Connecting the Smartshunt to both starter and auxiliary battery

[Franc]

Hi, I am about to add a Smarthunt 500A to my system.

I understand how I need to connect it to the auxiliary battery and loads, but I would also like to monitor the starter battery with the aux connection, but according to the manual, I would need to put the negative behind the shunt (and the shunt only, I would imagine). But where does this leave the connection from the alternator, that is now charging both batteries through a diode based battery splitter?
As it also seems like (although I need to investigate this further) that they have a combined ground (through the alternator or the hull of my ship?)

So where does the alternator fit in this picture

And would I need to isolate the charging circuit with a DC-DC loader of some kind?

[mac]

The smartshunt can only monitor SOC for one battery (or bank).

The aux input is used to monitor only voltage from a second battery. For this to work both negatives must be connected as is shown in your schematic.

[Theo]

In your case the starter battery and auxiliary battery probably are connected over ground/minus already. You could check this with a multimeter. Or, when you only connect the aux connection of the shunt to the auxilary battery’s positive, you should instantly see the auxiliary voltage in the app/VRM. If not, the batteries negative are not connected then.

[Franc]

So, If I understand correctly, seeing that the aux only measures voltage of the starter anyway (and not current - which is good enough for me), it is not really essential to move the negative side of the starter behind the shunt?

And knowing that the alternator charges both batteries with a simple battery isolator, they have to share a common ground (although I would like to change that when I switch to a LiFePO aux battery).

[mac]

@Franc

So, If I understand correctly, seeing that the aux only measures voltage of the starter anyway (and not current - which is good enough for me), it is not really essential to move the negative side of the starter behind the shunt?

Correct. In fact doing so will make current bypass the shunt so it does not work properly.

[Franc]

Alright, so I have been staring at the schematic and I realize 2 things… one of them makes me doubt mac’s answer (truly sorry, please correct me if I am wrong, I am not trying to be a smartass, just trying to understand):

1. It should not matter on what side of the Shunt I ground the starter battery as there is never a circuit with both batteries and Loads in it.
   There is A - a circuit my the aux battery, solar chargers, Loads and alternator which I want to monitor and
   B - a circuit with the starter of the engine and the alternator in it. The circuit with the alternator is essentially 2 circuits because of the isolator, so there will be no current going through the shunt from this circuit.
2. Chargers need to be behind the shunt in order for it to calculate and monitor, so I have to get the alternator negative behind the shunt also or it will never see the charging current

Am I right or am I wrong?

[mac]

@Franc

It should not matter on what side of the Shunt I ground the starter battery as there is never a circuit with both batteries and Loads in it.

If you 100% know what you’re doing and have a very good reason to, maybe, yes.

@Franc

Chargers need to be behind the shunt in order for it to calculate and monitor, so I have to get the alternator negative behind the shunt also or it will never see the charging current

The shunt is a real simple device, it measures the current flowing through it. If you want to monitor a battery SOC all current going in and out of the battery should go through the shunt. therefore the shunt should be connected as follows;

-on the battery side: only the battery, all other connections can influence the accuracy of the shunt.

-on the load side: everything else, including ground connections etc.

edit: not saying it can’t work any other way but in general best practice is to only connect the battery on the battery side.

[pwfarnell]

This FAQ answers how to connect the SmartShunt.

Shunt / battery monitor not reading the correct SOC/ Amps, missing solar, decreasing daily or other misreading problems

This topic covers the SOC / Amps reported by BMV battery monitors, SmartShunts, Lynx BMSs and Lynx Shunts and will just be called shunt below. It is also assumed that the shunt is monitoring the domestic / house / cabin battery bank and not engine start / bow thruster / winch / caravan mover battery banks. The problems may be one or more of the following The SOC decreases daily despite the solar charger or other charger(s) working. The SOC increasing when discharging and falling when charging…

[Franc]

Thanks veryone for the clarification.

One (final?) uncertainty is the circuit of my bow thruster. I am not entirely sure about the current it is drawing (it looks like there is a 280A fuse but hard to read), so would it be wise to put that current through the 500A smartshunt?

You only use it in short bursts, so does it really throw off the calculations if I don’t include the bow thruster in the mix?

[pwfarnell]

Some people have had the engine starter current going through the SmartShunt without any problems if they only have a single battery bank so a bow thruster should be OK.