Battery monitor counting backwards!

sailinglegend420 said:
I thought you'd be the first one along to question my post. After 40 years in the business I still don't understand how you could call yourself a marine electrical engineer, or why the moderators keep allowing you to post here. I really am sorry for this continual criticism of your posts but from what I can see on the threads I've posted on almost every time you open your mouth you put your foot in it - big time!
Click to expand...

Still didn't answer the question, just more of you rudeness.
 
PaulRainbow said:
How do diodes join the two banks together ?
Click to expand...
Sorry my error - thanks for pointing that out. The VSR's and diodes will charge both banks at the same time, but if the the common starter battery negative that usually goes to the service battery negative is not rewired to go to the load side of the shunt then there is no charging current going into the service battery through the shunt that will show on the monitor.

This was very clearly explained in my post and it is shown in the nasamarine link from Graham376.
 
sailinglegend420 said:
Sorry my error - thanks for pointing that out. The VSR's and diodes will charge both banks at the same time, but if the the common starter battery negative that usually goes to the service battery negative is not rewired to go to the load side of the shunt then there is no charging current going into the service battery through the shunt that will show on the monitor.
Click to expand...
You said "So the starter battery common negative - that connects the starter negative to the house bank negatives - must also be connected to the bus bar. "

The starter negative is connected to the starter battery negative. The starter battery negative is then connected to the shunt.

Your amended wording above reads correctly, your original post did not.
 
sailinglegend420 said:
Sorry pvb you need to read my post through again!
Click to expand...

Really?....

sailinglegend420 said:
Sorry my error - thanks for pointing that out. The VSR's and diodes will charge both banks at the same time, but if the the common starter battery negative that usually goes to the service battery negative is not rewired to go to the load side of the shunt then there is no charging current going into the service battery through the shunt that will show on the monitor.
Click to expand...
 
sailinglegend420 said:
I'm sure you followed the MPPT instructions to the letter and installed the new equipment “directly to the battery”, but not via the shunt bus bar.

Often extra equipment is added at a later date and the installer may well follow the installations instruction. Make sure you tell an electrician fitting new kit that you have a Battery Monitor installed.

The installation manuals are usually not very clear, especially the BEP. The shunt must be as close to the battery as possibly – for most brands on the negative post – and a new bus bar must be installed immediately after the shunt to take all the negative cables, and any new one that might be fitted later. The instructions are usually that all battery negative cables MUST go via this bus bar. This is in itself confusing as a stand alone house bank will always have a cable on each negative and positive stud to tie several batteries into one big capacity bank. With multiple batteries in parallel, and not all in the same location, it is often difficult to know what wires are going where. If not all of the devices drawing current or charging the service bank are going through the shunt then the Ah count will be wrong.

With a battery monitor installed that has the shunt in the negative cable the bus bar becomes the new negative post for the service bank. So the starter battery common negative - that connects the starter negative to the house bank negatives - must also be connected to the bus bar. This error is probably one of the major causes of battery monitors not reading the alternator charging amps going through the service battery.

This will happen when split charging VSRs or diodes join the banks together. This supplies positive volts to the starter bank and the service bank. Now try and follow the return path of the electrons to the alternator from the service battery. This will be via the service battery negative stud and then, via common earth, to the starter battery negative stud, which is usually wired to the engine block and hence to the negative of the alternator. This will bypass the shunt and not show any current into the service battery when the service battery is charging.

Moving the starter battery common negative to the other side of the shunt always confuses people because they think the shunt will then also be monitoring the charge current going into the starter battery!!!!
....
...l.
Click to expand...
Look at the NASA scheme.
It's quite simple.
The shunt goes between the -ve of the domestic battery and the -ve bus bar, which is wired as directly as possible to every other negatve. The shunt is the only connection (other than a sense wire) to the domestic battery's -ve stud. Therefore the shunt sees everything going in or out of the domestic battery, nothing more, nothing less.

The only problem with this is that the shunt will carry starter current if you flatten the starter battery and start from the domestic via an 'emergency' switch. The shunt may not be rated for that current. In reality I expect the shunt will survive as the starter battery will not be totally flat, but this kind of thing wouldn't survive a design review with some engineers I know....

If you want to avoid this (non?) problem, you have to make 'domestic earth' different from 'starter earth'. This will tend to result in the charge current of the engine battery register as charge current into the domestic battery, but in reality that error is smaller than the basic inaccuracies of battery monitors. And you need either an isolated starter or alternator and all other engine 'ground' connections isolated.

