help - Strange NASA BM1 battery monitor behaviour

Georgio

Georgio

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Wondering if there are any ideas from the forum to solve this puzzle.

Having had a BM1 installed for 6 or more years and been very happy with it the monitor has started over-reading the voltage by 0.1 to 0.2V and has been showing a small drain on the battery bank, this started about a year ago. If I leave the boat for a while it can show that more amps have been discharged from the battery than there is in the battery, but still showing 12.9V!

I have tested the voltage at the batteries and at the shunt on two different multi-meters and the boats instrumentation and they all agree that the BM1 is over-reading. Even when the battery isolator switch is off it does this. I sent the unit back to NASA about 6 months ago for testing and they could find nothing wrong with it so I reinstalled but still had the same problem.

I have tested everything I can think off, isolating different parts of the electrical system, checking connections but all to no avail

I have just sent it back again and while they can find nothing wrong with it again I have paid to have the internals replaced and will reinstall shortly.

If the reading was too low I would assume a bad connection somewhere but it is reading too high!

Can anyone come up with any explanation why this should be?

As I said it has worked perfectly in the past but not now, I have not changed anything in it's wiring although I have added a few bits of equipment in that time.

I welcome your thoughts.... (without blaming cheap Nasa equipment).
 
I fitted one to my last boat, good for the first season, then went very erratic.

I now prefer a digital voltmeter on each battery bank reading to two points of decimal. Cheap as chips and after 3 years very reliable and accurate.
 
Have no real experience with the exact model, I do know the whole idea behind these devices is to measure the voltage drop across the shunt. The point is that the voltage drop must be kept at minimum to prevent losses in the main circuit. So effectively the system needs to measure a very small voltage across two wires that run from the shunt to the device.
So basically two thing can happen; the measuring circuit is off. This can be tested by shorting the connection points for the two leads directly at the device, making sure there is no voltage across them anymore. The device should report exactly zero current.
The other cause is that the leads are acting as some kind of antenna or some form of induction is introduced to the lead between the shunt and the device. One way to prevent this (upto a certain degree) is by using twisted pair cables where the two conductors are twisted around each other to cancel out the effect. Shielded cabling may work as well. You can verify if some strong current cable is next to the aforementioned leads.

Cheers,

Arno
 
aluijten said:
Have no real experience with the exact model, I do know the whole idea behind these devices is to measure the voltage drop across the shunt. The point is that the voltage drop must be kept at minimum to prevent losses in the main circuit. So effectively the system needs to measure a very small voltage across two wires that run from the shunt to the device.
So basically two thing can happen; the measuring circuit is off. This can be tested by shorting the connection points for the two leads directly at the device, making sure there is no voltage across them anymore. The device should report exactly zero current.
The other cause is that the leads are acting as some kind of antenna or some form of induction is introduced to the lead between the shunt and the device. One way to prevent this (upto a certain degree) is by using twisted pair cables where the two conductors are twisted around each other to cancel out the effect. Shielded cabling may work as well. You can verify if some strong current cable is next to the aforementioned leads.

Cheers,

Arno
Click to expand...

Arno, thanks, that is a great explanation and a further thing that I an try.
 
Georgio,

You say "the monitor has started over-reading the voltage by 0.1 to 0.2V".

If this occurs even when no current flows then I cannot see how this can happen unless the NASA is faulty, or as "aluijten" states an additional voltage is being induced into the montoring circuit..
 
Georgio said:
...the monitor has started over-reading the voltage by 0.1 to 0.2V and has been showing a small drain on the battery bank, this started about a year ago.
Click to expand...

How are you measuring the battery voltage to determine that the BM1 is over-reading?

Have you gone through the zero current reset procedure?
 
pvb said:
How are you measuring the battery voltage to determine that the BM1 is over-reading?

Have you gone through the zero current reset procedure?
Click to expand...

