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Strange alternator/charging problem.
So I have a 2005 2900 coronet with twin 8.1 crusaders. The boat has internally regulated alternators. Port motor charges Engine battery bank and STBD charges house bank. All batteries are brand new.

The house bank charges fine, motors on, charges to ~14V
The engine bank charges to only 11.8V, the battery bank rests at 11.2-11.4V.

First thing the guys did was send the Port alternator to a repair shop, they dynod it, repaired the rectifier and sent it back saying it was fixed. Alternator was reinstalled and the same problem occurred. Then the guys swapped the Port and Stbd alternators to see if the problem followed, and oddly the problem stayed on the same side. Even weirder they say if only one engine is running, everything works fine. Meaning if the Port engine is running alone, the engine battery bank is charging properly. When you start the stbd motor the engine bank voltage drops.

They yard folks are going to look into it more in depth later this week. But do you guys have any ideas? Im stumped.
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Are you sure you don't have a bad battery or two on your engine bank?

I'd start with both engines off, battery charger off, battery switches off, and test each battery with a digital volt meter and a battery tester. A fully charged battery should read 12.4 volts give or take. A bad battery can cause all kinds of strange problems.

Sent from my XT1635-01 using Tapatalk
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The batteries were brand new as of my refit less than a month ago.

So I just got word from the yard, they think they've fixed the issue. It was very confusing to them too, but they basically went wire by wire and connection by connection and just disconnected and cleaned any corrosion. Now the problem seems to be fixed. Theyre getting 14V on both battery banks as it should.
[-] The following 1 user Likes wallyman424's post:
Misdirection (06-27-2019)
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Took the boat out today. Definitely better than before. Port motor is 12V at idle and gets up to 13 or so at cruise. Its still not as high as the stbd motor though... May need to just give the batteries a good charge.
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Strange alternator/charging problem.00