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Old 11-01-2021, 11:42   #1
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Alternator voltages - should they be fluctuating much?

Hi all,

I have a new Raymarine Axiom MFD that can show supply voltage at all times. I have been looking at it ever since the installation and would like to understand if what I'm seeing is a problem or if it is an expected behavior

After the boat has been sitting on a shore power and charger, i disconnect power, start the boat and head out under engine power
- MFD shows 14 or so
- spend about 2 hours under motor going at 2500-2800rpm with chart plotter, sonar and autopilot (used occasionally).
- i watched the MFD show voltage steadily drop to 13.2-13.3v while under power.
- when engine is not running, voltage drops to expected 12v ranges (12.5-12.6v)

Is this normal and expect because it is topping off the batteries and they are not needing more voltage or is this really not normal and is a sign of a problem?


I know that i need to break out my volt-meter and do some troubleshooting, but I couldn't do it yesterday and will need to do it next time i'm at the boat.

Alternator is a stock (not sure if it is 35 or 55A) for 2gm20. No external regulator as far as i can tell.
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Old 12-01-2021, 16:04   #2
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Re: Alternator voltages - should they be fluctuating much?

I would suggest that the alternator isn't charging the batteries because the voltage even with a basic alternator should be about 14v. Time for some investigating with the portable volt meter.
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Old 12-01-2021, 16:37   #3
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Re: Alternator voltages - should they be fluctuating much?

My guess is the alternator is overheating and winding back the voltage. Yanmar uses Hitachi alternators and most of them use internal temperature compensation (built into the internal regulator).

They can only output their rated current when cool and will quickly overheat is the rated current continues for some time (say 30 or 40 minutes). The internal regulator monitors the temperature and winds back the voltage in order to reduce the output current thus keeping the temperature down.

It isn't a fault, it just the way they designed it to prevent the alternator from burning itself out. It is OK for small electrical loads.

Maybe your batteries aren't getting fully charged by the shore power or there is some other problem which is causing the high current demand on the alternator.

Mainsail has a full write up explaining how it works and why it isn't suitable for cruising boats with high electrical demands.
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Old 12-01-2021, 18:07   #4
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Re: Alternator voltages - should they be fluctuating much?

Yes, I've read his post and possible thermal issue. There is a dedicated intake vent and exhaust vent with a fan. I can certainly stick a thermal probe to see or perhaps measure the V under power.

My electrical use is very modest - in fact I've been running without even using FM radio or any other modern niceties just to see.

Maybe the alt is dying or belt needs to be tensioned.
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Old 12-01-2021, 18:39   #5
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Re: Alternator voltages - should they be fluctuating much?

A voltmeter with clamp on ammeter will tell you if the alternator is generating any current. Though from the symptoms that is probably the case anyway. However it is odd that you aren't getting an alarm at the engine panel if the alternator really is not generating current.


If the alternator belt is turning the pulley you should also check the connections at the alternator.



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Old 14-01-2021, 12:05   #6
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Re: Alternator voltages - should they be fluctuating much?

Quote:
Originally Posted by George_SD View Post

After the boat has been sitting on a shore power and charger, i disconnect power, start the boat and head out under engine power
- MFD shows 14 or so
- spend about 2 hours under motor going at 2500-2800rpm with chart plotter, sonar and autopilot (used occasionally).
- i watched the MFD show voltage steadily drop to 13.2-13.3v while under power.
- when engine is not running, voltage drops to expected 12v ranges (12.5-12.6v)

Is this normal and expect because it is topping off the batteries and they are not needing more voltage or is this really not normal and is a sign of a problem?

it is an expected behavior


Charger has kept the bank charged. (You might want to find out why this is so high, i.e., 14V, when IF your boat has been plugged in overnight, it should be in float, around 13+v, not 14, unless you just plugged it in when you got to the boat and are just starting the charge cycle, un which case it makes sense.)


Motor around for two hours, alternator charging seems normal, especially if it;s a Hitachi alternator with temp compensation.


Turn off engine, sure.


Why do you think you have a problem?
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Old 14-01-2021, 12:35   #7
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Re: Alternator voltages - should they be fluctuating much?

Quote:
Originally Posted by Stu Jackson View Post
it is an expected behavior


Charger has kept the bank charged. (You might want to find out why this is so high, i.e., 14V, when IF your boat has been plugged in overnight, it should be in float, around 13+v, not 14, unless you just plugged it in when you got to the boat and are just starting the charge cycle, un which case it makes sense.)


Motor around for two hours, alternator charging seems normal, especially if it;s a Hitachi alternator with temp compensation.


Turn off engine, sure.


Why do you think you have a problem?
Hi Stu,

Sure, i see your point. Maybe i think there is a problem because I don't understand electrical systems well enough (still actively educating myself on them). So i could be completely wrong here. With that said, i do suspect there may be an issue (maybe not with alternator, but with electrical system in general). Here is why
- when i got the boat, i would get low voltage alarms after short periods of time of not being plugged in and with light use of the electrical items. My old MFD would alarm on voltage drop under 12v.
- I had to replace battery as it died (overheated, boiled off/released water and just died). Not sure if it was due to age, charging profile from Xantrex, alternator, low electrolyte (discovered in post-mortem), etc.
- on my last trip FM radio decided to power off and no come up until i was back to the dock and plugged to shore power. I've observed similar behavior right before the old house battery died.
- i think my battery bank consisting of house and engine single group 29 FLA 100ah each lasts less than it should and doesn't even give me a single day of moderate use before voltage drops significantly and i need to recharge them. I don't have a battery monitor that would show my actual draw, but my power budget calculation and use suggests it should last longer.

