Boat Forum / Cruising / July 2006

If you are worried about it install a "zap stop"

Some switches if switching from "1" to "both" to "2" will make before
breaking and not zap your alternator.  If you battery switch opens or if you
go through "none" then yes you may fry your alternator.

The ample power people are trying to sell THEIR products by badmouthing their
competitors. They advise against even having that selector switch, because they
sell a much more expensive one that "might" protect you from something that
almost never happens. If you know the make and model of your switch, you can go
to the website of whomever made it, and they will be able to tell you whether
THEIR switch is "make before break".  If it is, then switching while running is
perfectly safe as long as you do not switch it OFF. They probably won't be so
dishonest as to disparage other company's products to promote their own.

CWM

This is no wives tale.  Look at any regular battery switch and it will say
on the front "Stop engine before switching off."

When the load is removed suddenly from an alternator the regulator needs a
few milliseconds to respond.  During this time the magnetic field is still
generating power.  The formula for voltage is Power/current.  As the current
drops to zero the voltage rises to the point that a diode may break down.

There are two fairly simple solutions.  The Zap Stop is more or less a
sacrificial diode that sees the voltage rise and conducts it to ground
sometimes frying itself in the process.  But ZapStops cost about $30 at West
Marine so they are cheaper and easier to replace compared to rebuilding a
diode bridge inside the alternator.

The other way is a battery switch with a secondary switch for the alternator
field wire.  This extra switch turns off the field current before it turns
off the load so the alternator stops generating power before the load is
removed.  The BlueSea battery switches with automatic field disconnect cost
about $4 more than the same model without.

Way back, I smoked an alternator this way..... I had an old sailboat and the
marina put in the 1, 2, both switch for me.  So, my friends come on board
and ask me "hey, that switch is nice how does it work"?  I get all Captain
like and show them that I can have battery 1, battery 2, or both on line as
we sail/motor.  I am switching back and forth to show them....

Big Booo Boooo

Next thing, no alternator..

======================

How about starting with the switch on "Both," then switching to the house
bank when you shut the motor off?

AmplePower gave you good advice.  In theory the right technology will
allow you to switch batteries (make-before-break, ZapStop, etc).  In
practice they should be regarded as backstops that *may* save your
alternator if you switch accidently.

Most people start their engines and run in the "BOTH" position.  After
anchoring and shutting down the engine it is good practice to switch
to either "1" or "2".  That should leave you with one charged battery
for restarting later on.  If you have inadvertantly flattened a
battery, start the engine and warm it up on the remaining good
battery.  After warming up the engine, shut it down, switch to "BOTH"
and restart.  It's good practice to continue recharging the battery
that got flattened back at the dock since it is unlikely to get fully
recharged on the run back.

As you see, there are many answers, each of which is at least partially
correct.

Personally, the next time I make major mods to our battery banks, the
starting battery will be directly connected to the alternator, the house
banks connected to that battery with a simple on/off switch.

First time I saw that setup was on a charter boat. Dead simple: On when
motoring, off otherwise. The major mistake of course would be to leave
them connected as you drew the bank down, but fumble-fingers can't fry
the alternator.

I had the same concern - I did install a Zap Stop and did switch from
starting battery to house battery (without shutting down motor) after about
1/2 hr of motoring so as to charge the house battery. I did not fry my
alternator, but always had the concern that someone WOULD switch via the OFF
position. I had considered making a removable plate that could be put on the
Guest switch that prevented it from going to "off"  but in the end, I
decided to change the set-up and installed a Blue Sea Systems’ Dual
CircuitTM Battery Switch. See:

http://www.bluesea.com/product.asp?Product_id=174086

With this switch, the alternator is always connected to one of the
batteries. It can be set to "combine" to parallel the batteries so
alternator charge gets to both batteries. I believe that this can be done
without shutting down because there is no chance of an open circuit.

However, I also installed a Blue Seas battery combiner with alternator going
to house bank. See:

http://www.bluesea.com/product.asp?Product_Id=53476&d_Id=7458&l1=7458&l2=

Although it shows an outboard motor, this is essentially the set-up:

http://www.bluesea.com/Article_detail.asp?Section_ID=137&id=273

BTW - I have no connection with Blue Seas - Just found these new products
worked well for me.

GBM