There goes battery charger number 3!

[stranded]

There is nothing wrong with your logic and the fact that there was a partial break in the direct line to your battery earth on the DC line would indeed be at least a possible cause of your problems. It would mean that there was a high resistance at the least and it is quite possible that the break would cause an intermittent connection which could have caused the voltage to fluctuate.
Can't say with any authority that this would cause the problems you have had but as the battery chargers sense voltage and adjust the charge rate to suit it would certainly cause them to malfunction I would have thought? Again not an authoritative diagnosis but a viable theory perhaps? Is the shunt located on a common ground for both batteries or just the service batteries? As I said earlier, if it is on a common earth (which it shouldn't be) it could account for restricted volts to your starter motor also. Think of it as a bad or corroded contact and it might be easier to visualise.

Fantastically helpful, thanks Mike. I am beginning to enjoy this sleuthing. I would like to take over the whole investigation myself but think I need to give Berthon the chance to resolve any possible error on their part and don't want to complicate that/lay new false trails.

Appreciate it!

 

[RichardS]

Frankly I think you are being misled by other posts.

Hmmm ..... I take it that your reference is to my posts as I'm the only other recent poster other than the OP.

I'd be very interested to know where I might have "misled" ohnmaggie as everything you say in your post #38 repeats points I have already made.

Richard

 

[stranded]

Hmmm ..... I take it that your reference is to my posts as I'm the only other recent poster other than the OP.

I'd be very interested to know where I might have "misled" ohnmaggie as everything you say in your post #38 repeats points I have already made.

Richard

I am sure it is a case of me misleading myself (and everyone else!) rather than anything else. As you say, similar thing in a slightly different way. Given I have apparently bamboozled the top electrickery man at Berthon, no-one on here should be surprised if I can do the same to you!

 

[boatmike]

Hmmm ..... I take it that your reference is to my posts as I'm the only other recent poster other than the OP.

I'd be very interested to know where I might have "misled" ohnmaggie as everything you say in your post #38 repeats points I have already made.

Richard

Well I wasn't looking for an argument but suggesting that you can burn out an ammeter with the shunt removed when removing the shunt disconnects the ammeter from the supply completely is just daft. Calling it misleading was perhaps my effort at being polite.

 

[Poecheng]

Well I wasn't looking for an argument but suggesting that you can burn out an ammeter with the shunt removed when removing the shunt disconnects the ammeter from the supply completely is just daft. Calling it misleading was perhaps my effort at being polite.

I always thought that the ammeter was wired in parallel to the shunt. Therefore removal of the shunt (alone) from the circuit would force the entire current through the ammeter.
Richard was seeking to help and, even if he is wrong (and I have no reason to believe he is), it doesn't warrant that retort.

 

[stranded]

I always thought that the ammeter was wired in parallel to the shunt. Therefore removal of the shunt (alone) from the circuit would force the entire current through the ammeter.
Richard was seeking to help and, even if he is wrong (and I have no reason to believe he is), it doesn't warrant that retort.

Come on chaps - I've been really grateful for all the suggestions - electricity is sorcery - who knows who is right!

 

[RichardS]

Well I wasn't looking for an argument but suggesting that you can burn out an ammeter with the shunt removed when removing the shunt disconnects the ammeter from the supply completely is just daft. Calling it misleading was perhaps my effort at being polite.

LOL.

Dear Mike ...... you really really need to improve your understanding of ammeters and parallel shunts, and perhaps reflect on how ohmaggie's ammeter actually came to be fried. When you've done that I think you will want to do the right thing. :encouragement:

Richard

 

[RichardS]

Come on chaps - I've been really grateful for all the suggestions - electricity is sorcery - who knows who is right!

Sorry ohmaggie, but I genuinely wanted to know where I had misled you and being insulted for being correct was certainly not what I was expecting.

Richard

 

[pvb]

I don't think there's any chance that the ammeter was damaged by having a high current passed through it. In the vast majority of cases, shunted ammeters are simply small voltmeters, measuring the tiny voltage generated across the shunt when current flows (typically only 50-60 millivolts). In the OP's case, the shunt had 2 conducting rods, one of which fractured. However, the result of this would have been to double the voltage produced across the shunt (for a given current) and this may have damaged the meter. The fact that the meter reads max when the sensing leads have been disconnected isn't necessarily a fault though, as presumably it's still connected to power through other connections.

