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Old 07-07-2020, 02:12 PM   #1
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What does my Voltage Gauge tell me?

What exactly does my dash panel voltage gauge measure? I ask because on the starboard side the gauge generally starts out showing 12 volts (or just slightly less) and then over the hours declines down to around 11, although it will bounce around occasionally in there.

I've measured at the terminals with a multimeter and the gauge is showing what the meter is receiving. I've also measured (engine and charger off) at the batteries both before (when it shows 12) and after (when it shows 11 or less) and each time the batteries themselves show about 12.8.

The gauges at the 12 volt panel in the salon also match these readings. They never show below 12.6 - 12.8 even when the dash shows less than 11.

If I start the gen while underway and turn on the battery charger, the gauge pops up to over 12.6. If I turn it off, it drops immediately below, and resumes fading further. This leads me to believe that it's showing me the output of the charging system, except the gauge shows voltage even before the engine is started, when the charging system isn't running.

I've cleaned connections at the gauge with no change. I'm not sure where that wire is pulling it's readings from at the other end or I'd clean that end too.

I'm sure this is obvious to experienced folks, but it's a head scratcher for me right now.

Thanks
BD
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Old 07-07-2020, 02:18 PM   #2
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It sounds to me like it's measuring the output of the battery charger. If you're plugged into shore power, does the voltage also go up?

Ted
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Old 07-07-2020, 02:25 PM   #3
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Might be helpful to explain your batteries and how they get charged.

One engine or 2?
House battery bank separate from engine batteries?
Separate generator battery or shared?
How does the house bank get charged?
Do the engine(s) alternator(s) only charge the engine battery?
Do you understand what all your engine room big battery switches do?

Ted
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Old 07-07-2020, 02:37 PM   #4
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Is this gauge part of the engine gauge panel? If yes, it may be indicating that the engine alternator is not working. There may be a battery charger hooked to that engine battery. When you start the generator, it may be recharging that engine battery. As you run the engine without the generator, the voltage would drop as the battery powers gauges, panel lights and probably a run solenoid. When you plug into shore power, the battery would also get recharged.

Knowing if that gauge is in the engine panel or what it's hooked to would be helpful.

Ted
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Old 07-07-2020, 02:41 PM   #5
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Originally Posted by O C Diver View Post
Might be helpful to explain your batteries and how they get charged.

One engine or 2?
House battery bank separate from engine batteries?
Separate generator battery or shared?
How does the house bank get charged?
Do the engine(s) alternator(s) only charge the engine battery?
Do you understand what all your engine room big battery switches do?

Ted
I'll start with the answer to the last question because it's the most important - NO. I have not yet spent the time to trace and diagram the entire system, and it's on my list for this weekend.

That said:
- 2 engines
- six Group 31 lead acid batteries for each engine - shared for house and starting. Each bank feeds a separate side of the 12 volt panel so loads are divided that way.
- separate starting battery for the Gen
- Each engine alternator (I believe) charges it's own bank (not confirmed)
- Battery charger is 3 bank so charges each bank plus the gen batt.
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Old 07-07-2020, 02:42 PM   #6
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It sounds to me like it's measuring the output of the battery charger. If you're plugged into shore power, does the voltage also go up?

Ted
I haven't looked at it on shore power. I'll check that Thursday.
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Old 07-07-2020, 02:45 PM   #7
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Originally Posted by O C Diver View Post
Is this gauge part of the engine gauge panel? If yes, it may be indicating that the engine alternator is not working. There may be a battery charger hooked to that engine battery. When you start the generator, it may be recharging that engine battery. As you run the engine without the generator, the voltage would drop as the battery powers gauges, panel lights and probably a run solenoid. When you plug into shore power, the battery would also get recharged.

Knowing if that gauge is in the engine panel or what it's hooked to would be helpful.

Ted
Yes the gauge is part of the engine gauge panel. And the alternator not working was exactly my first suspicion too. Yet after running for 12 hours and the gauge showing 11 volts (or less) I can shut off the engines, go to the ER and measure the batteries directly and they'll show 12.8. So will the house 12 volt panel gauge. So it seems to me that the gauge isn't showing the battery voltage. Confused.
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Old 07-07-2020, 02:51 PM   #8
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Ok.

You didn't answer whether the gauge in question is part of the engine gauge panel. Posting a picture of the gauge in it's surroundings would be helpful to see what it's associated with.

Clearly the voltage rise is caused by the battery charger. The question is whether the battery it's associated with is getting properly charged when the engine is running.

Never mind, I got ahead of you.

