> I am now being asked to power the unit with a 12VDC
> car battery. How is the best way to do this and how
> do I guard against reverse polarity?

    To protect against reverse polarity you can
    place a diode in series with one of the power
    supply inputs.  I use...

                   |\ |            |-----|
        V+  -------| >|-------+----| Reg |----+----
                   |/ |       |    |-----|    |
                              |       |       |
                             ---      |      ---
                             ---      |      ---
                              |       |       |
                              |       |       |
        V-  ------------------+-------+-------+----

    In my case, the diode is a 1N5819 because I want
    to lose the smallest voltage possible (to maximise
    battery life).  In general, a 1N4001 is fine (this
    all depends on your current of course).

    To protect against dumps and surges (and lightning
    strike!) you can place any standard protection
    circuit across the inputs prior to the diode.

___Bob