 

[stranded]

Sorry ohmaggie, but I genuinely wanted to know where I had misled you and being insulted for being correct was certainly not what I was expecting.

Richard

Of course not. When I get this sorted perhaps I should buy a beer for Mike in the Solent before we depart then pass on his love to you in Croatia when we arrive there sometime next summer! Sleep tight gents - I will update when progress is made.

 

[boatmike]

Of course not. When I get this sorted perhaps I should buy a beer for Mike in the Solent before we depart then pass on his love to you in Croatia when we arrive there sometime next summer! Sleep tight gents - I will update when progress is made.

I smiled at that ohmaggie. Obviously Richard was offended by my reply but he did ask why I thought his post misleading and I guess he objected to being told. So here is a more comprehensive reply.

Firstly let me say that one of my jobs as an apprentice with Joseph Lucas was tuning ammeter shunts and I think at 73 years old now I have probably seen every variety of system for measuring amps on DC circuits that there are. I only mention this because whatever I say someone will say "mine is not like that". So lets start by saying the assumption of what follows is that the system you have is analogue (not digital) it has an external shunt (not internal to the gauge) is 12volt and mounted on or near the negative terminal of the battery.
The shunt is a device of known resistance that is meant to carry the main body of current in one end and out of the other. It will be rated regarding the maximum current it is designed to pass. While there are cheap low amp ones on the market, for safety I would suggest approx. 150amps is about right for a boat system and the device should be mounted in the negative line of the service batteries only. This is so the starter motor current from the engine battery is not routed through it. It is possible to have the shunt in a common line ground for both batteries but then a more heavy duty shunt and gauge is required. This is usually considered to be overly expensive and the gauge, which will be calibrated for high amps will be less sensitive on low amps. Most shunts of about 150amps are constructed so that there are plates or in your case rods of a known resistance with a block either end to which are mounted the terminals for the gauge wires. In some cases on cheap low capacity shunts the wires are attached to the same end terminals that the main line uses, but most are on the end blocks inboard of the main line terminals. This is where your gauge wires should be attached and nowhere else. It should therefore be impossible to remove the shunt without removing the gauge wires and neither should ever be done with a live battery. In fact it is essential to remove the positive terminal or switch off before attaching or removing the gauge wires. Not doing so is a common cause of gauge failure on digital instruments and while you may get away with it on analogue you risk these failing too which is my guess at what may have totalled your gauge. I think the other posts seem to assume that you have removed the shunt but somehow left the gauge wires in place without connecting the supply cable to the battery. This is not what I understood you have done as it would indeed be difficult anyway. If you have removed the damaged shunt entirely and attached the supply cable direct to your battery then I assume you have removed the gauge wires also or they are still attached to the shunt which is the same thing.
Now for this business of "tuning". when supplied the gauge and shunt are matched. In other words the resistance of the shunt is "adjusted" to give the correct reading on the gauge at its maximum reading. On shunts with a number of plates this is sometimes achieved by making a small sawcut in one of the plates(sounds crude but it works) Other systems are sometimes used and not all shunts are the same. This does not alter the voltage across the shunt as pvb suggests but it does increase the resistance thus "tuning" the instrument. Of course there are other ways of tuning used but this is still common. Pvb is correct in theory in that for a given current voltage would have to increase if one half of the shunt were broken which may have totalled the gauge.I agree however with your logic in that this seems to be a DC problem not on the AC side and my guess is it's a likely suspect for having boogred your chargers

 

[RichardS]

I don't think there's any chance that the ammeter was damaged by having a high current passed through it. In the vast majority of cases, shunted ammeters are simply small voltmeters, measuring the tiny voltage generated across the shunt when current flows (typically only 50-60 millivolts). In the OP's case, the shunt had 2 conducting rods, one of which fractured. However, the result of this would have been to double the voltage produced across the shunt (for a given current) and this may have damaged the meter. The fact that the meter reads max when the sensing leads have been disconnected isn't necessarily a fault though, as presumably it's still connected to power through other connections.

My understanding is that shunt has been disconnected with the meter still in circuit. This will easily blow the meter depending upon its range and the current applied.

The boat is now shut down and the battery switches are turned off ...... but the ammeter is still reading maximum current.

