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.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"ray lunder" wrote in message
...
The AmplePower people agitate for strict prohibition on this saying it
will smoke the diodes in your alternator. Is this true? What are you
supposed to do? Start the boat with the starting battery, let it run
for 10 minutes, turn the engine off and start it again with the house
batteries and charge them under way? What say all of you?