Choosing a Battery Monitor

Hi guys,

Thank you all for your invaluable help.

I'm comfortable making up cables etc so that woudn't be a problem.

The negative cable from the domestic bank goes to the negative terminal on the engine starting battery negative. Another cable leaves the engine battery negative and goes into the electrical system (either buss bars or main switch i think).

What i will do is disconnect the negative cable going out to the engine battery negative and attachs shunt battery side to negative of domestic directly and shunt load side to the cable i would have just disconnected.

My worry really is the size of the 200amp shunt which comes with nasa bm2. Since the negatives are joined I am afraid that during engine start some current may flow from the domestic negative into the starting battery through the shunt and overload my shunt.

Is this a realistic worry or am I going too far !?

Thanks once again
 
Current can only flow down the domestic battery negative connection during starting if there is a link from the domestic battery positive to the engine battery positive. I have seen boats with complex switching arrangements that made it relatively easy to cross connect the batteries - saw it on a Jeanneau Sun Odyssey about eight or nine years old quite recently. The rationale seems to be that, if your engine battery goes flat but you have a decent charge level in the domestic battery, you will want to be able to use it to start rather than have to call for help. This does make some sense and obviously could result in an overloaded shunt - I guess that you just have to be careful if you have that level of configurability!
 
tgalea said:
My worry really is the size of the 200amp shunt which comes with nasa bm2. Since the negatives are joined I am afraid that during engine start some current may flow from the domestic negative into the starting battery through the shunt and overload my shunt.

n
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That can only happen if the house battery is starting, or contributing to the stating of the engine. Normally it will be isolated, but if you join the house and start batteries ( usually because the start battery is flat) current will flow through the shunt.
The 200A should be a continual rating in which case the shunt will cope with the starting current of a medium to largish diesel even if the full starting current is going through the shunt.
 
tgalea said:
Hi guys,

Thank you all for your invaluable help.

I'm comfortable making up cables etc so that woudn't be a problem.

The negative cable from the domestic bank goes to the negative terminal on the engine starting battery negative. Another cable leaves the engine battery negative and goes into the electrical system (either buss bars or main switch i think).

What i will do is disconnect the negative cable going out to the engine battery negative and attachs shunt battery side to negative of domestic directly and shunt load side to the cable i would have just disconnected.

My worry really is the size of the 200amp shunt which comes with nasa bm2. Since the negatives are joined I am afraid that during engine start some current may flow from the domestic negative into the starting battery through the shunt and overload my shunt.

Is this a realistic worry or am I going too far !?

Thanks once again
Click to expand...


Yes what you are proposing is fine... If I understand correctly


There should be no current flow to/from the domestic battery negative during starting .. at least no part of the cranking current.

I think you will find that the 100 or 200 amp rating refers to the maximum current that the monitor will display and that in both cases the shunt will handle much higher currents (obviously there are limits but I dont know what they are) and the monitor will correctly interpret the higher currents even though they exceed the display lmit.

The question has arisen before where people have systems that allow the house battery to be used for engine starting. IIRC Nasa have advised that the starting current of small yachtengines is not a problem even for the BM1.

A call to Nasa should confirm ( or otherwise :o )

You are only likely to need the BM2 with a large boat with masses of 12 volt Dc equipment using loads of juice, or a high capacity battery with high charging currents of more than 100 amps otherwise the BM1 is adequate

http://www.nasamarine.com/proddetail.php?prod=BM2


.
 
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Your negatives SHOULD be connected

It is perfectly normal (correct) for both the starter circuit and the domestic circuit to be connected. Boat electrics by convention have negative ground. That means the they go to the block of the engine, metal bits like seacocks etc and to the sacrificial annode(s). If you disconnect the two negatives from each other then you may be leaving one without a ground, which could expose you to horrible galvanic problems. If both circuits are individually grounded then disconnecting them won't make a damn bit of difference, since they will anyway be connected though the boat's grounding. The starter current goes from the starter positive to the starter negative, and can't shove anything back up the domestic negative. You simply have to make sure that your shunt only goes in the domestic negative.

I have two BM1s (my engine is so small that I can put the starter current through a BM1 shunt) and two symmetrical batteries. I switch one on for both starter and domestic and reserve one for emergencies, alternating which is used for which purpose. The setup and the BM1s have served me perfectly. I must admit I don't much believe "hours left", as it's hard to know how an aging battery will perform. However volts and current tell me a lot. The batteries are four years old, and I have pessimistically set the capacity 30% below what it says on the label.
 
Okay so it looks like i got the hang of it.

I'm tossing between the NASA BM2 or the Victron BMV602S which has a larger shunt.

Any opinions ?

What can i say. You have all been of great help.

Thanks
Tyrone
 
tgalea said:
Okay so it looks like i got the hang of it.

I'm tossing between the NASA BM2 or the Victron BMV602S which has a larger shunt.

Any opinions ?

What can i say. You have all been of great help.

Thanks
Tyrone
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I'm surprised your electrical power consumption or battery charging system is large enough to need a BM2 let alone a 500 amp BMV602S

What ever do you do with all the electricity?
 
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well yes you caught me there! David read my thoughts! The boat is a bavaria 38 and has a couple of high draw items such as windlass and electric winches.
 
tgalea said:
well yes you caught me there! David read my thoughts! The boat is a bavaria 38 and has a couple of high draw items such as windlass and electric winches.
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I have a Bavaria 38 with windlass, but not electrical winches. Have 4 x 140 amp batteries (560 in total) and 60 amp starter battery and 2 x 100watt solar panels. Use a Nasa BM1, brilliant piece of kit, no problems with it.
 
tgalea said:
Okay so it looks like i got the hang of it.

I'm tossing between the NASA BM2 or the Victron BMV602S which has a larger shunt.

Any opinions ?
Thanks
Tyrone
Click to expand...

Have fitted both NASA BM1 & Victron for customers, but have just fitted a 602S to my own boat if that tells you anything, the 602S will also seperately monitor (at basic level) the starter battery so that's a bit of a bonus
 
David2452 said:
Have fitted both NASA BM1 & Victron for customers, but have just fitted a 602S to my own boat if that tells you anything, the 602S will also seperately monitor (at basic level) the starter battery so that's a bit of a bonus
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The Nasa Monitors also now monitor starter battery Volts except, IIRC, for the compact version of the BM1.
 
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I have had a BM1 on the boat for a couple of years and it has been a great bit of kit. You can tell at a glance the state of charge / discharge and hence set a charging regime to ensure the battery's last.
Unfortunately it has to go as my new Victron 12/3000/120 combi unit draws too much current for the BM1 shunt, so I have replaced it with a Victron BMV602S with a 500A shunt. The BM1 is now for sale!
 
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