# Nasa BM1 Battery Monitor



## MrGaz (Nov 8, 2009)

I would like to fit a Nasa BM1 battery monitor to my 2010 Autotrail....I have 2 batteries connected by 2 separate fly-leads and a Solar panel connected as per 1st attached drawing
There is no parallel connection (Neg to Neg & Pos to Pos) across the top of the batteries but I think this is made at the opposite end of the fly-leads
Can anybody advise on best way to connect the BM1 shunt...would you agree with 2nd attached drawing...any advice would be appreciated


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## philoaks (Sep 2, 2008)

With the proposed setup I think you would only be able to monitor the current flows in and out of battery 1. The voltage readings would give you the overall state of the bank of both batteries but the current readings would only tell half the story.

My Dakota has the same setup and (I presume) that both the flying leads are coupled together at the control/fuse panel inside the van. For the shunt to monitor everything it would have to be fitted to the circuit at the point after they are coupled together. I can think of ways of doing this but, if it was me, I would contact Sargent's to ask their advice on the best way of doing things. When I fitted my second solar panel they were really helpful


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## cater_racer (May 1, 2005)

Phil's right, however any shunt is waste of battery power, it creates a measurement via a voltage drop across it, turning electricity into heat.

Much better to monitor voltage only if that's possible.


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## listerdiesel (Aug 3, 2012)

cater_racer said:


> Phil's right, however any shunt is waste of battery power, it creates a measurement via a voltage drop across it, turning electricity into heat.
> 
> Much better to monitor voltage only if that's possible.


It is not a waste of power, the loss of voltage is something like 60mV or 75mV, depending on the calibration of the system, and you'd probably lose more than that in the wiring.

You cannot measure current flows without looking at volt drop across either a shunt or a similarly set up piece of cable. A direct-reading ammeter would probably lose even more voltage, that's why shunts are used.

Does the BM1 look at battery current in AND out?

Peter


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## cater_racer (May 1, 2005)

Well yes it's minor and you're right about the wiring. But to be pedantic you can measure current without a shunt, with the wire passing through a ferrite core you can measure the deformity in the hysteresis via the electrical displacement..................

Sorry but you need a physics masters to have got that far (mine's from UCW)

I'll get my coat.


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## listerdiesel (Aug 3, 2012)

You can use many other methods, but cheapest and simplest is a shunt. I believe our DC clamp meter has a Hall-Effect sensor in it.

Peter


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## philoaks (Sep 2, 2008)

listerdiesel said:


> Does the BM1 look at battery current in AND out?
> 
> Peter


Yes. The display reading alternates between "Charge" and "Discharge" dependant on which way it's flowing.

Phil (O level metalwork :lol: )


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## listerdiesel (Aug 3, 2012)

OK, then the shunt has to be on the main terminal of the battery and nothing else, that way it will monitor current flows both ways and keep track of the state of the battery.

We had something similar on some robotic fork lifts at a big plastics factory in Wrexham, but they needed resetting by the controlling computer as errors crept in over time.

Peter


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## DABurleigh (May 9, 2005)

Both battery negatives need to be connected with a thick cable, and nothing else to those negatives other than one end of the shunt.

If you have high current items then do make the cabling thick between the negatives and to/from the chassis and load. I like welding cable.

Dave


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## Techno100 (May 8, 2010)




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## MrGaz (Nov 8, 2009)

DABurleigh said:


> Both battery negatives need to be connected with a thick cable, and nothing else to those negatives other than one end of the shunt.
> 
> If you have high current items then do make the cabling thick between the negatives and to/from the chassis and load. I like welding cable.
> 
> Dave


Now I am really confused...there are no THICK cables in the current wiring setup and the 2 x Sargent fly-leads (see picture) are just small gauge
'Andy' setup looks really neat and purposeful...you don't fancy sorting ours out (We are Newcastle up north)  I think I will just leave well alone...Merry Xmas everyone


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## Techno100 (May 8, 2010)

Using large cable between batteries effectively makes them one battery and a fuse is not required unlike with the small cable in your picture.
Mine are linked with large cable as I have a battery to battery charger installed too which may deliver over 50 amps.

