After being all about diesel at the Detroit Auto Show this year, it looks like BMW is going the next step by adding in hybrid technology next month at the Geneva. A new X5-based concept, apparently called the Vision EfficientDynamics, will be on display and it will employ the mild-hybrid system that BMW is developing in cooperation with Mercedes-Benz. In this application, BMW has combined the system with the 201 hp 2.0L twin turbo diesel used in the 123d. Based on the diagram you see above, the hybrid system uses a 120V lithium ion battery that doesn't appear to be much larger than a regular 12V car battery. Electric assist comes from a 15kW motor paired up with an 8-speed automatic transmission. A solar roof is apparently used to pre-heat the transmission fluid, presumably to improve efficiency during cold starts. Mercedes already showed several vehicles equipped with this same mild hybrid system at the Frankfurt Motor Show last September and it's expected to go into production in 2009.