Installing Nasa BM-1 Battery Monitor

[TwinRudders]

Hi - I am getting confused staring at the batteries.

I want to install my new BM-1 monitor onto the domestic bank (2 x 110ah batteries).

However I want to be absolutely certain I am doing this right so please advise!


I also have an odyssey engine starter battery. They are selected with a rotary 1,2,both,off selector.

Looking at the wiring I apear to have the main positive connection from the startmotor connected to the rotary switch and then then No 1 positive connected to the domestic service bank and No2 connected to the engine start battery. All fine.

The negative ground starts at the solenoid and is connected directly to the domestic bank battery negative terminal - which is connected to the other domestic battery - and also the engine starter battery neg terminal.

That all works and I can select the 2 banks from the switch.
But looking at the Nasa instructions the negative ground should go the "nasa shunt" first and then to the domestic battery bank it is going to monitor... Does it need to do that or can I wire a lead from the shunt to the battery -ve terminal and leave the rest as it is.

Thanks for any help.

Jonny

 

[whipper_snapper]

[ QUOTE ]
But looking at the Nasa instructions the negative ground should go the "nasa shunt" first and then to the domestic battery bank it is going to monitor... Does it need to do that

[/ QUOTE ]

Yes

[ QUOTE ]
or can I wire a lead from the shunt to the battery -ve terminal and leave the rest as it is.

[/ QUOTE ]

No. The shunt must carry the current used for everything to be monitored. The device works by measuring the voltage 'dropped' across the shunt by the current passing through it.

However, if I understand your arrangement correctly, you could, and possibly should, put it in 'downstream' of the solenoid - ie not on the battery side - that way it will not carry the engine start current.

 

[TwinRudders]

OK thanks for that but still confused. When you say "downstream"?
From the solenoid the black -ve lead goes straight to the -ve terminal of the first of the domestic service batteries. From there that -ve terminal is connected to the other dom battery -ve and the engine start battery -ve terminal.
Do you mean leave the connection from the solenoid to the battery and put the shunt in after this?

Thanks, J

 

[VicS]

[ QUOTE ]
Does it need to do that or can I wire a lead from the shunt to the battery -ve terminal and leave the rest as it is

[/ QUOTE ]
I am not quite sure exactly what you have in mind there.
All the current flowing into and out of the domestic bank must flow through the shunt but the current flowing in and out of the engine start battery must not, it must flow directly to and from the starter battery. The positive connecton to the BM1 must be directly to the domestic battery positve but in your case the 1/2/both/off switch is a problem!

I am also puzzled by the statement that the negative runs from the solenoid. Normally with a negative earth system it is the positive that runs between battery and solenoid while the negative runs between a convenient bolt somewhere on the engine block (assuming it is not an insulated negative return system) and the battery. If that were the case the negative could be run to the engine start battery, rather than the domestic bank, then from there via the shunt to the domestic bank. (Remembering that the instructions to run two wires all the way from the BM1 to the same terminal on the shunt must be observed)

Your 1/2/both/off system complicates matters further because that allows you to start the engine from the domestic bank BUT the BM 1 is limited to currents of 100 amps, almost certainly less than the starter current. OK if you can be sure never to start the engine from the domestic bank.
The BM1 is better suited to a system with dedicated starter and domestic batteries with separate isolating switches charged via a splitter or VSR and completely separated engine and domestic wiring systems.

 

[whipper_snapper]

where does the domestic supply main neg come from ?

I took it that it shared a stud on the back of the solenoid. If so, take off the domestic supply from this stud but leave the lead to the battery on the stud. Put the shunt between the stud and the domestic neg supply.

 

[VicS]

[ QUOTE ]
If so, take off the domestic supply from this stud but leave the lead to the battery on the stud. Put the shunt between the stud and the domestic neg supply.


[/ QUOTE ] That's no good. Like that it will only monitor the domestic use but not the domestic bank charging current.

The shunt must go in the negative connection to the battery and there must be no other connection on the battery side of the shunt or the battery negative. (apart from the connection to the BM1 itself)

 

[TwinRudders]

The negative connection is from the starter motor - this goes directly to the -ve terminal of the domestic batteries. The engine battery is also connected to this negative terminal.

The +ve lead is form the SOLENOID to the rotary switcher.
J

 

[whipper_snapper]

[ QUOTE ]
That's no good. Like that it will only monitor the domestic use but not the domestic bank charging current.

[/ QUOTE ]


errm. what ?

Are you sure ? Unless the charge neg makes separate connection on the battery side, it will measure everything in and out except for starter motor.

 

[whipper_snapper]

[ QUOTE ]
The negative connection is from the starter motor - this goes directly to the -ve terminal of the domestic batteries. The engine battery is also connected to this negative terminal.