Ted
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Old 07-07-2020, 02:58 PM   #9
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The other question is whether the gauge and/or its wiring/connectors is any good, given the discrepancy between the reading at the battery and the gauge. Have you tried taking the multimeter and reading off the wires at the gauge? Where do those wires terminate before they go to the gauge? What's the reading there? And so on.
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Old 07-07-2020, 02:59 PM   #10
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Ok.

You didn't answer whether the gauge in question is part of the engine gauge panel. Posting a picture of the gauge in it's surroundings would be helpful to see what it's associated with.

Clearly the voltage rise is caused by the battery charger. The question is whether the battery it's associated with is getting properly charged when the engine is running.

Never mind, I got ahead of you.

Ted
Photo taken at WOT during the survey...
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Old 07-07-2020, 03:03 PM   #11
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Quote:
Originally Posted by BDofMSP View Post
Yes the gauge is part of the engine gauge panel. And the alternator not working was exactly my first suspicion too. Yet after running for 12 hours and the gauge showing 11 volts (or less) I can shut off the engines, go to the ER and measure the batteries directly and they'll show 12.8. So will the house 12 volt panel gauge. So it seems to me that the gauge isn't showing the battery voltage. Confused.
Are you sure that the 6 lead acid batteries are all wired in parallel? I would want to look for a dual battery isolator (has alternator wire going to 2 diodes to 2 separate batteries).

Does the volt meter go to 0 when you turn off the engine?

Ted
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Old 07-07-2020, 03:04 PM   #12
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The other question is whether the gauge and/or its wiring/connectors is any good, given the discrepancy between the reading at the battery and the gauge. Have you tried taking the multimeter and reading off the wires at the gauge? Where do those wires terminate before they go to the gauge? What's the reading there? And so on.
Yes I did. I mentioned in the original post "I've measured at the terminals with a multimeter and the gauge is showing what the meter is receiving." I even swapped out the two gauges to be sure - starboard shows that reading regardless.

So the wires to the gauge are getting that. Those wires go into the loom that go somewhere into the ER, which is sort of what I was hoping this thread would answer. Where do those wires start? Alternator? Battery? Something else? What specifically is that thing measuring?
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Old 07-07-2020, 03:07 PM   #13
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Originally Posted by O C Diver View Post
Are you sure that the 6 lead acid batteries are all wired in parallel? I would want to look for a dual battery isolator (has alternator wire going to 2 diodes to 2 separate batteries).

Does the volt meter go to 0 when you turn off the engine?

Ted
I mispoke there. The 6 batteries are TOTAL - 3 per bank. Photo of that bank attached.

Yes gauge goes to zero when engine shut off.

Thanks for the help.
BD
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Old 07-07-2020, 03:13 PM   #14
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Does the positive battery cable go directly to the starter?

What make engine and approximately how old?

Ted
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Old 07-07-2020, 03:18 PM   #15
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Do you know where the battery charger connects to that battery bank / engine?

Ted
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Old 07-07-2020, 03:28 PM   #16
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Does the positive battery cable go directly to the starter?

What make engine and approximately how old?

Ted
Not 100% certain but almost sure that it goes to the battery switch, and then to the starter.

Cummins 6CTA 8.3 450s. 2001. 1970 hours
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Old 07-07-2020, 03:30 PM   #17
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Do you know where the battery charger connects to that battery bank / engine?

Ted
Here - haven't followed the route from here back however.
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Old 07-07-2020, 03:42 PM   #18
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Ok.

I think you will need to trace the alternator output lead (large red wire) from the alternator to wherever it goes. Usually on a 6CTA, that wire runs from the alternator to the starter. The battery also connects to that same starter post along with the positive wire which goes to the ignition switch (which powers the voltage gauge). Don't think yours will be wired like that.

Do you know if you have a battery switch that combines both engines' batteries?

Ted
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Old 07-07-2020, 03:52 PM   #19
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Ok.

I think you will need to trace the alternator output lead (large red wire) from the alternator to wherever it goes. Usually on a 6CTA, that wire runs from the alternator to the starter. The battery also connects to that same starter post along with the positive wire which goes to the ignition switch (which powers the voltage gauge). Don't think yours will be wired like that.

Do you know if you have a battery switch that combines both engines' batteries?

Ted
I will be back onboard Thursday and I'll trace it. And diagram as much as I can.

I know that each bank has it's own switch - not a 1-2-Both switch. I'm not aware of any ACR and I can't imagine where one would be hidden. But I'll be looking for it.

Thanks again.
BD
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Old 07-07-2020, 05:41 PM   #20
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Thanks for your clarification. Often times I am a little dense, so excuse me if I misread your earlier post: does the reading on the gauge match any of the readings at the batteries taken at the same time and RPM?
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