Wiring up a shunt

[boatmike]

In adding a digital ammeter to my boat I was surprised to see that the instructions asked that I wire it to a shunt on the battery negative terminal. In fact, the instructions suggest that it would be damaged if the polarity was reversed. I have always expected that shunts were on the positive side but I guess there is no reason not to put it on the negative side as it will still register current flow. Has anyone else seen an ammeter shunt wired like this though? Being digital and having seperate positive and negative terminals to power the thing I guess that I should wire it as instructed, but is it just me or is this unusual?

 

[pvb]

Most battery monitor shunts are wired in the negative lead. It's generally easier because there are often various separate positive leads connected to a battery, whereas there's often only one negative.

 

[BERT T]

SHUNT

Here is an example:

http://www.keepandshare.com/userpics/k/e/v/i/n2009/2012-04/sm/5135_va-23505450.jpg

 

[VicS]

Yes wire it the way the instructions say.

As pvb says many, if not most, battery monitors have their shunts fitted in the negative battery connection.

What ammeter are you fitting ?
It's impossible to make any further comment or offer an explanations without knowing more about it

 

[boatmike]

Yes wire it the way the instructions say.

As pvb says many, if not most, battery monitors have their shunts fitted in the negative battery connection.

What ammeter are you fitting ?
It's impossible to make any further comment or offer an explanations without knowing more about it

No need really Vic. I think you and other posters have answered my question in that it's not unusual. Just that all those I have seen before have been on positive. I guess the only difference is that with 2 battery banks it will read both banks on the negative (common)side rather than just domestics as I intended, so will record starter current too, but as its range is + or - 150amps and its a 500amp shunt that's OK.

 

[pvb]

I guess the only difference is that with 2 battery banks it will read both banks on the negative (common)side rather than just domestics as I intended, so will record starter current too, but as its range is + or - 150amps and its a 500amp shunt that's OK.

If you mount it in the negative next to the domestic bank, it will only show the current in that bank.

 

[VicS]

If you mount it in the negative next to the domestic bank, it will only show the current in that bank.

And here is the diagram to go with those instructions

 

[boatmike]

I see that Vic, but regardless of what I said before,there might indeed be an advantage in mounting directly on the starter battery negative post as you have drawn it as the reading will then show the total amps charge or discharge to all banks which will tell me the alternator output total or indeed the charge rate from battery charger total when on shore supply through the combi charger/inverter or indeed when the genny is running and charging through the combi ( I think ) All negative connections would then be to the other end of the shunt.

I don't see a problem with this as its a 500amp shunt, do you ?

p.s. The battery banks are charged through a "smartcharge" giving me a readout of voltage on each bank and controlling a relay between them, so I already have an indication of the state of charge of each battery so perhaps the total charge/discharge amps might be a more useful added bit of information rather than the ammeter just showing one bank.... What do you think?

 

[pvb]

...there might indeed be an advantage in mounting directly on the starter battery negative post as you have drawn it

If you do that, you'll only see the current going in to or out of the starter battery, not the domestic battery.

 

[VicS]

The original diagram is to measure the current to and from the domestic bank only.

Amended diagram to show shunt positions for measuring starter battery current only and for measuring the total current ( except that the cranking current will probably exceed the range of the meter)

 

[halcyon]

p.s. The battery banks are charged through a "smartcharge" giving me a readout of voltage on each bank and controlling a relay between them, so I already have an indication of the state of charge of each battery so perhaps the total charge/discharge amps might be a more useful added bit of information rather than the ammeter just showing one bank.... What do you think?

The only reading the OP needs is net amps service battery, with max alternator voltage, and low amps that's the max recharge level.

Which means measuring the service battery either the positive terminal post, or providing a separate neg to the service battery so a resistive shunt can be fitted.

Brian

 

[Maine Sail]

And here is the diagram to go with those instructions

Amen!! So easy yet sooooo many mess it up....

 

[nigelmercier]

The original diagram is to measure the current to and from the domestic bank only...