[/ QUOTE ]

In that case, when I said solenoid I meant starter motor. It should be perfect with the shunt between starter motor stud and all other loads. But VicS disagrees! We should probably resolve that first!

 

[VicS]

[ QUOTE ]
It should be perfect with the shunt between starter motor stud and all other loads. But VicS disagrees! We should probably resolve that first!


[/ QUOTE ] It must have both the load currents and the charging current flowing through it. The alternator current will be flowing directly between the earth stud and the batteries if it is wired as you suggest.

Describing things in a different way

Starting from the negative terminal of the domestic bank there must be a connection to the shunt <u>and to nothing else!</u> The other side of the shunt is then connected to everything else. Engine earth, starter battery negative, domestics negative bus-bar, the lot!

I am still worried about where you will make the positive connection to the BM1. If you connect directly to the battery positive it will not be switched off when leaving the boat etc. If however the connection is to the common terminal of the 1/2/both/off switch it will be connected to the wrong battery when the starting battery is selected. OK its voltmeter function will then give a useful reading on that battery but i am uncertain if a volts reading on the wrong battery will upset its little brain when it comes to monitoring the domestics bank. Maybe not but Nasa might clarify that point. It will also interpret its own current usage as a charge going into the domestics battery but that will be small and short lived so that should not matter

The other problem resulting from the use of that type of selector switch will be that the BM1 will see a current outside its maximum range if the domestic bank is used for starting. I believe that has already been discussed with Nasa by another forumite and that they have said it is OK but I again I would want to clarify that with them. Why specify a limit if it is OK to exceed it?

 

[TwinRudders]

Thanks for that Vic,

Nasa say connect it to the +ve of the dom battery as it only draws 3mA. Designed to be permanently connected they say.
If I do it that way am I still in danger of putting too much through the shunt if I start the engine with domestic batts selected?
J

 

[VicS]

Fair enough on the permanent +ve connection then, but it would be advisable to fit a fuse in that connection as close as reasonably possible to the battery. Low enough value to protect the wiring to the BM1.

Yes on the starting issue, but as I said it has cropped up before. Maybe a search will find it. It was not many weeks ago. I remember my final comment was something along the lines that I thought I knew how these gadgets worked but now I'm not so sure!

 

[whipper_snapper]

[ QUOTE ]
The alternator current will be flowing directly between the earth stud and the batteries if it is wired as you suggest.


[/ QUOTE ]


If that is so, I agree. You mean the alternator neg is the engine block ground ? Is that normally the case ?


[ QUOTE ]
I believe that has already been discussed with Nasa by another forumite and that they have said it is OK but I again I would want to clarify that with them.

[/ QUOTE ]

NASA have said <span style="color:blue"> 100 Amps continuous load, it will withstand higher loads for short duration </span>. Several people have said they start reasonable sized engines through it. NASA also said <span style="color:blue">However it would be best to bypass the shunt when starting the engine</span>.

[ QUOTE ]
Why specify a limit if it is OK to exceed it?

[/ QUOTE ]

I take it to mean it can only reliably measure currents up to 100A, which is not the same as handle currents up to 100A.

 

[TwinRudders]

Yeap there is a fuse attached to the positive lead that attaches to the +ve terminal.
J

 

[VicS]

Yep I found the thread to which you replied about the 100 amps limit.

HERE

re the alternator negative. Normally earthed to the engine block (except with insulated negative return systems of course) with just one negative wire from there to the batteries.

Similarly the positive output from the alternator is often/usually taken to the battery connection on the solenoid, then just one connection from there to the isolator.

 

[whipper_snapper]

OK, thanks, I stand corrected.

My present boat has no alternator!

I never checked but always assumed the neg was wired back separately because the alternator hangs on a series of movable brackets and arms which I would not have expected to give reliable connection.

 

[VicS]

[ QUOTE ]
I never checked but always assumed the neg was wired back separately because the alternator hangs on a series of movable brackets and arms which I would not have expected to give reliable connection.

[/ QUOTE ] They may not so an earth connection from the alternator to the block may be fitted. Same as on cars, better ones have an earth connection others rely on the mountings.

 

[ChiPete]

Hi there,

I've just installed one of these to my Oceanis 323.

Agree with all the post re get the negative side right, so basically move the neg strap from your domestic battery(ies) on to the shunt and put the strap supplied from the other side of the shunt to the point at which you took the battery cables from.

Re the positive, I wired it straight to one of the domestic battery positives. The BM1 unit is designed to be left on 24/7. It draws 3ma, so will take fair while to flatten even one 70Ah battery. Simple work around if you must switch it off would be to put a simple on/off switch in line with the positive line to the BM1.

Hope this helps!

Cheers,
Pete