BM1 under reading

[sailaboutvic]

I have 4x105 ah batteries as house bank , there a bm1 monitor and a MPPT both connected to the banks .
The question is , why would my BM1 shown 12.5when my MPPT show 12.7 a two point different ? By the way my muti meter correspondent to the MPPT reading .
Other then the BM1 being faulty ?
Or is there some other reason ?

All connection have been checked

 

[dom]

Just to be sure, have you used the multimeter to test the voltage across the sensing wires of the BM1 exactly where they are connected into the system?

Also, the rounding algorithms could account for half of that difference

 

[ghostlymoron]

Is it possible to calibrate the bm1?

 

[[70521]]

What is the voltage across the shunt and at the unit?

There is some calibration on the BM1 but I think that is for amps you can Google the manual if you don't have it with you.

 

[RupertW]

What is the voltage across the shunt and at the unit?

There is some calibration on the BM1 but I think that is for amps you can Google the manual if you don't have it with you.

That should be too small to read but good point - if it is noticeable then there is a problem.

 

[VicS]

That should be too small to read but good point - if it is noticeable then there is a problem.

ITYWF that the PD across the shunt wil be 50mV at 100 amps

 

[Richard10002]

Is it possible that both should be reading 12.6, and one has resolved up and the other down?

 

[pvb]

What is the voltage across the shunt and at the unit?

The voltage across the shunt is only relevant for the current readings on the BM1.

 

[RupertW]

The voltage across the shunt is only relevant for the current readings on the BM1.

Really? You may well be right as I don't have the installation diagram in front of me but doesn't the negative voltage sensing wire come from the shunt not from the battery terminal. So a bad connection from battery to shunt, or a fault in the shunt might cause a voltage drop. If both voltage sensing wires go direct to the battery terminals, then you are right, of course.

 

[pvb]

Really? You may well be right as I don't have the installation diagram in front of me but doesn't the negative voltage sensing wire come from the shunt not from the battery terminal. So a bad connection from battery to shunt, or a fault in the shunt might cause a voltage drop. If both voltage sensing wires go direct to the battery terminals, then you are right, of course.

The negative sensing wire comes from the battery side of the shunt, so isn't affected in any way by the voltage across the shunt (which will be minimal anyway).

 

[VicS]

Really? You may well be right as I don't have the installation diagram in front of me but doesn't the negative voltage sensing wire come from the shunt not from the battery terminal. So a bad connection from battery to shunt, or a fault in the shunt might cause a voltage drop. If both voltage sensing wires go direct to the battery terminals, then you are right, of course.

You will find the wiring diagrams in the instruction manual at https://www.nasamarine.com/wp-content/uploads/2015/12/Clipper-BM1BM2.pdf

A poor connection between the battery negative and the shunt may well cause a falsely low voltage reading on the BM1 but it will vary with load and be zero at zero load. If this is the cause of the problem there will be no discrepancy at zero load on the domestic battery and could be confirmed by direct measurement with a multi-meter anyway

 

[[70521]]

The negative sensing wire comes from the battery side of the shunt, so isn't affected in any way by the voltage across the shunt (which will be minimal anyway).

If it is not minimal that will tell you something.

The reason I said check the voltage across the shunt was to eliminate that component from the equation.

 

[pvb]

If it is not minimal that will tell you something.

The reason I said check the voltage across the shunt was to eliminate that component from the equation.

The voltage across the shunt is a function of the current flowing through it, and is used to generate the current reading in the BM1 and the Ah calculation. As Vic said in post 6, the voltage across the shunt is only 50mV when 100A is flowing through the shunt. This has nothing at all to do with the voltage measurement process carried out by the BM1.

 

[[70521]]

The voltage across the shunt is a function of the current flowing through it, and is used to generate the current reading in the BM1 and the Ah calculation. As Vic said in post 6, the voltage across the shunt is only 50mV when 100A is flowing through the shunt. This has nothing at all to do with the voltage measurement process carried out by the BM1.

You misunderstand my logic. If there is voltage crossing the shunt there is a problem. I am suggesting the OP checks it in order to exclude it as a possible cause of the problem.

 

[pvb]

You misunderstand my logic. If there is voltage crossing the shunt there is a problem. I am suggesting the OP checks it in order to exclude it as a possible cause of the problem.

I certainly don't understand your logic. Where do you think this voltage is likely to come from?

 

[[70521]]

I certainly don't understand your logic. Where's do you think this voltage is likely to come from?

I am not even going there. I'll assume that everything is insulated to within 0.2 mm of any contact... meanwhile in the real world.

 

[pvb]

I am not even going there. I'll assume that everything is insulated to within 0.2 mm of any contact... meanwhile in the real world.

So you don't really have a theory then?

 

[VicS]

You misunderstand my logic. If there is voltage crossing the shunt there is a problem. I am suggesting the OP checks it in order to exclude it as a possible cause of the problem.

If there is a voltage across the shunt it will show as a current reading on the BM1. That's how it works. Every mV will show as 2 amps The BM1's max reading is 100 amps which will be shown when the PD across the shunt reaches 50mV. We are looking for a discrepancy of 0.2 volts ( 200 mV) which, if the BM! was capable of displaying it, would be a current reading of 400 amps!

 

[sailaboutvic]

Thanks for your input guys , found the problem , one of the wire must had. Been rubbing on the wire tie and wore most of it away .
Again thanks

 

[[70521]]

So you don't really have a theory then?

Just a jobbing engineer, with an engineering degree and some post grad stuff, from a long line of jobbing engineers who have always been taught if you can test something to eliminate it you test it.

When I bought my current boat I asked the previous owner to talk me through the electrical system as it looked like a rats nest, all I got was it works and I know nothing about it! I've made it safer, now working on getting it up to date, a lot safer and more efficient.

Thankfully the OP has found the problem and it sounds like poor insulation.