Bo Berglund wrote:

> Using the bridge you will ensure:
> 1) No possibility for polarity reversal
> 2) There  will always be a potential difference of 1.4V between the 0V
> of the circuit and the battery negative (two diode drops)

  I suggest you think a little harder about that one!  ONE diode drop.

James Cameron wrote:

> However, I'm worried that using a bridge would result in strange
> potentials relative to vehicle ground, but I haven't thought about it
> yet.

  Too right strange things happen.  Significant if the device has *any*
external inputs/ outputs other than the battery power.

  An often recommended procedure for fitting radios is to take both
live (positive) and ground direct from the battery terminals (to
minimise interference, it is proposed).  This really comes to grief
however when the antenna (shield) is grounded to the body and connects
to the ground inside the radio.  If the negative cable from battery to
chassis and/ or engine block comes loose, guess how many pieces of wire
and PCB catch on fire?

  (Yes, I know the good brands have two line fuses.)

  Cheers,
        Paul B.