0.35 volts on dead alternator output

[Oily Rag]

With the engine off and both engine and domestic battery breakers open, there is a voltage of 0.35 volts above the battery negative terminals registering on a basic digital voltmeter. This with a connection to the alternator main output terminal, B plus. I have not tried to measure the current that would flow through a wire connecting the two, but I'm left scratching my head.

The boat has hull, engine and shaft anodes and all I can think of is the difference in galvanic output of zinc and the cast iron of the engine block. Could my DVM be measuring the voltage caused by the anodes?

Just to be sure, when measuring the voltage across the dead alternator output and the battery positive, it is less than across battery terminals, by about the same small difference.

Any ideas?

 

[William_H]

Digital multimeters have a very high input resistance. So can smell a voltage that is not really there in any practical sense. Yes it can detect galvanic generated volts of course.
Suggest you disconnect the output wire at the alternator and see what you get. Nothing? Then check wiring back including possible leakage of breakers. (Isolation switches I presume) If you get the voltage at the alternator with wire removed. I would firstly suspect the meter. I have an elderly MM where occasionally volt readings are way out and even indicated before touching probes to anything.
In any case it is time to dismantle the alternator to get it working. Start if you can (depending on access) by checking brushes and slip rings for pressure and cleanliness. This area of transmitting the field current to the rotating armature is the main cause of alternator failure. ol'will

 

[john_morris_uk]

Wot he said. ?

 

[Alex_Blackwood]

Digital multimeters have a very high input resistance. So can smell a voltage that is not really there in any practical sense. Yes it can detect galvanic generated volts of course.
Suggest you disconnect the output wire at the alternator and see what you get. Nothing? Then check wiring back including possible leakage of breakers. (Isolation switches I presume) If you get the voltage at the alternator with wire removed. I would firstly suspect the meter. I have an elderly MM where occasionally volt readings are way out and even indicated before touching probes to anything.
In any case it is time to dismantle the alternator to get it working. Start if you can (depending on access) by checking brushes and slip rings for pressure and cleanliness. This area of transmitting the field current to the rotating armature is the main cause of alternator failure. ol'will

Why does he want to strip the alternator? the engine is not running , he doesn't say that the alternator is not working . I suspect he is getting a reading from the anodes.

 

[Oily Rag]

Thank you all. There is no problem with the alternator, but I've just had a week trying to defeat the alternator booster device.

Perhaps a bit of thread convergence with "Battle of the chargers", but my alternator booster (X-Alt by Driftgate) senses at the moment it is switched on ("ignition" circuit activated) the voltage on the domestic battery. If it is below 12.6 volts, then the booster commits to 5 cycles of 17 minutes at 14.8v and 3 minutes at alternator normal - say 14v. So for a battery at 12.55v, the booster gives over an hour of hammering to both engine and domestic batteries.

My problem is that the booster activates when the domestic battery is above 12.8v, so if I motor for an hour or so, the whole system gets an overdose.

The instructions I have point out that alternative sources of charge such as wind or solar can be connected to the system, but must be disconnected before starting the engine, else the booster circuit cannot measure true battery voltage.

Using that info, I managed to defeat the system at the weekend by deliberately starting the engine with the solar connected via a controller And it did not activate the booster. Success!

So the question is "Why does the booster activate when the domestic battery is at 12.8v?"

And that's when I discovered that the alternator B+ is showing 0.35v above the battery negative. The booster might be measuring not 12.8v, but 12.45v. And that's why I asked if it could be a galvanic voltage being sensed.

Any thoughts or experiences worth sharing?

 

[PaulRainbow]

Why does he want to strip the alternator? the engine is not running , he doesn't say that the alternator is not working . I suspect he is getting a reading from the anodes.

The thread title says "dead alternator" Alex.

 

[Oily Rag]

Apologies. My use of the word "dead" has caused confusion. Please see post #5 for details. I should have said "inactive alternator". ?

 

[Alex_Blackwood]

The thread title says "dead alternator" Alex.

Not the text Oily's second post does help. Outside my expertise though I shall leave that one to you!