Mystery voltage

[B27]

Can one use an LED ' bulb' for this task ? IIRC they can light up at lower V than normal incandescents.

You have to be aware of what the bulb is telling you.
Viewing a 2W LED bulb in daylight, I find it easy to tell whether it's managing to draw a watt or more from some wires.
It's telling me there is power there, it's not telling me it's 10, 12 or 14V
Very handy for checking connectivity, polarity etc.
If I wanted to check for voltage drop I'd maybe load up the circuit and measure the voltage drop.

There are LED bulbs with simple resistors to limit the current, thee vary in brightness in a simple relationship to the voltage. They generally don't light up if the polarity is wrong.
There are other LED bulbs which regulate the brightness and look the same from maybe 10 to 30V. Some of those have rectifiers built in and will not care about polarity and may or may not care whether it's AC or DC.

Bulbs and meters are 'indicators', it helps to understand what exactly they are indicating.

 

[Alex_Blackwood]

Current situation: the -ve side of the battery is connected to an isolator, in the off position, with the only other cable being to the engine block. The positive side isn't connected to anything yet.

At the -ve bus bar, I was getting about 9.4v, but not strong enough to create a spark. After disconnecting these cables, they individually give me similar readings, with one being under 5v.

I'm not seeing continuity between the engine block lead and any of these -ve cables (and I'm not expecting to).

So is it just an induced or capacitive phantom voltage? Am I worrying about nothing? I'd much rather that it wasn't there...

Sorry if I am being thick here but now confused, there seems to be some variation between the description in your #9 and #16.
Is the "other cable" from the output of the -ve isolator? You now say you are getting readings at the -ve Bus bar? what about the reading you mentioned between the "Cable" and the -ve battery terminal post?
Have you read and understood my #15?

 

[PaulRainbow]

Can one use an LED ' bulb' for this task ? IIRC they can light up at lower V than normal incandescents.

Not in this case, best to use a bulb that will draw a significant current.

 

[Sea Change]

Sorry if I am being thick here but now confused, there seems to be some variation between the description in your #9 and #16.

Yes, it's not the same configuration. I've been making and breaking connections in an attempt to isolate the source of the phantom voltage.

Is the "other cable" from the output of the -ve isolator?

On the input of that isolator.

You now say you are getting readings at the -ve Bus bar? what about the reading you mentioned between the "Cable" and the -ve battery terminal post?

Tbh I've lost track now...

Have you read and understood my #15?

I think so. I'm not aware of any boat ground, to an anode or otherwise. But I've only had a few days to look at this so far.

Today's mystery: the house circuit is losing about 2v somewhere, causing the LED lights to flicker. I'm guessing there's a bad connection or partial short somewhere. This exact configuration was working fine for the last two days...

 

[webcraft]

Yes, it's not the same configuration. I've been making and breaking connections in an attempt to isolate the source of the phantom voltage.


On the input of that isolator.


Tbh I've lost track now...


I think so. I'm not aware of any boat ground, to an anode or otherwise. But I've only had a few days to look at this so far.

Today's mystery: the house circuit is losing about 2v somewhere, causing the LED lights to flicker. I'm guessing there's a bad connection or partial short somewhere. This exact configuration was working fine for the last two days...

Have you considered gremlins?

— W

 

[starfire]

Can one use an LED ' bulb' for this task ? IIRC they can light up at lower V than normal incandescents.

You could, lower current so less load on the circuit, & its polarity sensitive.
I still prefer the low wattage bulb.

I have a car headlight bulb with leads & clips where I want to load a circuit a bit more.

 

[Alex_Blackwood]

Yes, it's not the same configuration. I've been making and breaking connections in an attempt to isolate the source of the phantom voltage.


On the input of that isolator.


Tbh I've lost track now...


I think so. I'm not aware of any boat ground, to an anode or otherwise. But I've only had a few days to look at this so far.

Today's mystery: the house circuit is losing about 2v somewhere, causing the LED lights to flicker. I'm guessing there's a bad connection or partial short somewhere. This exact configuration was working fine for the last two days...

That was #10 not #15 Are you on shore power? if so have you tried total disconnection of that and also your Solar, isolate them completely. Make sure that you have only your battery set operational with everything else removed. If you still have spurious voltages. Have a cup of tea, or a couple of beers take a day off, have a good think and then go through the system bit by bit, taking notes and readings as you go so that you know exactly what you have and where
Good luck with it!

 

[Baddox]

Do you have a DSC VHF wired directly to the battery? The NMEA wires that supply lat/lon to it may be providing voltage from places you don't expect.

 

[Alex_Blackwood]

Do you have a DSC VHF wired directly to the battery? The NMEA wires that supply lat/lon to it may be providing voltage from places you don't expect.

He said nothing else connected. However, good thinking and all worth checking

 

[penberth3]

You could, lower current so less load on the circuit, & its polarity sensitive.
I still prefer the low wattage bulb.

I have a car headlight bulb with leads & clips where I want to load a circuit a bit more.

That is a big problem with using an LED as a test lamp.

 

[dave1dpc]

Real head scratcher here. I'm working my way through a system, currently there are only two batteries connected, both at around 12.8v.
I've covered and disconnected the solar panels and disconnected shore power.
The main bank of house batteries was kaput and in place of the six dead lead acids, I've currently just got one good battery. There's also a battery jury rigged up to the main switch panel to let me run lights etc. This system is, I think, currently isolated from the main battery
wiring.

The batteries were in three pairs distributed throughout the boat so I was working my way along, connecting up the cables to bridge the gaps where the old batteries were.
I noticed a spark between two of the cables on the positive side, and I'm measuring around 13.4v between them. Bizarrely, this varies at different times from around 6v up to 13.7v. It's higher than either of the batteries onboard, and neither of them should be producing a voltage within the positive only side anyway.
The boat has a Cyrix battery relay which I've disconnected, no change.
There are no 24v systems onboard.

I'm utterly perplexed. Is it possible that there's some hidden device with enough capacitance to create this phantom voltage?

Had a similar problem once. It turned out to be my seperate bow thruster battery.