Ergo the meter has blown and shunt disconnection has presumably caused it ...... unless we wish to speculation about something which might have happened which we have no facts to support.

Richard

 

[pvb]

Now for this business of "tuning". when supplied the gauge and shunt are matched. In other words the resistance of the shunt is "adjusted" to give the correct reading on the gauge at its maximum reading. On shunts with a number of plates this is sometimes achieved by making a small sawcut in one of the plates(sounds crude but it works) Other systems are sometimes used and not all shunts are the same. This does not alter the voltage across the shunt as pvb suggests but it does increase the resistance so that a tiny percentage more current will go through the gauge wires to reach the right reading thus "tuning" the instrument. Of course there are other ways of tuning used but this is still common. Pvb is correct in theory in that for a given current voltage would have to increase if one half of the shunt were broken but in effect we only have 12v available from the battery so voltage wont increase, rather current will drop.

Oh dear! Maybe it's a long time since you were an apprentice. A shunt will typically be "tuned" to produce a given voltage across it when the maximum current flows. Often, this is 50mV. So, for example, if it's a 150A shunt, it would produce 50mV when 150A flows through it, and proportionally less with lesser currents. The ammeter gauge responds to the voltage and produces a reading.

If half the shunt is broken (ie if one of 2 rods breaks), the resistance across it is basically doubled, and so the voltage it produces for a given current also increases. For our notional 150A shunt, instead of producing 50mV it would produce 100mV.

 

[pvb]

My understanding is that shunt has been disconnected with the meter still in circuit. This will easily below the meter depending upon its range and the current applied.

The boat is now shut down and the battery switches are turned off ...... but the ammeter is still reading maximum current.

Ergo the meter has blown and shunt disconnection has presumably caused it ...... unless we wish to speculation about something which might have happened which we have no facts to support.

The ammeter will typically require 12v connecting to it, in order to work, so disconnecting the sense wires won't stop it doing sometrhing if the power is connected. See this little diagram, for example...

 

[RichardS]

I smiled at that ohmaggie. Obviously Richard was offended by my reply but he did ask why I thought his post misleading and I guess he objected to being told. So here is a more comprehensive reply.

At 73 years old Mike you really should have learned some manners by now.

You can be as comprehesive as you wish ..... but it doesn't alter what has actually happened, and why, to ohmaggie's ammeter.

Richard

 

[RichardS]

The ammeter will typically require 12v connecting to it, in order to work, so disconnecting the sense wires won't stop it doing sometrhing if the power is connected. See this little diagram, for example...

And what happens if I take a chisel and break that shunt in the middle between the two terminals whilst current is flowing?

Richard

 

[pvb]

And what happens if I take a chisel and break that shunt in the middle between the two terminals whilst current is flowing?

If you were able to break the shunt completely, if the engine were started it would doubtless ruin the ammeter. But the OP's shunt hasn't been completely severed, although the ammeter may have been subjected to excess voltage as a result of the partial breakage.

 

[VicS]

Oh dear! Maybe it's a long time since you were an apprentice. A shunt will typically be "tuned" to produce a given voltage across it when the maximum current flows. Often, this is 50mV. So, for example, if it's a 150A shunt, it would produce 50mV when 150A flows through it, and proportionally less with lesser currents. The ammeter gauge responds to the voltage and produces a reading.

.

ITYWF what you are saying is applicable to shunts for use with modern digital meters which themselves draw a negligible current. Boatmike is, I think, talking more about shunts for electromechanical meters, eg moving coil meters. which will themselves require a significant current, maybe tens of mA to drive them and whose parameters may be rather nominal compared with those of a digital meter. Hence the need to match the shunt to the meter.

 

[pvb]

ITYWF what you are saying is applicable to shunts for use with modern digital meters which themselves draw a negligible current. Boatmike is, I think, talking more about shunts for electromechanical meters, eg moving coil meters. which will themselves require a significant current, maybe tens of mA to drive them and whose parameters may be rather nominal compared with those of a digital meter. Hence the need to match the shunt to the meter.

All ammeters are matched to their shunts.

 

[RichardS]

If you were able to break the shunt completely, if the engine were started it would doubtless ruin the ammeter. But the OP's shunt hasn't been completely severed, although the ammeter may have been subjected to excess voltage as a result of the partial breakage.

Thank you.

I hope Mike takes note.

Richard