I've looked at the wiring and tried to clean contacts, re-crimp battery terminal connections, check for any chafing or exposed cables, cable corrosion, etc and correct whatever i could.
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Old 14-01-2021, 14:44   #8
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Re: Alternator voltages - should they be fluctuating much?

You need a clamp on ammeter to really understand what youíre doing to your single battery.

Iíll bet youíre pulling more power from your battery than you realize.
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Old 14-01-2021, 15:18   #9
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Re: Alternator voltages - should they be fluctuating much?

Quote:
Originally Posted by Sailmonkey View Post
You need a clamp on ammeter to really understand what youíre doing to your single battery.

Iíll bet youíre pulling more power from your battery than you realize.
There are two batteries - house and engine.
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Old 14-01-2021, 16:03   #10
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Re: Alternator voltages - should they be fluctuating much?

George, Sail Monkey is on to something.


You essentially have one house bank and one start bank with standard Beneteau wiring, which is a disgrace, IMHO.


Here's why:
Beneteau Wiring Diagrams - Maine Sail's corrections - Just in case you have a friend with a Beneteau and want to help him with his unusual switching Reply #8

https://forums.sailboatowners.com/in.../#post-1456512


None of us was born an electrician, so we're glad you're trying to learn.


Here's a start, including references to good books, too, and links to Maine Sail's website.


Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html


FWIW, a 100 ah house bank is horrendously undersized for a boat of your size even without a fridge. You only have 50 ah usable. Which is barely good enough for a daysail with the stuff you have running. That size bank is barely good enough for a 22-25 foot daysailer with a VHF and AP only.



You should:


1. Increase your house bank capacity to at least 200 ah


2. Investigate your wiring basics with the link I provided


3. Continue your education.


Good luck.
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Old 14-01-2021, 16:30   #11
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Re: Alternator voltages - should they be fluctuating much?

I do have 2 batteries - engine and house, FLA 100ah each group 29. They are wired together as shown the Main Sail diagram. I plan on adding 1 group 29 FLA and replacing one of them with 2 x 6V golf cart batteries (only thing the space allows me).

I do have a fridge, but don't use it. On day sails with minimal use, i should not be pulling more than 4-5ah max. So with two batteries as i have now - i should be able to get at least 4-5 hours of use without alarms going off.
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Old 15-01-2021, 05:06   #12
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Re: Alternator voltages - should they be fluctuating much?

Quote:
Originally Posted by George_SD View Post
I do have 2 batteries - engine and house, FLA 100ah each group 29. They are wired together as shown the Main Sail diagram. I plan on adding 1 group 29 FLA and replacing one of them with 2 x 6V golf cart batteries (only thing the space allows me).



I do have a fridge, but don't use it. On day sails with minimal use, i should not be pulling more than 4-5ah max. So with two batteries as i have now - i should be able to get at least 4-5 hours of use without alarms going off.


Are you pulling 4-5ah or pulling 4-5 amps?

Like I said, you need a meter to measure current as well as voltage directly at the batteries.

If youíre pulling 5 amps, youíve pulled 40ah over 8 hours. Couple that with likely undersized wire to feed the plotter and plotter inaccuracies measuring voltage youíre just working yourself into a meaningless pit.

Do you leave your engine battery turned on while sailing?
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Old 15-01-2021, 09:06   #13
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Re: Alternator voltages - should they be fluctuating much?

I'll measure, but 4-5ah is 4-5A average instant draw over a course of an hour...No?

I'll be going to the boat today to work on it, so will fire up the engine and run it at around 2500rpm at the doc and see what it is putting out.

Yes, engine and house remain on during sailing most of the time. On occasion I will disconnect the engine one if I'm concerned that we will use too much power.
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Old 15-01-2021, 11:41   #14
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Re: Alternator voltages - should they be fluctuating much?

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Originally Posted by George_SD View Post
I'll measure, but 4-5ah is 4-5A average instant draw over a course of an hour...No?

I'll be going to the boat today to work on it, so will fire up the engine and run it at around 2500rpm at the doc and see what it is putting out.

Yes, engine and house remain on during sailing most of the time. On occasion I will disconnect the engine one if I'm concerned that we will use too much power.


4-5ah over the course of a day sail is quite different than 4-5amp draw. Unless your daysail is only 1 hour long.
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Old 17-01-2021, 14:24   #15
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Re: Alternator voltages - should they be fluctuating much?

Got some partial results:

First of all, it does seem that voltage drop is causing me to worry, even though there may not be as big of an issue as it appears on the surface.

I've got three different voltage measurements
12.64v at the engine battery via multimeter (house bank was showing a very similar charge)
12.5v shown by the basic voltage monitor wired to the MFD breaker at the panel
12.1v shown by the MFD ?!

This last one really tripped me up, as at that point I would have assumed that the battery has no charge and is deeply discharged. It appears that the there is a hole 0.5v drop in a 14awg or 12awg cable. It is a bit long since it runs from midship at the panel all the way along the starboard side, then to middle of stern, turns back and up into the binnacle or about 20ft in total.

So i guess i can't trust MFD for the voltage reading and need to keep in mind that it is off by at least 0.5v. I didn't get to test alternator output unfortunately.


On the plus side, i think I've fixed a well known Yanmar start button issue that many suffer from voltage drop. I've removed all butt connectors in the chain from the button to the starter solenoid and crimped new terminals to freshly cut ends of the existing cable and the engine started right away on two tries.
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