Mystery voltage

[Sea Change]

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?

 

[MontyMariner]

There's also a battery jury rigged up to the main switch panel to let me run lights etc.

My 2d worth
I would remove that and use a torch then see if you get any sparks.
You say varying voltage, that sounds a bit like it's solar induced.

 

[B27]

If the grounds of the two batteries are not connected then all sorts of voltages are possible.

 

[Plum]

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?

By coincidence, two days ago I measured the voltage of one of my batteries with my usual digital voltmeter which read almost 14 volts! The battery had been resting over night so, like you, also perplexed. Checked the voltage with another multimeter which read 12.7. Other than ditching the faulty multimeter the only thing I could think of doing was to replace the 9volt battery inside the multimeter. Although not expecting a difference, surprisingly the multimeter then read 12.7 ! On checking the voltage of the removed 9v battery it was just over 6volts. I have no idea why, but the new internal battery resolved my perplexia......

 

[Sea Change]

I thought of that, and borrowed a (much better) multimeter from a neighbour... it was giving the exact same readings.

I've now stripped everything down to the barest possible, including disconnecting both poles of the jury rig battery and turning off the charger. All MPPT connections removed. Still getting 3v from somewhere, and the battery is at 14v despite not being on charge for a couple of days, and previously reading 12.8v earlier today.

I think I'm going to strip the whole lot down and start from scratch.

 

[MontyMariner]

Still sounds like some solar is getting in

 

[VicS]

I thought of that, and borrowed a (much better) multimeter from a neighbour... it was giving the exact same readings.

I've now stripped everything down to the barest possible, including disconnecting both poles of the jury rig battery and turning off the charger. All MPPT connections removed. Still getting 3v from somewhere, and the battery is at 14v despite not being on charge for a couple of days, and previously reading 12.8v earlier today.

I think I'm going to strip the whole lot down and start from scratch.

If you are reading that directly at the battery terminals there are are only two possibilities. Either the meter is reading incorrectly or the battery is charging from somewhere.

.

 

[William_H]

By coincidence, two days ago I measured the voltage of one of my batteries with my usual digital voltmeter which read almost 14 volts! The battery had been resting over night so, like you, also perplexed. Checked the voltage with another multimeter which read 12.7. Other than ditching the faulty multimeter the only thing I could think of doing was to replace the 9volt battery inside the multimeter. Although not expecting a difference, surprisingly the multimeter then read 12.7 ! On checking the voltage of the removed 9v battery it was just over 6volts. I have no idea why, but the new internal battery resolved my perplexia......

A digital volt meter uses a calibrated voltage standard fed by the battery against which the measured voltage is compared. As the battery gets low it is not able to keep the voltage standard up to mark even though there is enough voltage to seemingly make the volt meter work. Your measured voltage is compared to a standard which is in fact low so indicated voltage is much higher. The fix as you found is a new battery for the meter. ol'will

 

[Sea Change]

I appear to have invented free electricity.

The yard power is off just now, and I've turned off the battery charger and completely disconnected it from the jury rig battery, and the jury rig battery from everything else too.

I have one battery installed with only a straight run on the positive side to the isolator, which is turned off. All other cables removed from that terminal.

I'm getting a weak voltage difference (around 8v but it varies) between the bare negative terminal the battery and the end of the cable I want to connect to it. The crazy thing is, I've disconnected the other end of this cable too. It's literally connected to nothing. And I just ran it this morning.

What about an induced current?? Not sure where from though. No other power sources currently active.

Could the battery have some kind of fault?

 

[Alex_Blackwood]

I appear to have invented free electricity.

The yard power is off just now, and I've turned off the battery charger and completely disconnected it from the jury rig battery, and the jury rig battery from everything else too.

I have one battery installed with only a straight run on the positive side to the isolator, which is turned off. All other cables removed from that terminal.

I'm getting a weak voltage difference (around 8v but it varies) between the bare negative terminal the battery and the end of the cable I want to connect to it. The crazy thing is, I've disconnected the other end of this cable too. It's literally connected to nothing. And I just ran it this morning.

What about an induced current?? Not sure where from though. No other power sources currently active.

Could the battery have some kind of fault?

More likely Capacitive coupling of some sort from somewhere. Another one of these imponderables unless on site and having eyes and hands "On". Do you have a boat "Earth" connection? e.g. Button anode? Are you afloat?

 

[penberth3]

A digital volt meter uses a calibrated voltage standard fed by the battery against which the measured voltage is compared. As the battery gets low it is not able to keep the voltage standard up to mark even though there is enough voltage to seemingly make the volt meter work. Your measured voltage is compared to a standard which is in fact low so indicated voltage is much higher. The fix as you found is a new battery for the meter. ol'will

A good explanation. I remember early DMM's could do some very strange things when the battery was low. Not a problem with analogue meters, they're still a useful thing to have!

 

[Sea Change]

More likely Capacitive coupling of some sort from somewhere. Another one of these imponderables unless on site and having eyes and hands "On". Do you have a boat "Earth" connection? e.g. Button anode? Are you afloat?

On the hard. I'm slowly working my way through the wiring, making and breaking connections to chase the phantom voltage. Currently it's at ~7v and very weak

 

[penberth3]

On the hard. I'm slowly working my way through the wiring, making and breaking connections to chase the phantom voltage. Currently it's at ~7v and very weak

What do you mean "very weak"? 7 volts is 7 volts, or is it reducing as you measure it, the capacitive charge that's been suggested? This is where an analogue meter is the tool for the job!

 

[starfire]

A multimeter is such high input impedance you will get stray voltages.

Far better to use a test lamp which will load the circuit, I used a halogen 10W 12V or 24v bulb, wired into a holder, with a pair of Croc clips on the end.

The number of times I have been asked why am I using a bulb when I have an expensive multimeter when finding corroded connections.

 

[Alex_Blackwood]

On the hard. I'm slowly working my way through the wiring, making and breaking connections to chase the phantom voltage. Currently it's at ~7v and very weak

Which side is +ve. WRT the other, The cable or the battery terminal? I assume the cable, but? Is the cable running in close proximity or directly alongside other cables? That would account for capacitive coupling.

 

[Sea Change]

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...