My rear batteries are also linked with 35mm welding cable as they are used to supply very large current to the 1800 watt inverter.


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## MrGaz (Nov 8, 2009)

Techno100 said:


> Using large cable between batteries effectively makes them one battery and a fuse is not required unlike with the small cable in your picture.
> Mine are linked with large cable as I have a battery to battery charger installed too which may deliver over 50 amps.


Andy that looks really well put together you obviously know what you are doing...unlike me
I have read some posts on the B2B charger and thought it was a good idea but obvious not with my current setup...
I think I will have to pay the professionals


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## philoaks (Sep 2, 2008)

DABurleigh said:


> Both battery negatives need to be connected with a thick cable, and nothing else to those negatives other than one end of the shunt.
> 
> If you have high current items then do make the cabling thick between the negatives and to/from the chassis and load. I like welding cable.
> 
> Dave


The only problem with the Autotrail setup is that there isn't a single cable going from the battery bank to the control panel. Each battery has its own (relatively lightweight) cable that goes back to the control panel (positive and neutral wires - no connection to the chassis).

If the batteries were strapped at the negatives then the lightweight cables would effectively be strapped too. This would mean that (potentially) one of those cables could fail and the full current being drawn from (or charging) both batteries would have to be carried by a single cable.

I'm guessing that these lightweight cables must be connected to the same point on the control panel so (IMO) this would be the place to fit the shunt. That way each cable would only have to carry the current from one battery so overloading wouldn't be an issue.


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## DABurleigh (May 9, 2005)

Thin cables will work if you only have minor loads or moderate loads not sensitive to voltage. 

But frankly, too many converters scrimp on internal 12V load wiring. 

Dave


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## vicdicdoc (May 14, 2005)

cater_racer said:


> Phil's right, however any shunt is waste of battery power, it creates a measurement via a voltage drop across it, turning electricity into heat.
> Much better to monitor voltage only if that's possible.


Well, i can tell you that my NASA BM1 battery monitor shunt doesNOT give out enough heat to cook toast-infact I've never felt any heat come off it !

(PHD), F.I.I.K.W.I.T.A.M.O.T.T


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## MrGaz (Nov 8, 2009)

philoaks said:


> I'm guessing that these lightweight cables must be connected to the same point on the control panel so (IMO) this would be the place to fit the shunt. That way each cable would only have to carry the current from one battery so overloading wouldn't be an issue.


The 2 x Sargent fly-leads go back to a EM50 Interface behind the drivers seat (see attached) terminating see Page 2 on the top of the Black and White connectors (2 x N/U & 2 x W/O) 
The spade terminal are shorted out which is where I think the parallel connection is made to mak0e the 2 batteries appear a one big one (see green dotted lines on my original post)


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## philoaks (Sep 2, 2008)

MrGaz said:


> The 2 x Sargent fly-leads go back to a EM50 Interface behind the drivers seat (see attached) terminating see Page 2 on the top of the Black and White connectors (2 x N/U & 2 x W/O)
> The spade terminal are shorted out which is where I think the parallel connection is made to make the 2 batteries appear a one big one


That being the case, I would say that if you disconnect the 2 neutral cables coming from the leisure batteries and connect them to one side of the shunt and then connect the other side of the shunt to where the neutrals have just come from on the EM50 then that should cover it. A few suitable bits of cable and some male & female spade connectors and you'll be away  Don't forget to disconnect at the batteries before you start!


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## MrGaz (Nov 8, 2009)

philoaks said:


> That being the case, I would say that if you disconnect the 2 neutral cables coming from the leisure batteries and connect them to one side of the shunt and then connect the other side of the shunt to where the neutrals have just come from on the EM50 then that should cover it. A few suitable bits of cable and some male & female spade connectors and you'll be away  Don't forget to disconnect at the batteries before you start!


Okay so we have established that the 2 x fly-leads going to each battery are light weights (Drawing 1)
So how about doubling them up on Battery 1 and then strapping them to Battery 2 as per (Drawing 2)
Then connecting the BM1 shunt as (Drawing 3)...whats your thoughts guys :? [I think this is what Phil is saying]


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