for the plug-ins lie somewhere in the middle.”

The developers in Nabern bring together the defined requirements of the model, the installation space and the costs, and these criteria, in turn, are critical to the chemical composition of the respective power cells. Also, the battery in the new model needs to be assembled and cooled in such a way that all Mercedes-Benz models work perfectly in every kind of weather zone. “There’s no ‘absolute best’ with us, but simply the best compromise for a particular application,” says Wilstermann.



In the ‘catacombs’ of Accumotive, suppliers’ cells and the results of the development department are put through their paces on the test bench. The batteries are tormented for as long as it takes for them to meet all safety standards as well as Accumotive’s own requirements. The most extreme conditions are simulated here, conditions that no one would want to experience in an electric vehicle. The programme includes brutal tests in which the batteries are intentionally misused in ways that would be unthinkable in real life.

bored through, overcharged, deformed

In a materials laboratory, the developers open the cell up completely, test it and conduct electro-chemical analyses. In another laboratory, the battery’s electronics are hooked up to a supercomputer that simulates a car (which doesn’t even exist yet) in a wide range of driving conditions. “This allows us to test the electronics before battery and car come together,” says Wilstermann of the equipment that helps save time in development. The decision as to whether the battery, electronics and car are compatible is settled at an early stage of development here.

In another department, batteries are pressed, deformed, bored through with nails and gleefully overcharged – without the battery being allowed to become in any way hazardous. In yet more laboratories, the service life of the battery is tested under climatic conditions that simulate the real world. The ten years of service life are played through in fast motion.

“It’s like we are carrying out open-heart surgery but with high voltage,” says Wilstermann. “A single mistake might be your last one, and so all staff are trained accordingly.” For the tests on the open batteries, for example, the technicians use purpose-built ‘water tables’ with a working surface that in an emergency can be lowered into a basin. “This allows us to bring a damaged battery into a safe state without moving it.”


The way in which the battery is integrated into the car ultimately decides how the power output is defined. Wilstermann believes that in the future developers will be able to work to standard installation spaces for the drive units. It will then be possible to transfer these tight dimensions to various models.

For the development of powerful types of battery such as the lithium plus generation, there will continue to be different solutions for different vehicles. Only key peripheral elements such as electronic components will also be used in other batteries.

Wilstermann firmly believes that “from generation to generation the costs will carry on tumbling.” In the post-lithium-ion era, i.e. in about ten years, “we’ll be looking at double the range at half the cost.”



The batteries developed by Accumotive are designed to have a service life of at least ten years under all conditions. “And we’re definitely achieving that,” says Wilstermann. But that doesn’t mean that after exactly ten years the batteries will suddenly and without warning refuse to charge or discharge power. It just means that at worst they’ll be performing at 20 percent less than full capacity. The end will come rather more slowly. Under optimum conditions, however, a battery can go on storing and discharging energy for a quarter of a century.

When it comes to service life, it’s a huge benefit that Daimler was the world’s first company to use lithium-ion batteries, in the S 400. “We therefore have the most experience in the field, and that’s priceless. Electrochemistry is a complex field, and we now have, including the development phase, nine years of real-world experience and can now inspect batteries that have been working for that long in a car. What we have been able to learn from three battery generations will be applied in the fourth generation. Today we are supplying modularised, made-to-measure solutions for the various model series.”


  • The job of charge management is to optimise the service life of the battery and the range of the vehicle. The quality and capacity of the cells play a key role in this. Other parameters include:

    • where the battery is installed

    • the required cooling

    • the temperature conditions where the battery is installed

    • the requirements of the particular vehicle model

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