OK, I'm not an EE so forgive me, but it appears as though the 12v battery is a weak link in the reliability chain of the Model S/Roadster drivetrain. Failure of this battery (which has happened to several owners already, admittedly in most cases due to a software bug) renders the car completely unusable. So why is the 12v battery necessary? Couldn't a 12v DC feed pulled off the main battery be used instead? I know you'd need to step down the voltage obviously, but this would be one less subsystem to maintain/monitor. Sorry if this question's been asked before--a search showed a bunch of unrelated 12v battery threads.