On Apr 4, 10:59 pm, "charlie" wrote:
i have an i/o i want to install a second battery how do i wire it
using a 1 2 all switch


Connect all (-) negative grounds together. Use battery cables to
connect the (+) positive battery terminals directly to the big
contacts marked 1 and 2 on the switch. Wire the output (the remaining
big terminal ) to the engine where the single battery was connected.
There are, or should be, 2 other small terminals on the switch. They
are used to interrupt the field fire which controlles the alternator
output voltage and current. The field switch will disable the
alternator when (before, actually) the battery switch is set to off,
protecting the alternator from internal high voltage which is
generated should a battery be not properly attached.

I would install the batteries (+) terminals last to prevent accidental
shorts while wiring batteries. One instantaneous wrong connection can
be disasterous.

I would check before installing that the field circuit opens before
the battery terminals do. Some switches, it is said, do not.

Welcome to the arguement concerning equalisation of batteries while in
the "both" position. You will want a hydrometer and a supply of
distilled water. Use only steam distilled water to replenish the
batteries.

You will probably want to wire bilge pumps directly to a battery (+),
so that the pumps still work when ships power is turned off.

If you are nervous, you should be. If you are not qualified, hire a
skilled worker. Explosion and fire is a constant hazard when working
with storage batteries.

Terry K