battery drain

I have a 2006 Mainship 34T, twin screw.
It's my first full season with the boat. I put the boat back in in Oct of 2018. The batteries were cooked. I attributed that to the PO's lack of concern for the water level in the batteries. I added 3 gallons to 4 batteries. 3 8D's and 1 #27

The 8D's are a combination starting bank plus the house bank. he 27 is dedicated to the genny.

They are all brand spanking new. Just did a direct replacement. Set up as is.

The batteries will discharge down to 12.4 vdc in 1 hour from 13.9 vdc. I turned off the battery switches, same thing.

I disconnected the battery terminals, after a full charge. They dropped to 12.65 vdc after 4 hrs. and 48 hrs later they were 12.55 vdc. I think that's fine.

Previous boats, I could let sit, with everything turned off, for 2 or 3 weeks. Go to thew boat and start her right up. On the hook, I would get 24 hrs without worrying. That's with fridge, lights, fresh water, stereo, the usual stuff. The last boat I had a solar panel and would go 2 days.

This boat, I have to start the genny 2 or 3 times each day to keep things OK. If I leave the boat for 3 days, I can't start it. I have to plug in to charge up the batteries.

There is a battery isolator, but it's only on the one engine with 2 8D's. The other engine has an 8D, no isolator.

The genny has a completely separate battery. Tied into NOTHING.

Why would ALL the batteries discharge? The only system that ties the entire boat together is the bonding system. That is factory installed. The only after market items are the battery isolator and a separate 110v charger for the genny battery, (not hooked up to the charging system, only handles 3 batteries)

Seems to me that the system is just dumping power through a ground?
HELP ME!!!!!! (said the fly in the web)

I had the same boat but with a single engine and no generator. The battery switch disconnects the starter and main DC panel loads but some loads are wired directly to the battery terminals. That is what is drawing down your batteries but at a very high rate it seems. And I can't understand why one or two such loads (bilge pumps, CO detector, etc.) draws down all batteries.


Your boat obviously has a wiring problem. You can trouble shoot it youself with a voltmeter and clamp on DC ammeter if you have the electrical skills or hire a good marine electrician.


The later may be hit or miss. Many will just want to rewire the boat the way they were taught or think is best. You want someone who will think things through and diagnose the problem, not cover it up with revised wiring.


David

I had a similar problem lately that had me shaking my head. The G31 start battery would drop to 12.4V soon after shutdown of engine or charger. Went through 2 new batteries in as many years. Finally when the 3rd started doing the same, I got inquisitive and started disconnecting items on the start battery, one at a time. Starter, charger, windlass, alternator,...EUREKA! The alternator was causing the discharge.

Took it into the shop and it tested bad with a shorted, not blown, diode. The short was placing a load on the battery. New alternator and problem solved.

Molly said:

The 8D's are a combination starting bank plus the house bank. he 27 is dedicated to the genny.

There is a battery isolator, but it's only on the one engine with 2 8D's. The other engine has an 8D, no isolator.

The genny has a completely separate battery. Tied into NOTHING.

Why would ALL the batteries discharge? The only system that ties the entire boat together is the bonding system. That is factory installed. The only after market items are the battery isolator and a separate 110v charger for the genny battery, (not hooked up to the charging system, only handles 3 batteries)

Click to expand...

I'm a little surprised at your comment about the charger only handling three batteries. Your two 8Ds in one bank is 1 battery, your one 8D forming the other "bank" is 1 battery...

So if your charger is a 3-bank charger, you've only using two of its bank capacity...

Not a suggestion to change or anything's wrong, just a raised eyebrow at what you said...

-Chris

Remove the positive lead from both batts and hook a voltmeter in series with the battery and the cables, yes in series.

If nothing is draining the meter will read zero since there is no connection to ground. If it reads 12V start disconnection or turning off circuits until the meter drops down. The last item will be the culprit.

If the current draw is large you may damage the voltmeter putting it in series. May or may not have protection built into the meter. But if it works it will tell you what you want to know.

Comodave said:

If the current draw is large you may damage the voltmeter putting it in series. May or may not have protection built into the meter. But if it works it will tell you what you want to know.

Click to expand...

Not true, I said voltmeter not ammeter. Voltmeters can and are intended to be places across full voltage.

Brain fart today while I was in the hospital...

It is also possible that your battery set is "old" from too deep discharges.

A new batt will self discharge .5% of its capacity in a day.

An old or abused batt will loose 3% internally ,or more per day.

Charge it up , disconnect EVERY WIRE from the terminals and use a digital V meter to test it for a few days.

Report back?

Dave:
No problem

DC clamp-on ammeter. Something is drawing power or the batteries are not really fully charged. When you say "fully charged", how exactly do you determine that?



Maybe a second battery switch hidden somewhere for the bow thruster left on ALL?



Fully charged and disconnected the battery voltage should be 12.7 +/- a 0.1 on the following day.