Alternator Diode Rectifier Woe

[hanleyclifford]

I think I have learned a valuable lesson. For several years I had my boat in the barn with all batteries live on a charger. When I went to commission and launch I found the alternator fried. One expert suggested that an imbalance caused by resistance buildup on either the + or - side had done this. I should have disconnected the output cable from the alternator. Any thoughts?

[Dave Neptune]

That is something I haven't heard before. It does not make sense to me. How would the current be going through the alt? A diode can fail from a momentary surge while switching on off even in a charger. And I would assume that the charge level while in storage would be rather low for maintenance only IE no big amp ratings and even with big charging amps from a charger I have never heard of diode damage.

Maybe Joe or Neil would have an idea.

Dave Neptune

[hanleyclifford]

The man who told me this said it had something to do with an imbalance in the resistance between the - circuit from the housing back to batt - , and the resistance from alt+ back to the battery +.

[hanleyclifford]

This is above my pay grade.

[joe_db]

This makes no sense to me. I could go a month or two between outings if the January to March weather is nasty, my boat has a charger on 24/7 at the dock, and nothing bad has ever happened to the alternator. The diodes prevent current flowing backwards through it.

[ndutton]

Sounds to me like you jumped to a conclusion and now you're searching for reasons to support it. That's why things aren't adding up. For one example we have the high resistance guess. Resistance is not a factor unless current is actively flowing so if the guess is in play, why is current actively flowing through the alternator when it's at rest?

[Dave Neptune]

HMMMMM

Hanley, are the batteries good? And how/where was the charger hooked up?

I used a std 6/12V 2/10amp battery charger. I hooked it up to the output feed from the alt for the positive and to the main ground buss for the batteries, not the best way but most convenient due to access. The batteries were under a bunk and the engine was too and with the engine cover off both were exposed. I left the power switch to both and did this for 34 years. I also keep my battery in my hot rod hooked in the same fashion as the battery is under the seat. I clip to the output of the alt and an engine mount as both are exposed the the engine is not fully covered. No issues so far there either.

I would talk to another alt shop repairman to hear his opinion then tell him what you were told.

I kinda think the guy was trying to be impressive above his "paygrade" and knowledge. I've blown a few diodes in my time but never with a charger. This has peaked my curiosity.

Dave Neptune

[joe_db]

If a diode is ALREADY going bad, the charger could feed power back through the alternator and possibly damage it.
Without a charger the battery would do the same thing
* remember diodes can fail open or closed, open is less power and closed is reverse current flow

[hanleyclifford]

Sounds to me like you jumped to a conclusion and now you're searching for reasons to support it. That's why things aren't adding up. For one example we have the high resistance guess. Resistance is not a factor unless current is actively flowing so if the guess is in play, why is current actively flowing through the alternator when it's at rest?

Current is indeed flowing when the boat is in the barn with all batteries connected to the charger. I work on the boat, run the engine frequently, and sometimes leave lights and refrigerator running.

[ndutton]

Okay, what and how much current is running through the alternator when the engine isn't running?

[hanleyclifford]

Okay, what and how much current is running through the alternator when the engine isn't running?

That is what I do not know, together with the "why and how". Too late to check now since the boat is in the water with another alternator (functioning normally). Maybe I should consider a method to measure current flow thru the alternator?

[ndutton]

Maybe I should consider a method to measure current flow thru the alternator?

That information would be helpful for diagnostics.

[hanleyclifford]

That information would be helpful for diagnostics.

Would I be correct in saying that, at rest, no charger on, engine not running, but batteries and alt+ connected to the main buss bar, there should be zero current flow thru alt+?

[hanleyclifford]

Here's the sequence. When in the barn the batteries remain on, charger connected holding just over 13 volts EXCEPT when I run the engine. After shutting down the engine the charger is switched on again. One day when I started the engine the alternator would not charge - just like that, no noise, no smoke, no smell, no heat on alternator. Does this not preclude problems with the stator or field windings?

[hanleyclifford]

This is my alternator 110 amp: https://www.fisheriessupply.com/balm...ntinued/71-110