Can A Shunt Be Faulty?

[Old Son]

Before I changed the battery bank on my boat the Blue Seas Digital Amp Meter was working fine. Now it doesn't. I know the connections to the battery are exactly as they were. The amp meter is not registering small loads like fridge, macerator toilet etc. It stays on 0.0amps.

 

I ran a 2kw heater through the inverter today and it showed -56 amps (its a 24V system).

 

If the amp meter is working then can the shunt be the fault?

[Paul C]

Yes, and so can its wiring (the 2 little wires from the shunt to the ammeter).

[Dyertribe]

Ahhhh SHunt! You had me worried there!

 

What?

What?

[ditchcrawler]

Before I changed the battery bank on my boat the Blue Seas Digital Amp Meter was working fine. Now it doesn't. I know the connections to the battery are exactly as they were. The amp meter is not registering small loads like fridge, macerator toilet etc. It stays on 0.0amps.

 

I ran a 2kw heater through the inverter today and it showed -56 amps (its a 24V system).

 

If the amp meter is working then can the shunt be the fault?

-56 is a bit low

[Bobbybass]

You should have 2 little wires attached to it which go to the meter..and they are usually attached in their own small holes with small nuts and bolts.

Sometimes...people find that these tiny connections have 'come adrift' and the attach them to the end of the shunt where all the main cables attach....which gives false readings. Worth checking.

[Iain_S]

If a 2kW heater, 56 Amps is too low, (I make that 1344W) 2kW would be 83 Amps, making no allowance for invertor innefficiency.

 

Are the fridge, toilet, etc 240V or 24V? If 240V, possibly invertor output voltage is low, but most likely bad connection at shunt. (The meter connections: if it was a main cable connection, it would get hot )

 

Iain

[Paul C]

What does the ammeter show? As in, which current? If you run a large load while the engine is running, and if its wired in the negative line of the battery, then it will show the amount of current from the battery, but some of it will have come from the engine alternator, and some from the battery.

And also....do you have solar power? That will be another source of currnet which will supply loads, possibly variably depending on weather and state of charge of batteries and type of controller.

[Bobbybass]

If a 2kW heater, 56 Amps is too low, (I make that 1344W) 2kW would be 83 Amps, making no allowance for invertor innefficiency.

 

Are the fridge, toilet, etc 240V or 24V? If 240V, possibly invertor output voltage is low, but most likely bad connection at shunt. (The meter connections: if it was a main cable connection, it would get hot )

 

Iain

Well....I always allowed (roughly incl inverter loss) that 1 amp at 240 volts draws about 20 amps at 12 volts.

2000 watts for a heater is about (rough) 8 amps at 240 volts...so should be drawing about 160 amps from your 12 volts...UNLESS you have a bad connection and have a cable 'loss'.

Edited August 4, 2013 by Bobbybass

[alan_fincher]

so should be drawing about 160 amps from your 12 volts...

 

Except he says it is a 24 volt system, so your estimate would need halving.......

[Old Son]

Thanks for your replies.

 

I have a 24V system, no solar. The toiler and fridge are 24V. When I did the check I had everything switched off to start. The only current being drawn was by the inverter and the heater and the reading was -56amps. I then turned everything off again and only switched on the fridge. The amp meter registered nothing. I then added a toilet flush (normally around 4-5 amps) still nothing registering on the amp meter.

 

The only connection that was disturbed when we changed the batteries was the main earth lead to the shunt and a very small + lead to power the amp meter. I have checked all wiring and it is tight.

 

This is why I suspected the shunt might be at fault. The reading on the twisted pair connected to the amp meter was 56mv and the meter was showing - 56amp so the conversion seems right (even if I've got my units mixed up)

[Paul C]

A fridge sometimes draws 0 amps - it cycles between compressor running or not, its about 1/3 of the time on. Obviously if there's (ambient temp) food put in it, to cool down, or the door is opened more than average, it will run for longer.

[Bobbybass]

 

Except he says it is a 24 volt system, so your estimate would need halving.......

Duh...sorry..

 

I should have goone to Specsaversx

[Old Son]

A fridge sometimes draws 0 amps - it cycles between compressor running or not, its about 1/3 of the time on. Obviously if there's (ambient temp) food put in it, to cool down, or the door is opened more than average, it will run for longer.

I can hear when the fridge is running. We don't live aboard and at the time I tested the fridge was empty and the door was open. I can assure you it was running and the meter was showing 0.0amps.

 

Can I assume that the shunt could be faulty and it might be worthwhile buying a new shunt? How do the twisted pair manage to detect such small voltage across the shunt (non technical explanation please)

[by'eck]

I can hear when the fridge is running. We don't live aboard and at the time I tested the fridge was empty and the door was open. I can assure you it was running and the meter was showing 0.0amps.

 

Can I assume that the shunt could be faulty and it might be worthwhile buying a new shunt? How do the twisted pair manage to detect such small voltage across the shunt (non technical explanation please)

 

Since the shunt itself is a passive device not much can go wrong if the main battery connections to it are good and its inserted in the right place. I would strongly suspect the monitor/display or its wiring.

