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