Or you could use a second emergency switch to short across the shunt when emergency starting.

FWIW, my car has a shunt in one of the battery leads, it carries the starter current and everything.
 
PaulRainbow said:
...Your amended wording above reads correctly, your original post did not.
Click to expand...
You haven't read both posts properly!

My original post said :

Now try and follow the return path of the electrons to the alternator from the service battery. This will be via the service battery negative stud and then, via common earth, to the starter battery negative stud, which is usually wired to the engine block and hence to the negative of the alternator. This will bypass the shunt and not show any current into the service battery when the service battery is charging.

My last post included the words 'not rewired to go to the load side of the shunt'.. instead of saying 'This will bypass the shunt' - I was just trying to make this much clearer. Some people may not understand 'the load side of the shunt'.

...but if the the common starter battery negative that usually goes to the service battery negative is not rewired to go to the load side of the shunt then there is no charging current going into the service battery through the shunt that will show on the monitor.

As I have said you have to draw this out to fully understand what is happening.
 
halcyon said:
Would hanging a Hall effect shunt on service battery positive cable be a simpler and quicker route, save all this keyboard bashing.

Just a thought
Click to expand...

Which Hall effect shunt would you recommend which will work with most mainstream battery monitors?
 
Wouldn't the first consideration when sizing a shunt be the maximum possible load? I know it was when I bought mine. So starting amps plus all loads plus some for mum. I went with 500amps continuous and 600 peek. One end onto the neg (some can be wired to pos) the other end becomes the Neg stud of the house bank and bob's you uncle. Not rocket surgery.
 
lw395 said:
The only problem with this is that the shunt will carry starter current if you flatten the starter battery and start from the domestic via an 'emergency' switch. The shunt may not be rated for that current. In reality I expect the shunt will survive as the starter battery will not be totally flat, but this kind of thing wouldn't survive a design review with some engineers I know.
Click to expand...

Whilst in theory, starter current should blow a 100 amp shunt, in practice it doesn't. I've started from domestics by mistake several times over the years and the shunt has survived OK with no distortion usually seen with overheating.
 
Graham376 said:
Whilst in theory, starter current should blow a 100 amp shunt, in practice it doesn't. I've started from domestics by mistake several times over the years and the shunt has survived OK with no distortion usually seen with overheating.
Click to expand...

How about installations with V8 900 HP engines Graham ? <insert suitable number of smilies>

There isn't a one size fits all answer. For the vast majority of cases the NASA wiring is correct, even with other makes of monitor. For some installations the NASA monitor isn't suitable, although, with some non-standard connections a compromise could be acceptable. Having the odd load or charging current bypassing the shunt might mean the SOC is slightly adrift (something the BM1 isn't great at anyway) or it might mean that at times (for instance bypassing for engine starting or windlass/bow thruster use) the current reading is briefly wrong, but most of the time things are acceptable and costs are kept at a sensible level.

Some people might think that the BM1, with it's low current shunt (with some bypassed loads) would be better than the Victron with its 500a shunt, because the display is big and they can read it easily. If the shortcomings of the errors caused by the bypasses are understood, is there really a problem ? Don't think the boat will explode because of them :)
 
pvb said:
Which Hall effect shunt would you recommend which will work with most mainstream battery monitors?
Click to expand...

Not sure if anybody makes one for battery monitors, I supplied Sealine with a Hall effect shunt to drive the analogue ammeter 75-0-75 mV 30 amp or 60 amp range, see http://www.kddpowercentre.co.uk/spares_d/ST01.01.pdf so it is doable. Mentioned here tongue in cheek with all the posts re fitting a shunt in neg feed.

Never tried it with a battery monitor, I have a BM1 to play with, so may set one to match and try it out of curiosity.

Brian
 
PaulRainbow said:
How about installations with V8 900 HP engines Graham ? <insert suitable number of smilies>
Click to expand...

Mines only 1800cc but compression is what matters and remember, an 8 cylinder engine has twice as many firing strokes for the same cranking time as 4 cylinder so more likely to fire quickly.:)

Anyway, with a large engine, it would be sensible and easy enough to fit a starting relay to bypass the shunt if connected to domestics.
 
You must log in or register to reply here.

Share this page

Top