question

fritz01 avatar image
fritz01 asked

Smartshunt sizing

I have a combo start / house battery setup consisting of 2 lead acid Group 31 in parallel, providing :

1400 Cold Cranking Amps & 1720 Marine Cranking Amps

The Cummins minimum requirement on the spec sheet for starting the engines (6BT) is:

950 Cold Cranking Amps & 1188 Marine Cranking Amps

Other than the cranking amps, I have very little battery draw - just a few electronics, lights, and an AC/DC mini fridge.

Can I get by with the 500 or 1000amp Smartshunt or do I need to go to 2000? I can't find anything in the literature that says what happens if amperage of Smartshunt is exceeded for a brief time.

I know that having the start and house functions on the same bank is not ideal, but that's the way the boat was setup 30 some years ago and it's been working fine all these years so I'm not inclined to change it up. Thanks

SmartShunt
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3 Answers
snoobler avatar image
snoobler answered ·

Generally speaking, shunts should only be run at 60-80% of rated current continuously depending on manufacturer. However, they are also very tolerant of brief overloads.


What it comes down to is heat. The shunt is actually a precise resistor across which the voltage drop yields current. Heat will alter the readings, AND excessive heat can permanently change the accuracy of the shunt.


The downside to a high current shunt relative to your normal loads is that you lose resolution at lower currents. On a 500A shunt, each 1mV drop across the shunt is 10A. In a 1000A shunt, it's 20A... 2000A shunt, 40A.


What really matters is the actual current you'll be drawing. If you're really going to pull >1000A , but likely less than 1500A for a few seconds, I would think a 1000A is okay, but I can't back that up with facts specific to the smartshunt.

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fritz01 answered ·

Thank you, that's very helpful. I'm a little unclear on what you mean by losing resolution at lower currents. What is the practical implication of that? If I'm following the math, a 10 amp draw on a 2000A shunt would equate to a .25mV drop, and a 1 amp draw .025mV. Does the drop become so negligible that is can't be read?

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cardy01 answered ·
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