Bad Alternator

[Michael44]

Hi, I owned another cabin cruiser a few years back and just bought my Sundancer 240 a couple months ago. The previous owner had to charge the batteries to test drive. It showed 12.5 to 12.8 on the digital when i first started it, so I thought it was charging ok. I switched to the temp and oil and water gauges mostly to make sure all those were fine.
Well, I have it home, I hook up shore powder (as directed) and batteries are charging fine.
All other power is off, including the battery control switch. I go back the next day, get on the boat, turn the battery switch on and radio, lights, etc, all working good. I turn it all off, except the battery switch, and turn off the AC braker and unplug it from powder.
I go back 2 days later and the batteries are dead 2 to 3 v on both.
I turn off battery switch and plug AC back up and chanrge batteris, works fine.
But when I put a volt meter on battery 1 and turn switch to bat 1 it drains slowly, but bat 2 stays the same voltage. If I switch to bat 2 then bat 1 stays the same and bat 2 drains slowly.
I tracked down all i could for shorts or heat draws, nothing. So, I took the battery wire off the alternator (had a small spark). And the slow drain stopped. Alternator looks to have a bad diod in it. I have another alternator on the way.
Q? Question- is it ok to turn the battery switch on when on AC power and charging the batteries? My lights and other items do not work unless the battery switch is on.
I know, its long but thought i would answer most questions that everyone would be asking first.
Looking forward to chatting with you.
Thanks,
MAG.

 

[northern]

When we are on shore power I leave all the 12 volt items on. I turn on the 120 volt AC converter that charges the 12 volt batteries. We use the 12 volt: lights, water pump, shower sumps, heads

 

[Michael44]

Hi northern.
Yea, that's the way I did with my other cruiser but didn't know if I was missing something on this one.
I think (and hope) it's just the bad alternator that is causing it.

 

[Eddie B]

Did you ever find out if that was the issue? I’m having a similar issue where batt 2 is drawn down over a week or two. Also volt meter reading 12.5 in no wake zone then 13.5 while running at cruising speed.

 

[Nater Potater]

Also volt meter reading 12.5 in no wake zone then 13.5 while running at cruising speed.

Boat engines, like airplane engines, are designed around steady-state running at speed. Consequently, all the accessories are optimized for cruise rpms. Compare the pulleys on the engine and alternator versus on your car, and you may find that the ratios are such that the alternator doesn't turn nearly as fast on the boat as the car. All that being said; with a fully charged battery, you should see around 14.2 to 14.5 volts coming out of the alternator when the engine is at cruise rpm.