rock_doctor wrote:

"C.Lohnes" wrote in message
. ..

I installed 2 batteries using the 1-2 all switch and now my volt gauge

reads

+14 when i am at high rpms ,Is this normal ?


Yea, 13.7 to 14.2 is normal charging voltage. Just make sure you never
switch batteries when the engine is running, the alt puts out rectified DC
and w/o a battery connected when the engine is running the rectified DC can
fry electronics.

mark


Passing through the "both" position to change batteries will not
hurt. Properly wired field control terminals present on good
quality battery switches will prevent alternator damage from turning
batteries off while running the engine, buy stopping the excitation
in the field rotor before opening the battery line. Electronic
controlled alternators should not burn out even if the battery is
disconnected while running, but don't trust this unless you know
your alternator will survive for sure. I consider this unreliable.
An alternator might survive several switch-off occurrences, but then
fail on the next one. Don't think that surviving one or two is any
reassurance, especially since damage to only one alternator diode
may go unnoticed by the uninitiated, apprentice tech, until the
battery goes tits up.

A good over voltage (15 volt) surge supressor should protect even
sensitive alternators. Surge suppressors have a habit of failing
unnanounced, going AWOL if you will, deserting their alternator
buddies to strange fates. Even a relatively small voltage spike may
damage other electronics if you turn off batteries while running the
engine, dependant on wiring.

Terry K

Some assume that because they have switched batteries while
engine is running, then switching will never destroy
alternators. Manufacturers tend to disagree. Switching
batteries while engine is running can cause a problem known as
load dump. Load dump can damage alternator and can damage
other electronics. Get the battery change over to occur just
right and the alternator can suffer. It will not happen every
time. It is a rare event. But 10 miles out is no place to
suddenly lose both alternators. With no alternator, boat
engine will only run as long as battery charge remains. And
this assumes the boat operator knows alternators have failed.
Sometimes the failure can remain undetected by monitors.

From SGS Thompson - a manufacturer of these surge protection
devices for the load dump problem:
"The overvoltage is named the load dump and can be defined
by the following figures:
- Peak voltage 80 to 100 volts
- Duration 300 to 400 milliseconds
- Series resistance 0.2 to 1 ohms"

In 1996, SGS Thompson states the solution still leaves the
problem unsolved:

"For the first protection mode, there are several existing products
able to clamp this overvoltage at the board level, for example the
LDP24 or RBO series. The protection at the alternator level is
a quite new concept and all the technical problems do not seem
to be completely solved."



Terry Spragg wrote:
Passing through the "both" position to change batteries will not
hurt. Properly wired field control terminals present on good
quality battery switches will prevent alternator damage from turning
batteries off while running the engine, buy stopping the excitation
in the field rotor before opening the battery line. Electronic
controlled alternators should not burn out even if the battery is
disconnected while running, but don't trust this unless you know
your alternator will survive for sure. I consider this unreliable.
An alternator might survive several switch-off occurrences, but then
fail on the next one. Don't think that surviving one or two is any
reassurance, especially since damage to only one alternator diode
may go unnoticed by the uninitiated, apprentice tech, until the
battery goes tits up.

A good over voltage (15 volt) surge supressor should protect even
sensitive alternators. Surge suppressors have a habit of failing
unnanounced, going AWOL if you will, deserting their alternator
buddies to strange fates. Even a relatively small voltage spike may
damage other electronics if you turn off batteries while running the
engine, dependant on wiring.

Terry K

But 10 miles out is no place to
suddenly lose both alternators. With no alternator, boat
engine will only run as long as battery charge remains.

Yet another reason to buy a diesel

Doug
s/v Callista

In article ,
"Doug Dotson" wrote:

But 10 miles out is no place to
suddenly lose both alternators. With no alternator, boat
engine will only run as long as battery charge remains.

Yet another reason to buy a diesel

Thanks for anticipating my response.

But any quality switch I've dealt with (some 30 years old) was
make-before-break, a non-issue. Of course the engine loads down heavily
when tasked with charging the weaker bank, but that's self-correcting.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

If switching from a weaker bank of batteries to a fully
charged bank, then the armature coil inside alternator cannot
discharge fast enough. Load dump results.

Both SAE J1455 and ISO 7637-1 defined load dump for 12 V
automotive alternators as up to 270 volts and energy up to 50
joules. Still think alternator can safely switch batteries
when fully charging? What happens to 12 volt radios when
confronted with something less than 270 volts?

Provides were product lines for protection from load dump.
Does every electronic device onboard have such internal
protection (... or why marine stuff may cost more)?

Jere Lull wrote:
...
But any quality switch I've dealt with (some 30 years old) was
make-before-break, a non-issue. Of course the engine loads down
heavily when tasked with charging the weaker bank, but that's
self-correcting.

"w_tom" wrote in message
...
If switching from a weaker bank of batteries to a fully
charged bank, then the armature coil inside alternator cannot
discharge fast enough. Load dump results.

Both SAE J1455 and ISO 7637-1 defined load dump for 12 V
automotive alternators as up to 270 volts and energy up to 50
joules.

You can certainly get this when switching from a discharged battery to
"OFF".

Even a fully charged battery will absorb a tremendous amount of current if
the voltage rises very much. If the battery is any good, I doubt that you
could get the voltage up above 16 volts. Just the capacitive load of the
battery could absorb a fair amount of the energy.

The problem occurs when the alternator was pumping amps into the battery and
it is switched to OFF. It is a basic inductor problem, the current can't
change instantaneously. It WILL go someplace, and it is likely that it will
damage things in the process.

Rod

Rod McInnis wrote:
"w_tom" wrote in message
...

If switching from a weaker bank of batteries to a fully
charged bank, then the armature coil inside alternator cannot
discharge fast enough. Load dump results.

Both SAE J1455 and ISO 7637-1 defined load dump for 12 V
automotive alternators as up to 270 volts and energy up to 50
joules.


You can certainly get this when switching from a discharged battery to
"OFF".

Even a fully charged battery will absorb a tremendous amount of current if
the voltage rises very much. If the battery is any good, I doubt that you
could get the voltage up above 16 volts. Just the capacitive load of the
battery could absorb a fair amount of the energy.

The problem occurs when the alternator was pumping amps into the battery and
it is switched to OFF. It is a basic inductor problem, the current can't
change instantaneously. It WILL go someplace, and it is likely that it will
damage things in the process.

Rod


After reading the comments regarding load tripping, I was going to
write just about what you said.

Good on ya.

The surge supressor previously mentioned would protect the diodes in
the alt from inductive field collapse induced overvoltage on dropped
load as long as it lasted. Sometimes surge supressors die quietly,
leaving the system vulnerable, sometimes they work perfectly for
ages. Eventually, all electronics go poof.

This brings us full circle back to the topic overcharging. In the
above posting's scenario, the alternator would overcharge the
battery switched to for about a millisecond. No big deal.

A warning: make before break switches fail too; a little dirt in arc
pitting can transform such a switch into an intermittent break
before make device, putting us back into a blown alternator scenario.

Alternators immune to the phenomenon would include an internal
crowbar type ciruit to protect internal diodes from excessive peak
reverse voltage.

Terry K