On Wed, 4 Aug 2004 22:28:51 -0400, "Doug Dotson"
wrote:

Or spend $8 for a snubber and install it. Eliminates the potential
for frying the diode pack.

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

Are they a stock item, and if so, where do you get them? Do they have
any destructive failure modes?

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

I bought a combiner from West Marine for $70, leave the battery switch
at "House" and believe I don't have to switch the thing at all unless I
wear the house bank down too far to start the engine. And then I'm
switching when the engine is off. I turn it to 'off" when I get tied up
to the dock. I always remember to turn it back to "House" because
nothing works until I do.

Capt. Jeff