Choosing a Battery Monitor

[tgalea]

Hi All,

I decided to go for a battery monitor and have been doing some research but would like your opinion on the following.

I have one starting battery and two 160AH batteries forming the domestic bank. The domestic batteries are paralleled, BUT. For some reason the negative cable coming OUT of the domestic bank is daisy chained to the starter battery negative.

I am only interested in monitoring by domestic bank and like the looks of the NASA BM2 the new 200amp shut version.

Considering my setup, which battery monitor do you recommend.

On a technical note. Since my negatives are chained:

1) Is the shunt connected to the negative output of the domestics before joining the starter negative or should it be at the very end of the chain, i.e, betweent the starter negative and the rest of the negative circuits ?

If the shunt is to go at the very end of the chain, I am correct in assuming that this shunt will need to handle the starting currect of the engine starter motor and therefore 200AMP shunt could be a problem ?

Thanks in advance.

Tyrone

 

[aluijten]

Shunt should only be between the domestic bank negative and "the rest". So none of the starting current should be allowed to pass the shunt.
The Monitor is meant to monitor the Domestic batteries only. Some models can also measure (not monitor) the voltage of the starting battery.

 

[Malish]

I just fitted a Merlin smart gauge, it does not need a shunt. On a previous boat I fitted a BEP monitor that required a shunt but as I recall a common -ve was not an issue on that setup - ? Talk to NASA

 

[Tranona]

Just fitted a BM1. The shunt goes in the -vefrom the house to the busbar on the domestic panel. Instructions are very easy to follow.

 

[tgalea]

Thanks for the info so far.

Still in a dilemma however as to the wiring since my negatives are joined with the starter battery negative.

Anyone has a similar setup and possibly has a solution for me ?

Regards
TG

 

[VicS]

If you use a BM1 wire exactly as stated in the instructions. Reconfigure your negative connections as necessary t allow you to do this .

Diagram below might help ( although it does not show the orange wire connected to the starter battery now added to modern ones)

Where it says no other connections it means just that!

 

[Vara]

I fitted the NASA BM, about six months ago. It works extremely well once it is set up, easy to install, (well I did it).
Does give you an insight into your consumption and battery state.
My negative is common and it doesn't cause any problems.

 

[BarryH]

I looked at the NASA thing and thought it an overcomplicated, overly expensive box of electronics for a simple task. I got one of these instead. Its been excellent doing what it says on the tin.

 

[Richard10002]

In the diagram the starter battery negative is connected to the negative bus bar, which is connected to the shunt.

Surely this is the equivalent of the negative of the starter being connected to the shunt?

 

[Playtime]

As per VicS's diagram, connect the shunt directly to the negative terminal of the domestic bank and connect the other side of the shunt to the starter battery negative (which, presumably, is then connected to the negative bus bar, engine block etc. on your boat). Do not connect ANYTHING else to the domestic battery negative before the shunt otherwise it will not be monitored.

The BM1/2 is good value. The most useful function is cumulative amp hours charge/discharge. This can be reset to zero when the charging stops (i.e. engine or mains charger off) and the actual amp hours consumed monitored thereafter. The % capacity reading can be a bit suspect so treat that with caution.

 

[Vara]

I looked at the NASA thing and thought it an overcomplicated, overly expensive box of electronics for a simple task. I got one of these instead. Its been excellent doing what it says on the tin.

That's a voltmeter, i've got one of those on my switch panel, the NASA BM gives you much more information.

 

[maby]

In the diagram the starter battery negative is connected to the negative bus bar, which is connected to the shunt.

Surely this is the equivalent of the negative of the starter being connected to the shunt?

Not really since the bus bar is implicitly the boat earth. My concern with this layout would be that it implies that the negative return for the starter motor goes through that negative bus bar and that's a whole lot of current for the average bus bar. In the absence of obscure wiring faults, there is no reason for any current between the two battery negatives other than a possible small leakage current going through the anodes (depending on how they and any skin fittings etc are wired). I think I would prefer to leave the engine battery wired as it was before the monitor was fitted.

 

[tgalea]

Thanks for all the info guys. So i will go ahead and install as you suggest.

One last question.. so the 200amp shunt will be ok for me installed between the domestic and starter battery and then to a common negative bus bar ?

Thanks once again

 

[GrahamM376]

I looked at the NASA thing and thought it an overcomplicated, overly expensive box of electronics for a simple task. I got one of these instead. Its been excellent doing what it says on the tin.

