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Charging Voltage Level

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ncrunch32
Join Date: Dec 2003
Posts: 762 Kingston, NY
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2006-07-08          131864

This is a truly dumb question about my boat. When the boat idles the volt meter reads 13V which I assume is a good charging level. However, when I increase RPMs the volt meter reading declines to 12V. Is this normal? Why wouldn't the voltage continue to be about 13V as I increase RPMs? I am afraid the battery isn't being properly charged.

What prompted me to get obsessed with the voltage is the fact that I installed a new stereo in the boat and I noticed that, as you increase the volume output, you can actually see the volt meter reading drop. Yesterday my wife had the stereo really cranked up while we were underway and I noticed that ALL the gauges in the boat were dancing to the music. When we reduced the volume the gauges returned to normal. (Note: voltage decline described in first paragraph occurs even when radio is not turned on.)


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greg_g
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2006-07-08          131865

First guess might be that you wired the stereo in backwards, second would be the stereo is defective. But I can't proceed without knowing more. Is this a car-type stereo you're talking about (DC) or a home type (AC)? Or does the boat have an inverter?

If you don't get the help you need here, there's also a forum for electrical and one for generators (see the lower "Jump to..." dropdown just to the right).

//greg// ....

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ncrunch32
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2006-07-08          131866

Thanks Greg. This is a DC stereo in a 21' boat. No inverter here. I'm not sure how I could have wired this in backward. I will say that the stereo I replaced had a small white plastic box on the power input ( which I didn't replace since I couldn't figure out what it was for). I wonder if it was some type of isolator. ....

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kwschumm
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2006-07-08          131873

Simplify by eliminating the stereo from the equation. Lots of electronics these days do not shut off completely when you press the off switch. Disconnect the stereo from the electricals completely, maybe by pulling the fuse. Does the voltage still drop when the engine revs are increased? ....

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Peters
Join Date: Feb 2002
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2006-07-08          131877

There are two possible problems. One is the alternator actually charging. At low RPM you are looking at the battery line voltage 13 volts. If it is charging it should read about 14 volts. When you increase the RPM the alternator is using more voltage. Your alternator needs voltage as the magnetic coils are not perminate magnets.
The second possiblility is that there is a small short in one of the battery plates. You should see a slow discharge on the battery, but it will allow the battery to charge and have near proper voltage. If you took it in to do a load test you would see the problem. At rest you have 13 volts at low RPM you are not charging the battery and the altenator is actually a negative load. When the alternator is charging the battery is shorting. Normally the battery does not last very long in this condition at this point so I would think it would have failed. ....

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bloggins
Join Date: Apr 2006
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2006-07-08          131884

Car audio systems use a lot of power. Depending on the condition and size of your battery and alternator (and it's rated output), you may be demanding too much from your electrical system. Start with the battery, then alternator, then regulator (if seperate from alt), then grounds/connections, then relays (must be dry). Fair Winds! ....

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earthwrks
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2006-07-08          131888

I'm with bloggins, it sounds like the battery is to small and can't handle the draw or the alternator's too small and can't recharge quick enough. Another thing to look for is a loose alternator belt--it doesn't have to squeal to be loose. ....

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ncrunch32
Join Date: Dec 2003
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2006-07-08          131890

I sent some time with the boat running in the driveway today with rabbit ears on the drive. I disconnected the power lead to the stereo and the voltmeter behaved the same way (declined from 13V to 12V as rpms increased). I agree about the power required by this stereo - that is a separate problem I will have to address. I could install a second gell cell battery with isolator switch to handle that.

The battery previously went dead on me this year - I charged it and it load tested OK. It has managed to start the boat since then but it did act weak a couple of times. Today I put my load tester on the battery cables while the engine was running to read the charging volts. It showed a charge rate of 15V, even while the voltmeter on the dash showed 12-12.5V. That was kind of puzzling to me. The belts are tight - there is nothing slipping. So I wonder if Peters is on to something about a short in the battery. ....

