> To be completely sure to not connect a device's power
> 'the wrong way round', you may use a two-way bridge
> rectifier on the power input of the device.

    It is true that using a full-wave bridge rectifier
    at the power input will eliminate the distinction
    between the positive and negative terminals.

    In my application (which is the powering of elec-
    tronic scales for on-farm use from automotive 12V
    batteries) I have three reasons for rejecting this
    solution.

    The first two are obvious:

        Cost.

        Power loss.

    The third is less obvious:

        User psychology.

    Everybody is used to the idea that car batteries
    have a positive and a negative terminal, and that
    nasty consequences result from mixing them up.

    Farmers, especially, are used to using car batt-
    eries for all sorts of day-to-day purposes. They
    are used, therefore, to having to worry about the
    polarity of their connections.

    If you give them a piece of equipment that doesn't
    have clearly marked power terminals then they will
    just call you up and ask which way they need to
    connect the battery.  You can tell them a thousand
    times that it doesn't matter, they will _still_
    ask which way they need to connect the battery.

    So you gain nothing from producing equipment with
    non-polarised power inputs.

    Provide the simplest protection against accidental
    reverse connection (a series diode), mark the
    terminals clearly '+' and '-', color the terminals
    red and black respectively, and the intended User
    will understand what's required.

    So, coming back to Troy's original question, the
    best way to power equipment with a 12V car battery
    depends upon the intended User of the equipment.

___Bob