Where do I put my Shunt?

[CGirvan]

I have a quick question. I'm installing a battery monitor that has a shunt that should be installed in the negative line next to the battery (Pos A on diagram).


The way my boat is wired is that the battery bank is situated mid-ships at the base of the mast. There is a positive cable that feeds the main switch panel at the nav station, but the negative cable is connected to the engine.


My question is, if I install the shunt at position A would it measure the amps in and out of the main switch panel?


Would I be better off installing it at position B in the positive cable?


Thanks for your help.

[witzgall]

You want it connected so that the ONLY negative wire connected to your battery bank is going to the shunt. That way you are ensured of having all current flow through the shunt.

Here is what I tell people who ask me about installing our SG200. "Take the negative-most battery cable off of the battery bank. Connect it to the stud on the SmartShunt labeled Cable. Then put a new battery cable from the SmartShunt stud marked Batt back to the battery."

Chris

[CGirvan]

I understand the proper way to connect the shunt which would be as I stated position A on diagram.


My question is whether the current would flow through the engine and be measured at the shunt?

[Sailmonkey]

Originally Posted by CGirvan

My question is whether the current would flow through the engine and be measured at the shunt?

of course the current will flow through the engine. The battery is the source of the current, not the engine.

When the engine is running, the battery is still where all of the electricity you're concerned with is going and coming from.

[witzgall]

Sorry if I misunderstood the question.

If the engine is a part of the negative path, then yes, but I would want to run a wire separately from the panel to a bus bar near the batteries. I have seen too many poor connections on engines to consider this a reliable pathway. Others may differ.

Chris

[CGirvan]

OK, that makes sense, thanks for your help.


I'm trying to avoid running extra cables because of the expense and the lack of room in a conduit that it would have to run through.


What I was thinking of doing was disconnecting both negative connections to the engine, connecting these together off the engine along with a third short negative cable back to the engine.


Alternatively I could move one of the engine connections to connect to the same bolt as the second connection thereby providing a direct path back to the battery without having to travel through the engine itself.

[captstu]

I’m not sure the answer is exactly correct. If the shunt is installed between the engine and the negative terminal of the battery, any current generated by the alternator may go directly to the load without going through the shunt..

The meter will correctly show the current going to and from the battery, however if the total current is desired the shunt will not show that.

[guyrj33]

position A to measure current to/from battery.

[Zil]

My alternator only connects positive to the battery. The negative is through the engine to the battery. If you are taking any positives from the alternator, with separate negatives as well, you don't indicate that on the diagram. So my system like yours, with the alternator to the battery positive and my shunt at 'A' measures all the energy for all my shtuff.

[CGirvan]

Originally Posted by captstu

I’m not sure the answer is exactly correct. If the shunt is installed between the engine and the negative terminal of the battery, any current generated by the alternator may go directly to the load without going through the shunt..

The meter will correctly show the current going to and from the battery, however if the total current is desired the shunt will not show that.

I thought all alternators were grounded through the mounting bolts, I've never seen one with a negative cable to the battery.


From all your comments I'm understanding that the engine acts as a negative cable and transfers the current in my case from the negative cable from the main switch panel to the negative cable going to the batteries.

I think I would be better off having those 2 cables connected to the same bolt.
Then I'll place the shunt in position A next to the battery and see what happens.


Thanks for all your help.

[Stu Jackson]

Originally Posted by CGirvan

I thought all alternators were grounded through the mounting bolts, I've never seen one with a negative cable to the battery.>>>>>>>>>>>>>>>>>>>>>>>>>>

Simply not true. Many OEM alternators were grounded to the engine block. Many of us have added separate negatives to their alternators.

[Cat36Mahalo]

Isolated alternators ( no connection between case and zero volts (ground)) are very common on steel and Aluminum boats. Folks who like belts and suspenders like Stu add an additional ground to improve the ground path.

[OS2Dude]

I've read many items that say the battery negative should go to a bus bar where all the other major grounds connect (including the engine.) It is recommended that the engine block NOT be a part of the negative circuit.


A side question: Why a shunt in the negative side? Does it matter? On our vessel the shunt is in the positive side. I guess if you want the alternator output to be apart of the meter the negative side would be easier. If that is the case, wouldn't having the shunt between the alternator and loads be the correct position? (Given the engine block is a part of the negative circuit.)

[Empty Pockets]

Originally Posted by CGirvan

What I was thinking of doing was disconnecting both negative connections to the engine, connecting these together off the engine along with a third short negative cable back to the engine.


Alternatively I could move one of the engine connections to connect to the same bolt as the second connection thereby providing a direct path back to the battery without having to travel through the engine itself.

^^^ This. And position A for your shunt.

[thunderhoof]

I seems to me that the reason for the shunt placement in the negative lead is for two reasons. The main one is that if a lead to your amp-meter is shorted to ground then there will be only a little extraneous current and no shock hazard if it is in the negative lead. The other is installation dependent - there may be ac currents injected onto your power bus from inverters, transmitters, etc., that might be radiated by the leads from the shunt. In any event the leads from the shunt to meter should be twisted together.