I've been using one of these for 5 years. Mine was manufactured for BMW by Batterytender.
http://www.turnermotorsport.com/p-41...ng-system.aspx
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2006 Z4MC - #LL93158
SS Race Cans, RE X Pipe, DKF Sec 1, Fabspeed Headers, OE STB, AS Shifter, Dave Z CDV, GC Coil Overs, Stoptech lines, Hawk DTC Pads, RE Clutch Stop, RE Transmission Mounts, Vibra-Technics Engine Mounts, Poly FCAB & RTAB, LeatherZ M Shift Boot, Tekarbon Rear Spoiler, CF Grills, Advan RS DGM