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oneace
Join Date: Mar 2004
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2006-07-08          131897

I would not rely on the gauge in the dash. Check you voltage right at the alternator. Do you have a digital multimeter? Why I ask is you checked with the load tester and showed 15V. That is too high and the dash meter is showing too low. Are they both wrong, or one right and the other wrong? ....

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harvey
Join Date: Sep 2000
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2006-07-09          131903

My first thought is a loose belt. Low rpm no load it works hi rpm and load its slipping.

Tighten the belt it's the easiest and cheapest to start with.

New belts never hurt and keep tight. ....

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ncrunch32
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2006-07-09          131904

Thanks Harvey - I will check again for loose belt but I don't think this is it. I just had the belt off to replace the water pump and put it back on pretty tight. Oneace - I broke my digital multimeter but maybe its time to get another. I am also wondering if the gauge in the dash is working correctly. I hear these gauges in boats go bad quite often. ....

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wr5evk8jj
Join Date: Feb 2005
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2006-07-09          131905

Much earlier in my working life, in my late teens and then while in college after serving from being drafted, I was a marine mechanic. I was certified on MerCruiser (Mercury Stern Drives) while my brother was certified on the outboards. My father was a Zone Sales Manager for Mercury.

The first thing that went through my mind on this was the volt gauge having been wired wrong from the start; it sounds as if the gauge is wired in at the dash and not being wired to read the voltage at or near the battery. It seems to be sensing the voltage drop when things are being used (e.g.: the high current stereo). It also points to there not being a heavy enough gauge of wire going from the battery up to the console area -- it is doing exactly what it is supposed to be doing by sensing the voltage, and in this case the voltage dropping. If the supply wire was a sufficient enough gauge wire from the battery up to the console, there should not be any appreciable voltage drop -- maybe a few tenths of a volt, but not much more. But, if there is 18 gauge wire going up there for such a long run, that could be a problem with a sizable current draw such as the stereo. That's my read on the situation at least. ....

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ncrunch32
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2006-07-09          131907

Wr5evk, thanks for your input. Obviously you have good experience in this area. This and oneace makes me think that I should double check my reading at the alternator, see if the in-dash gauge is working or wired correctly, and run a direct power line from the battery to the stereo. The stereo is 200 watts (50x4) and drives 2 - 6 inch speakers and 2 - 6x9 inch speakers. Thanks to all who have posted! ....

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kwschumm
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2006-07-09          131909

It sounds like Wr5evk might be on the right track. Since your entire instrument cluster is dancing to the beat the voltage supply to the panel/stereo (wiring to the cluster/stereo, battery, alternator, belt) could well be inadequate, but the ground could be inadequate as well. Make sure the grounding is of a heavy enough gauge wire and connections are tight and clean. ....

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Peters
Join Date: Feb 2002
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2006-07-09          131911

Wr5evk could be right if the wire if the connections are corroded and the wire is small. I guess with the sterio disconnected it still leaves a question mark why the gauge would read low. I am assuming that this is a new condition and read properly before.
Is the battery a rolled cell or plate gel battery? ....

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ncrunch32
Join Date: Dec 2003
Posts: 762 Kingston, NY
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2006-07-09          131917

Peters, it is not a gell cell battery. Then is it a rolled cell battery? As I have been talking about this with my wife we are wondering if the voltmeter has always read like this. The stereo issue just caused me to focus on what was going on with the voltmeter for the first time. Sorry for confusing these issues - one problem is what causes me to find 10 others :) The boat seems to be running fine - I will just pick away at this voltmeter curiosity for a while and hopefully solve it. ....

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Peters
Join Date: Feb 2002
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2006-07-09          131918

Rolled cell is batteries like Optimas. The cases look like six cans of beans in a plastic case. You can tell by the case design. If the battery is a normal flooded lead acid you can check the for a short by checking the acid in the cells. One cell should be low after sitting a while. ....

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ncrunch32
Join Date: Dec 2003
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2006-08-27          133711

I ran a separate line from the battery to the stereo. It fixed the "dancing gauge" problem on the instrument panel when I crank the stereo up. I still need to spend some time with the voltmeter wiring as the voltmeter still reads 13V when the engine is idling and gradually drops towards 12V as the rpms are increased. This will take some time because I only get in the boat every couple of weeks and usually just want to get right to the water. Thanks to all who have posted here. ....

