Originally Posted by Saa Bear
Does anybody have a comment on this this note I received
Direct Current Generators to produce a direct flow to batteries. The problem with them is that they are not a efficient as the newer Alternators. The alternators on the other hand do produce Alternating Current that needs to be modified to Direct Current before being sent to the batteries. I understand there are three diodes that are designed to cut off the half of the current that "flows backwards". When the engine ignition is turned off before the alternator a back flash of current can destroy some or all of the diodes. IF this happens they can be replaced quite easily. I have found a local expert who became so used to my problem that his price for repair was significantly reduced. The diodes themselves are worth very little, -it is the time to take the unit apart and test it that costs.
The alternator itself is in good shape. My friend actually, on one occasion improved its output by adding, I believe, some permanent magnets to its casing. Someone in the business can actually do the diode replacement in less than an hour.
It is probable that at least one of the diodes is dead and should be replaced. Only someone who knows the business and has a test bench can actually tell you the condition of the output.
As you said the engine will continue to start as long as there is power in the startiing battery. If you have power at your mooring, the battery charger can be left on to keep the battery up to power. If you were to spend too much time at a mooring, you may be without starting capability.
The diodes referenced are in a configuration called a "bridge rectifier". How many diodes depends upon the design. There is no current "flowing backwards"....AC current alternates between + and - (its a wave form). The bridge rectifier modifies the phases of the AC current wave form so that DC current results as output. Think of it as chopping off the +/- parts
of the wave and redirecting just those parts
to output as DC current (+ on one lead and - on the other).
Damage to these diodes occurs when the output of the bridge rectifier has no where to go. Such as disconnecting the B+ "battery +" lead (normally by a switch). This could happen on some ignition switch configurations but is not typical.
Diodes are easy to test. Most multimeters (even cheap
ones) have a diode test mode.
Bridge rectifiers are realatively easy to replace. Not a bad spare to carry, especially if you have a history
of this issue on your boat.
No harm of course done by shutting down with the ignition on, but in most installations accidentally turning it off won't do any harm.