Caliber Mengsk said:
O-o a second lead acid battery in the car? If it were drained regularly, that would kill the alternator sooooooo fast. I"d just get this http://www.sparkfun.com/products/99
and use the GPIO pins to get the time when needed. Assuming you could figure that out. (by that, I mean the GPIO pin programming and interfacing an I2C connection through them. I don't know what libraries are already out for the rPi.)
You must get your alternators wher nmcc gets his batteries.
If you really have some electrical draw, get a for-real alternator and a smart charge controller. But, really, a nice LiPo pack to keep the Pi up overnight (or a week or more if sleep can be implemented) will not be a heavy draw to recharge.