NASA BM1 Used on a single battery.

J

jakeroyd

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I am considering fitting a BM1 to my boat.
The boat has a single battery which of course is used for both starting and domestic purposes.
The main domestic draw is a Waeco CF18 portable fridge.
I have looked at the NASA site and studied the manual which states it can not monitor the starting battery.
However , it seems to say it is not required rather than saying the unit technically can not monitor a starting battery.
I will contact NASA and see what they say but would like to ask if any formumites have fitted a BM1 to a singel battery system and if so did you get good data from it ?

Thanks in Advance
 
I suspect that the problem is the 100amp maximum but, as you say, the manual is a little ambiguous.
Do let us know what they say.
 
I have a BCM 600 battery monitor fitted and asked Merlin the specific question if it ok to start the engine using the domestic battery (ie through the shunt). The response was that the shunt supplied has a 500amp rating (compared to the NASA 100amp rating) and therefore it will cause the shunt no problems at all.
 
Yes as others have said the limiting factor is the 100 amp maximum rating, probably the limit of the shunt rather than the monitor itself. It therefore depends upon your engine and starter motor.

Normal cranking current may well be lower than 100amps but if stalled the motor will draw several hunderd amps possibly.
 
I suspect the problem may be that the capacity of the battery, as measured in ampere-hours, only applies where the current drawn is relatively small. Thus a 100 Ah battery will supply 2A for 50 hours, 5A for 20 hours, or even 10A for 10 hours. However if you try to draw 100A it will will not last for 1 hours. The BM1 may not be able to cope with this.

John
 
A follow on thought is that if you did fit the NASA unit and your engine/starter combination drew well in excess of 100amp, then "blowing" the shunt would be exactly as blowing a fuse in the starter circuit, just at a time when you may need the engine most. Just a thought.
 
I doubt shunts 'blow' like fuses, mor likely they overheat & lose calibration. The simple answer is to use the BM1 to monitor the house battery after you have fitted it. Would only run a single battery if I either had a reliable hand start or had NO electrice except nav instruments and VHF radio (and that includes nav lights) plus I was happy not relying on the engine.
 
I recently asked NASA this question.

My email:

I am looking at the BM-1 battery monitor. I have a single battery
bank for domestics and engine start. Starting the 10HP engine will
pull current in excess of the 100A the shunt is designed to measure.
Will this cause problems ? What is the max continuous current rating
for the shunt ?

NASA replied:

100 Amps continuous load, it will withstand higher loads for short duration. However it would be best to bypass the shunt when starting the engine, after a few minutes of load on the battery the BM1 will 'learn' the new battery state after engine start.

NASA Marine

I bought the BM1 but have not fitted it yet. Taking the neg to the starter motor before the shunt will actually make installation much easier. In fact, I will use the shunt stud on the battery side for this purpose. So although it may not be necessary, I will follow their advice.
 
[ QUOTE ]
after a few minutes of load on the battery the BM1 will 'learn' the new battery state after engine start.

[/ QUOTE ] If they say so. i thought I knew how these things worked in outline at least ... perhaps I dont.
 
I have two batteries but only one BM. The BM is attached to the 80 amp hour 'house' battery only. The 75 Ah starter battery is Battery 2 on our switch.

However, if I choose to start the engine on the No 1 battery (ie the 80Ah one, on the BM) the BM copes fine.

Engine is a Yanmar 1GM 10.

Now going to post about a 'possibly fried' alternator - completely separate issue!
 
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