Used car batteries stabilise renewable energy microgrids

December 09, 2015 // By Christoph Hammerschmidt
The (albeit slowly) growing popularity of the electromobility increasingly raises the question of the whereabouts of used traction batteries. Nissan now has announced a partnership with Power Management System vendor Eaton that gives these batteries a second life – in energy storage systems designed to stabilise microgrids from renewables. Is this approach a new trend?

Nissan and Eaton used the ongoing 21st UN Conference on Climate Change (COP21) in Paris to introduce their plans that, according to the companies, will create a win-win situation for owners of electric cars and operators of photovoltaic installations alike: Eaton will introduce an energy buffer system that stores the electricity from intermittent energy sources like solar roofs and wind turbines. In phases when demand exceeds power generation, the system feeds the stored energy back into the grid.

Technically, the system consists of a rack that accommodates multiple car batteries and a second rack with the control electronics, i.e. DC/DC converters as well as AC/DC and DC/AC converters that can be activated according to the power demand and generation situation. Eaton claims that this part of the installation has an energetic efficiency of up to 96 % depending on the load situation. The company did not reveal many details; a spokesperson just said that the power components used are not made of silicon carbide (SiC) but of conventional transistors. A major differentiator certainly lays in the control algorithms that can take into account the multitude of different tariff structures in European countries.

The batteries that store the electric energy have already a career as traction battery in Nissan’s Leaf behind them: Originally sold with an energy content of 24 kilowatt hours, they are transferred to their second existence once their energy content has fallen to a value that Nissan declined to specify exactly. In any case, they still have at least 70 % (and probably more) of their initial capacity. In the energy storage rack they spend a second life of several years, probably with less thermal and mechanical stress; Mikhail Bakunin, Manager New Business Development for Nissan said he expects a second life cycle of up to ten or even twelve years.