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Art White
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2006-08-27          133715

Normal charging should be around 14 volts at the higher speed. To have grounding problems on a boat is not unusual! ....

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ncrunch32
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2006-10-04          135677

Just an update. I finally got to checking my voltage at the battery while engine is running and charging - 14.5 volts. And the voltage is stable as engine is revved. Perfect. So I can only conclude that curent wiring to the dash/voltmeter is insufficient as wr5evk8jj suggests.

I might add that the battery seems to be getting weak too quickly. If my boat sits for 3 weeks the battery is too weak to start the boat and I have to pull out my portable "mighty might" battery at the marina. Something seems to be drawing power off the battery at a very slow rate. I wonder if all these problems are related. I wonder if some component like the depth finder is drawing excessive power from the dash area and/or not completely shutting down. How can I detect a small draw on the battery? ....

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Peters
Join Date: Feb 2002
Posts: 3034 Northern AL
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2006-10-04          135682

ncrunch,you need to check that you have all the circuits off and then check for resistance on the battery terminals. If there is an open circuit it will show up.
Does the boat have an automatic bilge pump. If you have a leak this will draw down the battery in a hurry.
Be aware if there is current to a metal part on the boat you can get electrolysis happening and corrosion of the metal part. This is more problematic in salt water as a small bleed of current can cause problems, but with 12 volts and a ground metal in contact with the water anymetal can corrode.
Most likely it is a small circuit like a clock which is staying energized or a small short in the battery allowing it to self discharge. ....

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ncrunch32
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Posts: 762 Kingston, NY
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2006-10-04          135683

Thanks Peters, the boat is out of the water on a trailer when not in use - so it should not be the bilge pump. I will check the resistance. Thanks for the advice! I guess another option I might have would be to just put a battery switch in to disconnect it when not in use. ....

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greg_g
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2006-10-04          135685

Remove the cable from your negative (-) battery post. Set a multimeter for DC amps. Put the negative (black) test lead on the battery (-) post. Put the positive (red) test lead on the end of the (-) cable you just removed. . Anything greater than 34 milliamps (0.034a) is considered parasitic drain, and is responsible for slowly discharging your battery. Keeping the multimeter in series between the battery and the cable, open up the fuse box. Pull one fuse at a time, while watching the multimeter for a drop in the amperage reading. That may not find the exact location of the short, but it sure narrows things down a bit.

//greg// ....

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DRankin
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2006-10-04          135688

I had the same issue with my Suzuki Samurai.

Even a new battery work discharge inside a week.

I finally tracked it down to an aftermarket Tach. I disconnected it and the battery now holds a charge. ....

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ncrunch32
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2006-10-04          135691

Greg, thanks for the precise instructions. I can put my multimeter to work. DRankin, I wonder if it has to do with my voltmeter malfuctioning or maybe my depthfinder. Definitely this battery should not discharge so quickly. ....

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kwschumm
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2006-10-04          135692

Awhile back I read about a new BMW model that had a similar problem with the battery going dead in a week if the car wasn't driven. It was in the electronics - many of them don't really shut off anymore. I'm working on a product right now that doesn't shut off even when the user presses the off button. It's a lot of extra code to make behave like it's shut off when it's really not. Sort of dumb but that's the way processors with "power management" work. Some part of the cpu stays awake to monitor inputs that tell it when to turn back on. That takes some power. It shouldn't take much but it adds up over time, esp. if there's a lot of electronics in a vehicle. It could be your depth finder or the fancy radio you installed. A battery cutoff would make sure off is off. ....

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oneace
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2006-10-04          135702

Greg is for the most part right. But you want to use the positive side of the battery. and the turn you meter to amps. and hook it up in series with the post and the cable. The start pulling fuses to you read zero amps. ....

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ncrunch32
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2006-10-15          135953

Yep, its the new fangled stereo that is draining the battery. I measured 43 milliamps being drained off. Thanks guys! ....

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