Stephen Trapani wrote in news:snwki.452
:

So I hook the alternator and AC charger to the common terminal of the
big battery switch, ditch the extra wire the PO had coming off of each,
and I have terminal 1 on the big battery switch go to the starter
battery and 2 terminal go to the house battery. Is that right?



There is a problem hooking it up this way. If you EVER screw up and plug
in the charger with that switch in the OFF position, the full open
circuit voltage of the battery charger, somewhere around 20-25 pulsating
VDC, will be applied to everything connected to the COMMON side of the
switch....without the battery regulating the maximum voltage of it. This
will blow every electronic gadget hooked to it on a live circuit....EVEN
IF IT IS NOT TURNED ON! Electronic gadgets use electronic switching, not
real power switches, those push button on gadgets. It will destroy them

So, I don't advocate doing it. I advocate using an isolator for both.
The diode isolators are fine. Connect the alternator to one and the
charger to another...SEPARATELY... There are 3 terminals....BATTERY 1,
BATTERY 2, SOURCE (the alternator on one and the charger on the other).
This will CHARGE both batteries from whatever charging source is running,
even both, WITHOUT inadvertently parallelling the batteries because one
of the diodes will be reversed biased when one battery tries to load the
other when running on batteries.

The other way to do it is with continuous-duty 12VDC and 115VAC
contactors available from auto parts places more cheaply. These look
just like a starter relay, except they have coils made for continuous
duty (always on).

When I hook these up, I use a manual switch feeding 12VDC by the engine
controls for the alternator charging and a 115VAC contactor for the
single-ended battery charger that works automatically any time you plug
the boat charger in. The heavy current contacts of the 12VDC contactor
parallel the + of the starting battery with the + of the house batteries
and have large capacity short cables between the two battery banks so I
can parallel all batteries for starting, sort of like jumper cables, if
the starting battery fails and the house batteries aren't dead. It gives
the starter all the power I have for starting a recalcitrant engine.
Normally, I would leave the charging switch off, starting the engine on
the starting battery alone without loading it with the partially-dead
house batteries we used last night. As soon as the engine starts, I flip
the switch, closing the big contactor, parallelling the batteries on the
single alternator charging the lot of them. To prevent me from
forgetting to open the switch, inadvertently leaving the house wearing
out the starting battery all night, the 12V power for this manual
charging switch comes from the engine ON switch. Switch off the engine
and power is ALWAYS removed from the alternator's contactor, no matter
where the charging switch is, disconnecting the parallel circuit used for
alternator charging. Placing the manual charging switch right next to
the engine keyswitch reminds most, but not all, sailors to check the
switch when turning the engine on and off.

The normal procedure would be:
Charging switch off
Engine switch on
Crank engine
Charging switch to on for charging and running it all from alternator.

Shutdown is:
Charging switch off
Engine off
just to get you used to switching the charging to off for the next start.

When I plug into a dock, or crank the AC power plant if you have one, the
AC voltage automatically closes the AC contactor, parallelling the
battery banks for charging from the single output shore power charger.
Unplug the boat, the contactor opens the parallel circuit, automatically.
AC contactors are found at electrical wholesale stores. Be sure to get
one that is explosion proof in case there is a gas leak, even in a diesel
boat. Sealed contacts corrode much later than open contacts. Both
batteries will charge, without question, from the AC charger,
automatically placed in parallel for charging by this contactor any time
115VAC is applied to the boat/charger circuit. Get the power for the AC
contactor from the charger side of the charger's AC circuit breaker. If
you turn off the charger with the breaker...the battery contactor
separates the batteries, automatically. Simple and very effective.
Damned near "Captain Proof"!

All this is done SEPARATELY from the battery switches....directly to the
batteries, themselves. The contactors are right on the side of the
battery boxes to reduce cable length/resistance. Hooking it up
separately eliminates any possibility of the alternator or charger being
connected up to expensive electronics without a proper battery in the
circuit. You can charge from either source with both battery switches
OFF this way. It matters not where the battery switches are set.

Be sure to FUSE the batteries' primary circuits, too! WAY too many boats
have no circuit protection in the battery primary circuit! A shorted
starter makes a battery go BOOM! It doesn't have to be that way. I'm
using #2 cables with 250A fusible links available from West Marine. The
starter doesn't blow them....even if the engine is locked unless you hold
the starter on over a few seconds..

EACH Battery (-) through fusible link to common (ground).
115VAC CONTACTOR
|-------| |--------|
| |
STARTING+-----|-------| |--------|------------+HOUSE
12VDC CONTACTOR

+12V-----][-------][-----12VDC CONTACTOR COIL-----GND
ENGINE CHARGE
KEYSW SWITCH

115VAC (HOT)------][------115VAC CONTACTOR COIL---AC NEUTRAL
CHARGER
BREAKER

(Sure wish we could post pictures of SCHEMATICS!)

Because the interbattery contacts NEVER leave the load disconnected from
a battery, switching them hot charging never pulses anything. You see
the lights get brighter...(c; If you don't care that the house batteries
may also be used by the starter, just leave the CHARGE SWITCH on all the
time. The engine keyswitch always turns off the 12VDC contactor,
anyways.


Larry
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