Battery voltage has been tested both direct at the batteries and at the point where the monitor is connected (battery and shunt), this has been done with two different multimeters and the tacktick kit also tells me the batt voltage, they all agree with each other but disagree with the monitor.

Yes I have reset the current monitor to zero, but weirdly after doing that and saving the setting it goes back to showing a slight discharge within a minute or so.

I should perhaps wait until I refit the BM1 before commenting further as the internals have all now been replaced although a bench test by Nasa showed is working accurately.

The most annoying part of this is that having paid for an inspection + postage 6 months ago then inspection + PCB replacement +postage this time I could have bought a new one for similar cost. I generally like Nasa but this has been really annoying.
 
Last edited:
Georgio said:
Battery voltage has been tested both direct at the batteries and at the point where the monitor is connected (battery and shunt), this has been done with two different multimeters and the tacktick kit also tells me the batt voltage, they all agree with each other but disagree with the monitor.

Yes I have reset the current monitor to zero, but weirdly after doing that and saving the setting it goes back to showing a slight discharge within a minute or so.

I should perhaps wait until I refit the BM1 before commenting further as the internals have all now been replaced although a bench test by Nasa showed is working accurately.

The most annoying part of this is that having paid for an inspection + postage 6 months ago then inspection + PCB replacement +postage this time I could have bought a new one for similar cost. I generally like Nasa but this has been really annoying.
Click to expand...

Your experiences with the Nasa battery monitor are not unique. I suspect the trouble lies with the connections and/or the wiring between the instrument and the shunt and battery.

If the problem persists with the refurbished instrument I would direct my attention to that wiring, checking, double checking all the connections, checking and possibly replacing and/or re-routing the wiring.
 
VicS said:
Your experiences with the Nasa battery monitor are not unique. I suspect the trouble lies with the connections and/or the wiring between the instrument and the shunt and battery.

If the problem persists with the refurbished instrument I would direct my attention to that wiring, checking, double checking all the connections, checking and possibly replacing and/or re-routing the wiring.
Click to expand...

Thanks Vic, if the problem persists I will certainly re check the connections.

The strangest thing is that the voltage is increased not reduced. I wondered if some existing equipment was somehow boosting the voltage even when switched off (internal battery discharge?) although that seems very unlikely.
 
Georgio said:
Thanks Vic, if the problem persists I will certainly re check the connections.

The strangest thing is that the voltage is increased not reduced. I wondered if some existing equipment was somehow boosting the voltage even when switched off (internal battery discharge?) although that seems very unlikely.
Click to expand...
are you charging 2 banks & the engine batt is equalizing to therefore showing a charge / higher voltage via the shunt
 
As commented many times before, a very likely cause of this sort of problem is a TINY resistance somewhere in the permanent positive supply to the BM-1. The inline fuse holder is a very likely culprit. (replace it with something like a blade-fuse).
 
VicMallows said:
As commented many times before, a very likely cause of this sort of problem is a TINY resistance somewhere in the permanent positive supply to the BM-1. The inline fuse holder is a very likely culprit. (replace it with something like a blade-fuse).
Click to expand...

I have replaced the fuse holder supplied (because it was carp, poor connection and therefore low voltage reading), however I would expect any further resistance in that circuit to result in a lower reading not a higher reading which is what I have experienced.
 
Georgio said:
Battery voltage has been tested both direct at the batteries and at the point where the monitor is connected (battery and shunt), this has been done with two different multimeters and the tacktick kit also tells me the batt voltage, they all agree with each other but disagree with the monitor.

Yes I have reset the current monitor to zero, but weirdly after doing that and saving the setting it goes back to showing a slight discharge within a minute or so...........
.
Click to expand...



I have recently fitted one of these monitors and it too registers a 0 - .1 amp discharge even with everything ostensibly switched off. Calibration to zero does not seem to affect it much.

The measured voltage does seem ok. It looked too high by my two five quid multi meters but I am now pretty sure it was the meters at fault.
 
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