"Ed" wrote in message
.. .
I thought he was talking about the switch with the alternator disconnect on
it. (requires 2 wires run to the alternator that disconnects the field
when put in the off position)

If he is just talking about the make before break then HELL YES... spend
the $10.




wrote:
Ed wrote:

It's not the $10. it's the pain of runing the extra wires. Go for the
regular one....

JimH wrote:

I am putting in a 2nd battery on my boat and will need an A/B switch.

Is the field disconnect one (such as this one http://tinyurl.com/pa9hn )
always the way to go, or is the one that is $10 cheaper but lack field
disconnect a better option? I do not switch to the other battery while
under power.....never have and do not plan to now. The switch will not
be
out in the open for anyone to fool with.

So save the $10 bucks or get the higher priced one?




What extra wires?

The switch in JimH's tiny url is just a make before break switch. You
have only the normal battery connections to deal with. The third option
"All", or "Both" is created not by wiring an additional circuit, but by
the architecture of the switch itself. The cables from the batteries
connect to arc shaped contact plates within the switch. One of the arcs
is closer to the perimeter of the circular shape of the switch than the
other, and these inner and outer arcs overlap a common radius for maybe
an inch or so. Under the top cover there is a straight "pointer" with
two contact that runs parallel to the external handle/indicator. When
battery "A" or "B" is selected by the indicator, the pointer is
positioned so that one of the
two contacts completes a circuit with only the contact plate associated
with battery/bank "A" or battery/bank "B". When "Both" is selected, the
pointer is positioned on the radius where the inner and outer arcs
overlap and both of the contacts on the pointer are engaged to complete
the circuit.

There is very little risk of removing the battery load from the
alternator when the engine is running with a switch like this. To get
to either A or B you must pass through "Both" with the selector, so the
connection to the second battery is "made" before the conection to the
first is "broken". (Make before break). It is useful to exercise the
switch a time or two before relying upon it if the boat has been
sitting long enough that the contact surfaces might have become dirty.

JimH: You can't possibly be thinking of trying to save $10 on a boat
part, can you?
Unless you put your battery switch under lock and key, some dunderhead
will find it and
switch from A or B to "off" while you're underway (with or without your
permission- or the dunderhead might even be you in a moment of
accidental carelessness). Have you priced
replacing the diodes in your alternator? That $10 is comparatively
cheap. :-)

The value of the switch in your URL is that it is easy to charge both
batteries at once when running. You will find cases where this practice
is discouraged, (I think I saw a marine supply company catalog at one
time that suggested it was better to buy two switches, one for each
bank, than a single A/B/Both switch). The pratice is also specifically
encouraged by experts such as Charlie Wing, author of "Boatowner's
Illustrated Electrical Handbook," now in its second edition. Just like
most other things related to boating, there is plenty of room for
differences of opinion on this topic and no shortage of well-reasoned
arguments to support diverse conclusions.

Spend the $10. You can hardly buy a six-pack of any decent beer for
less.




Both are make before break design Ed.