Battery Voltage Drop - Any suggestions?

[CaptBobO]

Here is a question for you electrical pros out there.

I have 2007 Mercruiser 8.1S Horizon engines in my 410.
Last weekend my voltage dropped on my port motor to 12.3 and the engine died.
I couldn't get it started using the emergency switch. I started up the generator and let it charge my batteries. Then, the voltage dropped on starboard and that died as well.
Thankfully after about 30 min the batteries charged up just enough to start the SB motor and start heading back in. the alternator was able to send a charge back to the SB bank.
Then, the Port motor finally started up due to my generator helping out.

My question is, what is the world could have drained my battery that quick?
Side notes: my stereo is on its own dedicated bank of batteries, so that was not part of the drain. Also, my batteries are only 8 months old. I replaced all batteries in the boat. My port motor also has a brand new alternator.

Does the alternator send the charge directly to the batteries, or does it go through the built in battery charger first? Reason I'm asking, is could an old/defective battery charger be the cause? mine is original to the boat.

Once I finally got home, I left it plugged in to shore power, and the next day, it started right up!
14.3 volts across the board. unplugged it from shore power, and still held 14.3 at idle.

What are your thoughts? what could have drained my battery that fast after a long run and then an idle cruise for about 20 min?

Thank you!

 

[Pirate Lady]

No electrician, but the alternator does not go thru a battery charger, ever, eliminate that idea.
I dont think your batts drained that quickly, not sure if even possible.
Batts only 8 months old, story time, had an issue, buddy kept asking about batts as we tested everything. End of day we took the batts to be tested, both defective after 4 months.
I never trust them now. I would get those 8 month old new batteries tested.

 

[scrumper]

Very odd that the motor died, that's not right at all and I'm not sure is the battery's fault either. The alternator runs the engine once it's turning, and should be outputting more than 12.3V (should be 13+). Check all your cable connections for corrosion, in particular the main grounds. Other thing maybe the alternator's voltage regulator, but cruddy grounds seems more likely with 14yr old motors on a 21yr old boat.

 

[capecodcruiser]

Very odd that the motor died, that's not right at all and I'm not sure is the battery's fault either. The alternator runs the engine once it's turning, and should be outputting more than 12.3V (should be 13+). Check all your cable connections for corrosion, in particular the main grounds. Other thing maybe the alternator's voltage regulator, but cruddy grounds seems more likely with 14yr old motors on a 21yr old boat.

The alternator does not "run the engine". The batteries supply voltage for the engine to run. The alternator keeps batteries charged.
I agree, check water in the batteries and than get them tested.

 

[ttmott]

Let's see - Port engine died and battery reads 12.3. Not related; the engine will run just fine with 12.3 volts. The only issue here is the battery voltage should be greater than 13 volts if the charging system is operating correctly. Then the port engine would not start even with the emergency start switch depressed.
Then the Starboard engine died and that side indicated low voltage.
Batteries are recent and one of the alternators has been replaced..
Does that boat have battery isolators?

 

[CaptBobO]

Let's see - Port engine died and battery reads 12.3. Not related; the engine will run just fine with 12.3 volts. The only issue here is the battery voltage should be greater than 13 volts if the charging system is operating correctly. Then the port engine would not start even with the emergency start switch depressed.
Then the Starboard engine died and that side indicated low voltage.
Batteries are recent and one of the alternators has been replaced..
Does that boat have battery isolators?

Great Question. The boat does NOT have isolators.

 

[ttmott]

At this point I would disconnect each battery and do a load tests. New batteries can be defective..
Parallel to that go through all of the ground conductors and connections from the batteries to ground buss to engines.