 

BTW 2kW provided by 24 volt system and presumably inverter would have current draw in the order of 90 amps allowing for inefficiency. The reason for lower may be result of low battery voltage however.

[OldGoat]

I can hear when the fridge is running. We don't live aboard and at the time I tested the fridge was empty and the door was open. I can assure you it was running and the meter was showing 0.0amps.

 

Can I assume that the shunt could be faulty and it might be worthwhile buying a new shunt? How do the twisted pair manage to detect such small voltage across the shunt (non technical explanation please)

 

 

Umm - that's because that is how a shunt "works". It's a very low and stable resistor. Current flowing through it produces a very, very small voltage drop which is measured by the "ammeter" and presented as a current in amperes. Quite often the meter has some sort of simple computer chips to do the conversion - especially as all those that I've seen have other functions. I believe the 56mv is the voltage presented when 100 amps flow through the shunt (could be wrong).

 

I suspect either the wiring at the meter end (mine has rubbish connections) or the meter itself, especially if it's a cheap import.

[Justme]

 

The reason for lower may be result of low battery voltage however.

 

 

A lower bat voltage would result in a higher amperage for the same load.

[RLWP]

 

if the main battery connections to it are good and its inserted in the right place

 

If it is inserted in the right place...

 

I suspect that this one isn't. If the fridge runs and no current is shown - it HAS to be wired into the wrong place

 

Richard

[Old Son]

If it is inserted in the right place...

 

I suspect that this one isn't. If the fridge runs and no current is shown - it HAS to be wired into the wrong place

 

Richard

But it was working fine before I changed the batteries. I have only disconnected the negative to the shunt and the very thin positive wire feeding power to the meter. Nothing else has been touched. There is either a problem with the shunt or a problem with the meter.

 

Old Goat - Its a Blue Seas item, £180!!!

[by'eck]

 

 

A lower bat voltage would result in a higher amperage for the same load.

 

Not in practice. The AC volts will usually be lower resulting in less than the 2kW rating of the load being drawn.

[OldGoat]

 

Old Goat - Its a Blue Seas item, £180!!!

 

Duh! - sorry.

I do try to read a thread from the beginning - but only on the first visit....

 

I have an older American import and it does some strange things - reports 150 amp draw when I start my Mikuni boiler - despite the fact that the Mikuni is protected by a 25 amp internal fuse and a ditto circuit breaker. Everything else appears fine. I have a sneaky feeling that the unit is affected by noise. However, that doesn't explain your problem.

[ditchcrawler]

But it was working fine before I changed the batteries. I have only disconnected the negative to the shunt and the very thin positive wire feeding power to the meter. Nothing else has been touched. There is either a problem with the shunt or a problem with the meter.

 

Old Goat - Its a Blue Seas item, £180!!!

This could be your answer ( I have only disconnected the negative to the shunt and the very thin positive wire feeding power to the meter.) Are you 100% sure they have been reconnected both in the right place and making perfect contact?

[smileypete]

IF the connections are OK, sounds like the meter has gone phut.

 

The shunt should have a rating like 200A 75mV, so the millivolts across it can be compared to a known load.

 

Still, digital ammeters can be had from about £10-15 on Ebay direct from our far east friends, look for one from a specialist seller, and one that can run off the same supply as being measured.

 

Wonder if one of the sense wires was disconnected before the supply wire which did it a mischief.

 

I have an older American import and it does some strange things - reports 150 amp draw when I start my Mikuni boiler - despite the fact that the Mikuni is protected by a 25 amp internal fuse and a ditto circuit breaker. Everything else appears fine. I have a sneaky feeling that the unit is affected by noise. However, that doesn't explain your problem.

 

Maybe twisting the sense wires together (twisted pair) would help there.

 

cheers, Pete.

~smpt~

Edited August 5, 2013 by smileypete

[RLWP]

IF the connections are OK, sounds like the meter has gone phut.

 

{snip}

 

cheers, Pete.

~smpt~

 

Not really the behaviour I'd expect from a phut digital meter. Nothing at all is the usual thing, or smoke

 

It's exactly the behaviour from an incorrectly wired in shunt that had just been removed

 

Richard

[Old Son]

Not really the behaviour I'd expect from a phut digital meter. Nothing at all is the usual thing, or smoke

 

It's exactly the behaviour from an incorrectly wired in shunt that had just been removed

 

Richard

The shunt has not been touched. The connections from the shunt to the meter have not been touched. I had a wiring diagram of the original installation so I know that it's wired correctly. The earth was reconnected when there was no load so that could not have been an issue but could connecting the live side of the meter have caused the meter to break and show incorrect readings? Could there have been a surge or something. It doesn't seem likely but I'm definitely no expert!!

 

I have been in contact with Blue Seas and they have offered to replace the meter even though its just over 3 years old. I don't want to ask for a replacement meter and find out I've still got the same issues.

[weeble]

Why not disconnect the sense wires and stick a meter set to mV across the shunt and see what indication you get when running a high load. The sensitivity of the shunt is usually marked on it so you should be able to 'roughly' calculate the current being drawn and check that it's in the right ball park.