Strange Nasa BM1 problem

[YELLOW BOAT]

Hi,
I'm afraid I made some mistakes in my post on this thread yesterday at 14:59
I looked at the wrong picture but now I deleted it.
So have another look at it.
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[pvb]

Hi,
I might have found the cause of the problem.
The negative of the two batteries have to be connected to the engine mass seperately to avoid them influencing each other the shunt is connected next to the domestic battery. Otherwise a slight difference in rest voltage ( caused by different resistance in the negative connections) will give a strong charge or discharge depending on which negative is highest.N.b.
Have a look at the picture. Its how its connected in my boat for 5 or 6 years and working great.
n.b. Only the domestic battery is measured.
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http://commondatastorage.googleapis....l/47038702.jpg

I had a second look at your post, as you requested.

There's no need to connect the negatives separately to the engine, as long as the cables are properly sized and securely connected. It won't make any difference to the current in the domestic circuit.

Your diagram shows the negative connection of the charge splitter going to the battery side of the BM1 shunt. For correct operation, the only wires on the battery side of the shunt should be the battery negative cable and the black & white sense wires for the BM1. Some charge split devices (eg VSRs) draw a constant small current, so should be correctly connected to ensure the BM1 results are accurate.

[YELLOW BOAT]

You're correct about that.
I zero'd it when installing the BM1 but may be I'll change it.
About the negative wires. Of course when the wires are thick enough this is the same as directly on the engine. Its just that I did because that was the easyest way for me.
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[pvb]

About the negative wires. Of course when the wires are thick enough this is the same as directly on the engine. Its just that I did because that was the easyest way for me.

OK, but it's important to realise that some people could have thought that your statement "The negative of the two batteries have to be connected to the engine mass seperately to avoid them influencing each other" was true. That's how a lot of confusion creeps in to these forums.

[VicS]

NASA answer follows

Dear Sir,

After reading the thread it appears you have been given some good advice: The backlight draws about 10mA. This current is supplied through the red wire and returned back to the battery through the black wire. If the black wire is not properly connected then the return will pass through the white wire which is one of the shunt sense wires. It only takes 50mV accross the sense wires for the display to read 100A. So even a few mA throughthe sense wire will cause a problem.Check all the connections. Make sure the terminal screws are tight and on the wire rather than the plastic sleeve. The offset is caused by stray voltages generated in the sense wire(white and yellow wires) terminals and should never be more than 0.1 to 0.2 Amp. It can, of course be ballanced out but if it's as high as 2amp then there is something wrong and ballancing it out should not be attempted. If you can't sort the issue recommend you reyurn the unit to us for test.
Regards,

NASA Marine
Boulton Road
Stevenage
Herts
SG1 4QG

Well they have not come up with anything new. Not sure whether to be disappointed or pleased that the might of the forum has thought of everything.
Back to checking and rechecking that all the connections are good.
They have told us that the backlight takes approx 10mA so to produce a discrepancy of 3 amps on the display it only needs a resistance of 0.15 ohms in the wrong place. That is very small!

[Stu Jackson]

Very unusual thread, since all the advice freely given seems to have taken some time to sink in.

Maine Sail, a contributor to this board in the past, has posted this helpful thread:

http://forums.catalina.sailboatowner...tery%20monitor

It's really not so hard to follow directions, manuals and advice. Shunts are pretty tricky. I made this article, too. which links back to Maine Sail's: http://c34.org/bbs/index.php/topic,6032.0.html

Good luck. I agree wholeheartedly with Vic's and others' advice. And even the manufacturer says so.

[pieter2]

Today I renewed the black-white socket.
Nothing changed!
Still a voltage shown of 0,3
When I disconnect the red wire (just to see if there is some interference with another equipment close to the wiring or so): there is no voltage to be measured.
The only thing that might be checked would be a complete new wiring I suppose.

[VicS]

The red wire is the positive power supply to the BM1 when it is disconnected the thing should go completely dead. No display, no backlight .. nothing.

I'd experiment with disconnecting the white and the yellow wires. With either disconnected you should not get any current reading. I am not sure if disconnecting them, either or both, will kill the volts reading but the instrument should not go dead with them both disconnected.

Are you still getting a voltage difference between the black and the white wires when the back light is on as you were in post #14? If you are then finding out why is the key to the problem I am sure.

[Playtime]

Today I renewed the black-white socket.
Nothing changed!
Still a voltage shown of 0,3
When I disconnect the red wire (just to see if there is some interference with another equipment close to the wiring or so): there is no voltage to be measured.

Peiter, how did you 'renew' the black- white connection? If you are still measuring 0.3V between black and white then the connection is still faulty. It should be zero.

Disconnecting the red removes 'power' from the meter, which is therefore effectively turned off.

[pieter2]

I totally renewed the crimp of the connector of the black and white wire at place of the shunt.
Putting off the red wire: no voltage difference
Putting on the red wire: again 0,3 V on the black and white. At the BM-1 dispaly I still have the difference in reading the Amp (1,9 A with backlight off / 0,1 with backlight on)
So, the only thing I can come up with is that the black or white wire might be damaged or faulty in some other way.
I will rewire the black and white totally.

If that does not help I will return the BM-1 to NASA (or the fishes).

I don't see what the yellow wire could do for this.