Battery Volatge Drop - What do you think?

[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!

 

[km1125]

RE: "Does the alternator send the charge directly to the batteries, or does it go through the built in battery charger first? "

The alternators are completely separate from the battery charger. THe alternators should be charging the batteries when the engines are running, and the battery charger should be charging the batteries when you're on generator or shore power.

I'm assuming you have a separate bank of batteries for each engine, but then you also have another bank for the stereo? Does that other bank do all your house loads or just the stereo?

Why does the port motor have a new alternator? Do you observe voltage on each bank when the engines are running? Do you have any sort of charge isolator so that either of the alternators can charge all of the batteries?

 

[CaptBobO]

RE: "Does the alternator send the charge directly to the batteries, or does it go through the built in battery charger first? "

The alternators are completely separate from the battery charger. THe alternators should be charging the batteries when the engines are running, and the battery charger should be charging the batteries when you're on generator or shore power.

I'm assuming you have a separate bank of batteries for each engine, but then you also have another bank for the stereo? Does that other bank do all your house loads or just the stereo?

Why does the port motor have a new alternator? Do you observe voltage on each bank when the engines are running? Do you have any sort of charge isolator so that either of the alternators can charge all of the batteries?

Great advice.
So I checked my batteries today while the boat was ONLY at idle and with all lights and bilge fans going. (Shore power OFF)
Port: 12.3 volts
Starboard: 12.5
I turned off all the accessory lights and fans, and it read this:
Port:12.5
Starboard: 12.7

I think I may have a bad batch of batteries. The other thing that I noticed, is that the batteries for both motors are Deep Cycle hooked up in Parallel. The generator battery is a starting battery.
When I purchased the boat, I immediately replaced them with what was in there. So that is why I put in deep cycle.

Since that time, I have removed the stereo and amps from the boats electrical, and have it hooked up to its very own battery bank that charges through a 110 battery charger when its plugged in to shore power or with the generator going and providing 120v.

The deep cycle batteries in there now only have 750 CA and the Starting batteries I'm thinking of exchanging them for have 1000 CA each. Keep in mind, each motor has two batteries hooked up in parallel.
You all think I should swap those out to starting batteries?

to answer your isolator question, I do not have one. What would that do if I installed one?

Thanks for all your feedback

 

[douglee25]

Make sure your amps aren't pulling from your starting batteries. I have two amps that pull from my port motor which forces me to run my generator anytime I'm away from the dock.

 

[km1125]

If those are your readings while the engines were running, then your alternators are not charging the batteries. Even with bad batteries you should still be reading over 13.2 when the engines are running (and could be up to 14.6 or so). If they were bad batteries they just wouldn't be accepting a charge. You should see if there are any breakers or fuses between the alternators and the batteries. If the engines are COLD, then feel the alternators to see if either or both are even slightly warm... that's an indication of bad rectifiers in the alternator which is also discharging the battery, but also means the alternator won't deliver a charge properly. When the engine is off you should also be able to read battery voltage at the back of the alternator (on the big wire). If you don't, then you know there is a problem (tripped breaker, blown fuse or blown fusible link) between the alternator and the battery.

Having two deep-cycle batteries in parallel probably provides PLENTY of CCA for your engines to start. At this point I would not be suspecting the batteries.

 

[Carpediem44DB]

You may need to start from scratch and determine voltage at each battery and alternator in all conditions. I would disconnect shore power, disconnect the batteries and let them sit a bit. Then check each battery's resting voltage. You should see 12.5 volts on fully charged battery. Also feel the batteries and ensure they are cold or at least not hot. A healthy battery at full charge should be room temp, if not one could be shorted and draining the other. If all of the batteries are showing good voltage, reconnect them and if you have an amp gauge installed in your power distribution panel turn every DC switch off and ensure there is no amp draw by something connected directly to the buss. People do the strangest things. Finally check the voltage at the batteries with either and both engines running and alternators charging to look for 13.2 to 14.6 volts. I would want to have Isolators installed myself and since you charger is original I would replace that with a Pro Nautic marine charger. You got your money out of the original. As to your Deep Cycle battery issue, I would think installing cranking batteries and using the deep cycle just for house and accessories. I would think that a singe group 31 battery for each engine would be plenty for starting batteries. My best guess is you will likely find some goofy wiring issue or defective battery.
Good luck in your trouble shooting.

 

[km1125]

...
to answer your isolator question, I do not have one. What would that do if I installed one?

I forgot to answer that question.

An isolator would allow either engine to charge both batteries. NOT required by any stretch of the imagination but useful if you have an alternator failure enroute. You could also use a VSR for that purpose. However, there are downsides. Having either of those in place will allow one alternator to do the job of both, so you might not notice a individual alternator failure until both alternators fail, which is even a worse situation. Having a manual switch or relay would allow you to combine them for as long as you need until you can resolve an issue.

But knowing whether or not you have either is important to troubleshooting the issues you are having now.