I wish I'd had that diagram when I first bought my boat, in particular the note not to connect anything else directly to the battery negative (apart from the shunt sensor wires of course). Whoever wired the boat up didn't know this, and I was nervous about changing it. It took months to figure out why my ammeter gave odd readings.

 

[VicS]

I wish I'd had that diagram when I first bought my boat, in particular the note not to connect anything else directly to the battery negative (apart from the shunt sensor wires of course). Whoever wired the boat up didn't know this, and I was nervous about changing it. It took months to figure out why my ammeter gave odd readings.

You only had to ask

 

[William_H]

Amp meter shunt

Amp meters come in many styles. For a simple moving coil amp meter it will not matter if the shunt is in the negative or positive wire. Not withstanding all the discussion on which battery you are metering. The only concern for shunt in +ve is that the meter wires and meter are all at +ve potential and any short to negative can wreck the meter or cause a fire. So fit fuses in the shut wires.
If you are using a digital panel meter of the simplest style with its own 9v battery and power switch then again it can be put in +ve wire and all the meter is floating at 12v positive but this doesn't matter.

However if you go to a meter with its power supply coming from the ships battery then the metering wires will be near negative if in the negative wire but near 12v + ve when in the positive line. This can cause troubles in the meter. (let the smoke out)
So IMHO the maker has simply said put it in the negative line so no concern whether it is important or not.
Good luck olewill

 

[cimo]

............... the instructions suggest that it would be damaged if the polarity was reversed. .........................

That's correct. The ammeter will display whatever current flows through the shunt. It's entirely up to you where you want to install it.

What the instructions are getting at: should you place the shunt in the negative cable, the shunts' positive is that end furthest away from the battery negative terminal. Alternatively, if you decide to place the shunt in the positive cable, the shunts' positive is that end nearest the battery positive terminal.

You need to wire the shunts' +ve & -ve to the meters' +ve & -ve accordingly.

 

[boatmike]

If you do that, you'll only see the current going in to or out of the starter battery, not the domestic battery.

No that's mistaken I believe. Vic's diagram is absolutely correct in showing the alternative positions and I am chosing to meter all domestic and engine battery current as he has shown in diagramatic form (thanks Vic) In practice I have all battery posts linked on negative and the link wire shown on Vic's diagram as going to "all negative connections" will be removed from the battery post and the shunt bolted directly to the post with this wire then connected to it doing precisely what Vic suggested. I agree that all his other alternatives work too, it's just a choice regarding what I want the meter to show.

 

[pvb]

No that's mistaken I believe. Vic's diagram is absolutely correct in showing the alternative positions and I am chosing to meter all domestic and engine battery current as he has shown in diagramatic form (thanks Vic)

It isn't mistaken at all.

You said "there might indeed be an advantage in mounting directly on the starter battery negative post", and I said that "If you do that, you'll only see the current going in to or out of the starter battery, not the domestic battery."

If you look at Vic's diagram in post #10, you'll see that with the shunt directly connected to the starter battery negative (as you suggested) Vic has written "To measure starter battery current only", which is what I said would happen.

 

[nigelmercier]

You only had to ask

Yes, but I didn't know I needed to at the time. I also found it odd that the shunt was in the negative connection, as this seemed counter intuitive. Since then I have realised that the battery and the shunt should be considered as a "black box" with the far end of the shunt being the battery negative for all intents and purposes.

 

[boatmike]

No that's mistaken I believe. Vic's diagram is absolutely correct in showing the alternative positions and I am chosing to meter all domestic and engine battery current as he has shown in diagramatic form (thanks Vic) In practice I have all battery posts linked on negative and the link wire shown on Vic's diagram as going to "all negative connections" will be removed from the battery post and the shunt bolted directly to the post with this wire then connected to it doing precisely what Vic suggested. I agree that all his other alternatives work too, it's just a choice regarding what I want the meter to show.

There is no need to take umbridge. The "mistake" was in assuming I was wired strictly as Vics diagram. I am not. you would of course be correct if I had.