Can an alternator draw from battery overnight?

[gspeak]

We are finding the starboard starter battery on our catamaran seems to discharge over night so that it sometimes struggles to start the motor the next morning. Every thing is fine when we first leave the dock after having the charger plugged in & in the afternoons when the solar panel recharges while at anchor or obviously if we have been motoring.

The starboard battery is only connected to the starboard motor for loads. Nothing else draws off this battery. It is combined with the port house bank & starter battery for charging only via two Yandina Combiner 160's.

I have tested the battery (which is just 12 months old) by disconnecting the leads over night & have also used a load tester on it. It appears to be holding full charge.

I have heard that faulty diodes in the alternator can draw back. However, wouldn't that also mean that the alternator wouldn't charge? Have tested at the battery with starboard motor only running, solar & charger turned off & get 13.6V charging.

Thanks in advance.

Dave

[44'cruisingcat]

I had this with a car - stuffed diodes would cause the battery to go flat overnight, and yes it did charge when running.

[Wraith_Mac]

Originally Posted by 44'cruisingcat

I had this with a car - stuffed diodes would cause the battery to go flat overnight, and yes it did charge when running.

Me too.
I suggest you charge the start battery and when the engine is stopped disconnect the alternator.
Before you restart, reconnect it.
Get to an alternator doctor ASAP or replace it ASAP.
Why disconnect? Because if the fault goes catastrophic it will probably kill your battery very quickly.
It did mine.
Mac

[Nostrodamus]

You say it is the starboard start battery... if you have a port one you could swap the batteries over to see if it is the battery or alternator or am I right off the point here.

[El_Guero]

You left out something to help us:

When you disconnect the battery cables overnight, does the battery hold the charge?

You have a short somewhere. I have had shorts just about everywhere in my Old Ford F100, taught me more about truck electrics than I ever wanted to know.

The short can be in your battery - that is hard to find. You have to place it under a special load - the load MUST test the battery for a short.

You can have a short in the alternator, regulator, or cables, or IGNITION .... that truck gave me a few learning experiences ....

You can find a normal short (not the battery) with any ohm meter, or a lighted screwdriver test, like they use on airplanes ....

I think the load test for the battery went something like this, set up the ohm meter, check for normal voltage range. Then apply the load - start ignition. I want to say there was a specific drop in voltage output. I think it was 2 volts (12 volt system 12.5 volts dropped to 10.5). This was a long time ago, I am on fumes when it comes to memory.

But, the drop while under load shows an internal battery short ....

Any electrical mechanic, worth his salt, could bring you up to speed on anything I missed.

wayne

[Wraith_Mac]

El Guero,
The OP said in the initial post:
"I have tested the battery (which is just 12 months old) by disconnecting the leads over night & have also used a load tester on it. It appears to be holding full charge."
Cheers,
Mac

[van demon]

Originally Posted by Wraith_Mac

El Guero,
The OP said in the initial post:
"I have tested the battery (which is just 12 months old) by disconnecting the leads over night & have also used a load tester on it. It appears to be holding full charge."
Cheers,
Mac

most likely is the diode pack - a diode pack is like an electrical one direction switch when they stuff up closed circuit they pass power both directions hence the battery going flat over night

get a new diode pack and solder it in its an easy enough job for most home mechanics

regards steve

[colemj]

I have had an external regulator fail in a manner in which it drew 3amps when "off", yet still drove the alternator at charge when "on".

If you have an external regulator, you might try disconnecting the power from it.

Mark

[rw58ph]

So why not install a battery on off switch.?

The Eagle has three, house, gen set and main engine. When at anchor and/or dock the engine are turned to the off position, and when we leave for a long period of time the house is also turned off as the bilge pumps are hard wired to the battery.

[David M]

Alternators should not be drawing current from the battery. Don't compromise by using an alternator shutoff switch to stop the current draw. Get your alternator fixed instead. You can confirm a bad diode by using a multimeter to detect any current running from the battery to the alternator.

[zeehag]

why not take it off and have it tested.
there are shops fro this function.
they can also be repaired ....

[gspeak]

Thanks everybody for your input & confirmation that the battery can be drawn down by the alternator.

I'm going to take the alternator off on the weekend & get it tested / repaired.

Will post the outcome later.

Dave

[btrayfors]

Why guess?

Put a clamp ammeter on it and see if current is flowing with the engine stopped.

Don't have one?

Shame. Every boat should have one for very basic electrical troubleshooting.

Bill

[hpeer]

Outside chance it could be your start switch.

With an external regulator you draw a field current when turned "on".

If, for some reason your switch was still supplying voltage to an external regulator when turned off it would drain the battery.

Simple to check. Is the alternator front magnetized when engine is shut off?

Should not be.

Might get same problem with an internal regulated alternator also.

Probably not the first thing to check, but something to check if nothing else obvious.

On my boat I shut the engine down with a cable that shuts off air flow. So I sometimes forget to turn off the engine, resulting in a dead battery.

[sy_gilana]

When that happens the alternator is usually warm to the touch when it should be cooled to the same as the rest of the engine. Its common, it is the diode pack that is either zapped, or dirty with carbon dust.