I had one of those years ago, pretty useless piece of kit. It packed up after quite a short time, I suspect it's designed to monitor 12V and didn't like what it saw as overvoltage when engine running.

BMs do an entirely different job and reasonably priced. They tell you voltage and current in/out to one decimal point, as well as hours to charge or discharge at present rate. Also gives a fairly accurate indication of battery capacity left.

The OP shouldn't have any worries about the negs being daisy chained, just put the shunt between the neg cable and domestic bank neg.

 

[maby]

Thanks for all the info guys. So i will go ahead and install as you suggest.

One last question.. so the 200amp shunt will be ok for me installed between the domestic and starter battery and then to a common negative bus bar ?

Thanks once again

Not quite... Forget the starter battery - however it is currently wired, leave it the same.

Take the negative cable from the domestic battery and follow it along to the other end - it will be a thick multistrand cable that probably terminates in a ring connector that is bolted down to some kind of "bus bar" already, though it may not be as formal as the things you can buy in the chandlery. You need to disconnect that wire from where ever it is currently conected and attach it to one side of the shunt. You then need to link the other side of the shunt to the place where the negative from the domestic battery was previously connected. this new link cable needs to be decently thick since it will be carrying the full current drawn by the domestic supply - but a lot less than the hundreds of amps that the starter motor will draw from the engine battery. Finally, you connect the ground, power and sense wired from the monitor to the shunt as shown in the installation instructions.

The bus bar that the previous poster referred to was a bit of a red herring - he had added it to help tidy up the other wiring - and that is very laudible - but it is not an essential component of the battery monitor installation.

 

[Playtime]

Take the negative cable from the domestic battery and follow it along to the other end - it will be a thick multistrand cable that probably terminates in a ring connector that is bolted down to some kind of "bus bar" already, though it may not be as formal as the things you can buy in the chandlery. You need to disconnect that wire from where ever it is currently conected and attach it to one side of the shunt. You then need to link the other side of the shunt to the place where the negative from the domestic battery was previously connected. this new link cable needs to be decently thick since it will be carrying the full current drawn by the domestic supply - but a lot less than the hundreds of amps that the starter motor will draw from the engine battery.

Why bother tracing the negative cable etc? It will be easier (and clearer for any subsequent owner), to attach the shunt directly to the domestic battery negative terminal (maybe via a short length of heavy duty cable) and then attach the negative cable to the other side of the shunt.

Where the negative cable goes after the shunt is actually irrelevant as far as the monitor is concerned. However, if I understand the OP correctly, in his case it actually goes to the engine battery negative ("daisy chained") and not to any bus bar.

 

[Vara]

Why bother tracing the negative cable etc? It will be easier (and clearer for any subsequent owner), to attach the shunt directly to the domestic battery negative terminal (maybe via a short length of heavy duty cable) and then attach the negative cable to the other side of the shunt.

Where the negative cable goes after the shunt is actually irrelevant as far as the monitor is concerned. However, if I understand the OP correctly, in his case it actually goes to the engine battery negative ("daisy chained") and not to any bus bar.

Yep that's what I did works fine.

 

[maby]

Why bother tracing the negative cable etc? It will be easier (and clearer for any subsequent owner), to attach the shunt directly to the domestic battery negative terminal (maybe via a short length of heavy duty cable) and then attach the negative cable to the other side of the shunt.

Where the negative cable goes after the shunt is actually irrelevant as far as the monitor is concerned. However, if I understand the OP correctly, in his case it actually goes to the engine battery negative ("daisy chained") and not to any bus bar.

That will, of course, work, but it involves cutting the existing cable to attach a terminal that will connect to the shunt. I was just trying to suggest a solution that minimised the disruption to the existing wiring, not knowing the OP's level of competency in making up new cable connections.

 

[Vara]

That will, of course, work, but it involves cutting the existing cable to attach a terminal that will connect to the shunt. I was just trying to suggest a solution that minimised the disruption to the existing wiring, not knowing the OP's level of competency in making up new cable connections.

I got my extra cable made up for very little cost £9 IIRC. Local auto sparky.

 

[Playtime]

That will, of course, work, but it involves cutting the existing cable to attach a terminal that will connect to the shunt.

Not necessarily - it depends how the cable is currently rolleyes connected. On Playtime, for instance, the cable was already terminated at the battery with a ring connector, ideal for connecting direct to the shunt.

I used a short (6") heavy duty cable with rings either end to connect the shunt (screwed to the locker front) to the battery negative.