Wolfram Walter with his self-converted Porsche EV, charging it up with solar power. Image: ASD Sonnenspeicher.
Depending on the technology used, at 1.2 to 3.7 Volts per cell, today’s batteries have relatively low nominal voltages. To keep electric cars running, however, the batteries used need between 300 and 600 Volts in total. How can this be achieved? Thus far the only and therefore the traditional way of achieving this has been to connect many battery cells in series until their combined voltage covers consumption. This means, for example, series-connecting six 2-Volt cells to achieve a nominal voltage of 12 Volts. In series connection, the voltage of each individual cell is added up to make up the total voltage.
Now you may ask yourself what the problem is with this solution. As an engineer and co-founder of a power storage system manufacturer (ASD Sonnenspeicher), I know that connecting a large number of cells in series to achieve the kind of voltage required for e-mobility is associated with several problems. This applies, in particular, to lithium-ion technology, which has become ever more popular. But older, lead-based battery technologies also reach their limits as the number of cells that are series-connected increases.
To be precise, series connection comes with three problems: Series-connected cells always have to be identical, which incurs considerable cell production costs. Reject rates of up to 40% in cell production are not unusual. Series-connected cells in electric vehicles face another dilemma, because different cell types are best suited for optimum acceleration and range respectively. High-performance cells are best suited for acceleration, while high-capacity cells are better for long range. As the cells need to be identical, engineers have had to resort to a kind of compromise by selecting cells which can be used for both acceleration and range – albeit to a limited extent. In my view, the third obstacle for e-vehicles is the limited vehicle design options. The series-connected cells must be built as a block, reducing design flexibility.
Some commonly encountered problems with series connection. Image: ASD Sonnenspeicher, cropped by Andy Colthorpe.
All of this led me to start developing a solution almost two years ago, which I now sell under the name of Pacadu through my company, ASD Sonnenspeicher. The Pacadu technology is based on parallel-connected rather than series-connected battery cells. This has the advantage of not requiring identical cells. If a cell is defective, it can simply be replaced with a cell from a different manufacturer, or even different battery technology. With series connection, the weakest cell determines the output of the entire battery pack. As a result, a defective cell means that the entire battery has to be exchanged. The Pacadu technology has been awarded the Environmental Technology Prize by the federal state of Baden-Württemberg.
Why I'm testing it on my own car first
Currently, we are only selling stationary storage systems, but I firmly believe that our technology will also be used for e-vehicles one day. For parallel connection eliminates the issues faced by e-mobility: High-current and high-capacity cells can be combined in a single vehicle to fully perform their own, different functions. And the fact that parallel connection does not require a block layout will give vehicle designers much more flexibility.
There are, as yet, no commercially available e-vehicles with our Pacadu technology. But there is a prototype: I retrofitted my Porsche Boxster with Pacadu technology, turning it into an electric vehicle. I drive around 16,000 kilometres per year, powered by electricity from my own solar installation.
Read Wolfram Walter's August 2014 guest blog on his original project to convert the Porsche Boxter to run on solar and how frustration at the lack of available solar storage solutions in the early days of the technology led him to set up ASD, here.