"GC" wrote in message
...
I'm about to wire up a battery switch so that I can switch between two
batteries. (one or the other or both)

Don't use the "both" setting ... read on

Just wondering if I should put the switch on the positive or negative??
Or
doesn't it matter?

Positive is the norm around here (geez ... hurts the head to even think
about switching the negative since that's normally the electrical reference)

Also, do both the batteries have to be the same age/rating??? I was
hoping
to use my old battery for powering accessories when I'm stopped, but
charge
it when the engine is running with the other battery.

In your situation, I would ... (get myself a deep cycle for the accessories
.... e.g. my accesory is my electric trolling motor)

a) make sure the switch is a make before break type which they normally are
so that the alternator is never without a load if switched while running;
b) when stopped select the old battery (if it's a regular starting type
battery it won't tolerate many cycles of being discharged), before starting,
switch to the new battery;
c) while running charge batteries by switching between the two (hmmm ...
when are they charged ... well, maybe your batteries have indicators of the
green it's charged variety?);
d) never connect the batteries ... it's my understanding that the charged
one could pump a lot of current into the discharged (my theory ... a
battery (let's just say for the sake of argument rated 800 cold cranking
amps) has no current limiting feature like a battery charger does) which
could result in 800 amps going through wiring/switch resulting in fire,
exploding battery, ...


Thanks in advance!


My 2 cents worth ... mainly because I'm up at 5:00 in the morning mulling
over a work related situation ... er ... challenge :-)