Victron Smart Shunt conflicting instructions.

[mrlorin]

Victron provides conflicting technical information on the correct way to connect a DC Smart Shunt in dual battery system.
I plan to use the aux input to monitor the starter battery voltage.

Which side of the Victron SmartShunt should the starter battery negative terminal be connected?

I think this is correct with the starter battery connected to the smart shunt load minus.

pic attached
https://www.dropbox.com/s/86t70756l1...ctron.jpg?dl=0

However this picture from a Victron instruction video shows the starter battery negative connected to the smart shunt battery minus.

pic attached
https://www.dropbox.com/s/istly33coe...ctron.jpg?dl=0

https://www.youtube.com/watch?v=0HG4GiH9su8

[Wotname]

Welcome aboard CF, great first post / question!

Interesting, I hadn't noticed this discrepancy before.

Both might give the same result but that depends on how 'smart' the shunt really is.

Intuitively I think it should be connected to the smart shunt load minus.

[jt11791]

Makes no difference. It's just sensing voltage, no measurable current, so can be on either side of the shunt.

[Wotname]

The problem is the voltage across the shunt varies as the house load current varies. The voltage across the shunt could be as high as 100mV (I forget ATM what shunt voltage is used by Victron).

If the Victron smart algorithm copes and adjusts for the varying shunt voltage, then all is good otherwise connecting it to the smart shunt battery minus in the same as putting a small variable voltage source in series with a voltmeter.

[mrlorin]

I also plan to install a voltage sensitive relay to enable the alternator to charge the system battery and the solar panel to charge the start battery.

https://www.kickassproducts.com.au/B...groupid=&adid=

If the start battery is linked to the shunt battery negative any current flowing through the VSR would not be measured by the shunt.

Have I just answered my question?

[wholybee]

The starting battery negative needs to be connected to the load side of the shunt. If it is connected the the battery side with the house battery, then any current going into or out of the starting battery would be totaled with the house battery, and throw off the SOC calculation.

[smac999]

Eng bat needs to go on the load side. Top pic is correct

[Wotname]

Originally Posted by wholybee

The starting battery negative needs to be connected to the load side of the shunt. If it is connected the the battery side with the house battery, then any current going into or out of the starting battery would be totaled with the house battery, and throw off the SOC calculation.

No. The starting battery current will not flow through the shunt in either case.

[Wotname]

Originally Posted by smac999

Eng bat needs to go on the load side. Top pic is correct

While I agree it should go to the load side, it is hard to argue that the other pic is wrong when the information is supplied by the manufacturer.

It is possible that both ways are correct.

[AKA-None]

Which side is common between the two batteries

That’s where is will measure as 0

The shunt goes from measured battery ground out to common ground so it will measure voltage from common ground to battery plus

If it weren’t for them tying common ground to one side of the shunt then you would be able to put the shunt on the battery plus side. But if you try that the all your voltage readings would be backwards


I think I said that correctly but let me think about that a bit :-)

[wholybee]

Originally Posted by Wotname

No. The starting battery current will not flow through the shunt in either case.

It would depend on what device. As drawn, no the starter current wouldn't flow through the shunt. But a charger connected to the Starter battery and grounded to the ground bus would if the battery were connected to the battery side of the shunt. Any other loads that might be connected to the Start battery and grounded to the negative bus would flow through the shunt.

And if connected to the battery side, then with a VSR current would flow from the alternator to the house battery without going through the shunt.
Both of those situations would cause an incorrect SOC reading. Absolutely, no question, the start battery negative needs to go on the load side.

The picture that shows the connection to the load side is directly out of the smart shunt manual, and is correct.

[smac999]

Originally Posted by Wotname

While I agree it should go to the load side, it is hard to argue that the other pic is wrong when the information is supplied by the manufacturer.

It is possible that both ways are correct.

No it’s not correct. In the 2nd pic the amp meter would only show a neg draw. Even if the engine was running. It will start at 100% Soc and only go down. No mater how much you run the engine the battery monitor would never gain Soc or show pos amps. Eventully the display would always show O% even if the battery was full.

[Wotname]

Originally Posted by smac999

No it’s not correct. In the 2nd pic the amp meter would only show a neg draw. Even if the engine was running. It will start at 100% Soc and only go down. No mater how much you run the engine the battery monitor would never gain Soc or show pos amps. Eventully the display would always show O% even if the battery was full.

I think it is safe to assume Victron meant to show the green box as "Loads and Chargers" just like the green box in the 1st picture otherwise the main battery would never get charged.

[CharlieJ]

The Victron first document is absolutely correct; the second document is a special case. In general, there is no intent to measure the 1 to 2 Ahrs that it takes to start a marine diesel. House bank B- on the high side of the shunt (marked Battery); everything else on the low side of the shunt. It is that simple.

[Wotname]

Originally Posted by CharlieJ

The Victron first document is absolutely correct; the second document is a special case. In general, there is no intent to measure the 1 to 2 Ahrs that it takes to start a marine diesel. House bank B- on the high side of the shunt (marked Battery); everything else on the low side of the shunt. It is that simple.

The start current is not flowing through the shunt in the second document (nor in the first document).
What is the special case shown by the second document?

I only see drafting errors in the green panel on the second document.

The final note on the second document states the negative of both batteries must be common. The first document does not achieve this requirement.