How do Shunts work? - Page 4 - SailNet Community
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post #31 of 38 Old 09-22-2012
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Re: How do Shunts work?

The way this Microlog DMM-1 monitor is wired, with it's dual shunt, seems rather unusual to me. Even when the alternator is charging, it's going to monitor load current through the left side of the dual shunt as shown in the drawings. If the battery is charging at 10 amps and the load on your DC panel is 10 amps, then there's going to be 20 amps going through the right half of the shunt and 10 amps through the left half. The monitor is going to have to subtract this 10 amp load current to be able to give an accurate battery charging current. What happens when you select discharge current mode while the alternator is running? The current through the left half of the shunt is no longer "discharge current" because the battery is not discharging. It is now load current supplied by the alternator. Will the display show it when "discharge current" is selected? This is not the "standard" method of installing battery monitors and shunts. Very odd.

Eric
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post #32 of 38 Old 09-22-2012
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Re: How do Shunts work?

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Originally Posted by fairbank56 View Post
The way this Microlog DMM-1 monitor is wired, with it's dual shunt, seems rather unusual to me. Even when the alternator is charging, it's going to monitor load current through the left side of the dual shunt as shown in the drawings. If the battery is charging at 10 amps and the load on your DC panel is 10 amps, then there's going to be 20 amps going through the right half of the shunt and 10 amps through the left half. The monitor is going to have to subtract this 10 amp load current to be able to give an accurate battery charging current. What happens when you select discharge current mode while the alternator is running? The current through the left half of the shunt is no longer "discharge current" because the battery is not discharging. It is now load current supplied by the alternator. Will the display show it when "discharge current" is selected? This is not the "standard" method of installing battery monitors and shunts. Very odd.

Eric
Yeah you're right. Another way to put it is that one shunt measures the alternator and the other measures the load. Battery current has to be inferred from those two (and does the meter do that?). When I wrote my last post I said the first one was measuring the battery which it isn't.

I agree this is odd because I'd consider battery charge/discharge current then load current to be the most interesting data points so why not measure them directly? I don't see anything preventing it from being wired that way. Battery right, alternator center, load left would do it.
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post #33 of 38 Old 09-22-2012
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Re: How do Shunts work?

You'd be reversing the polarity of the voltage measured across the "charge" shunt and you can't just reverse the brown and green wires that measure that voltage.

Eric
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post #34 of 38 Old 09-23-2012 Thread Starter
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Re: How do Shunts work?

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"then all you see is " Yes, you lose the granular information. But then what you are getting is a view of the system as a whole, instead of looking at two separate pieces.

Not a question of right or wrong, better or worse, but just a question of which information means more to you.
Don't know what post this is responding to, and I don't understand it.
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post #35 of 38 Old 09-23-2012
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Re: How do Shunts work?

Ammeters & Shunts 101 Ammeters & Shunts 101

Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)
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post #36 of 38 Old 09-24-2012 Thread Starter
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Re: How do Shunts work?

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Ammeters & Shunts 101 Ammeters & Shunts 101
Thanks for that. It provided another link to a post regarding wiring Battery Monitors, which included the following paragraph:
Keep in mind that many marine alternators are case grounded and thus the system ground, which on most boats is the engine block, is the ground path for the alternator. While I much prefer an isolated ground for alternators many boats just do not have alternators with this feature and they use the case as the ground. Due to this, the ships main ground connection should be connected to the -LOAD side of the shunt and NOT ahead of it or on the -BATTERY side.
Microlog addressed the first half of this, but the second half would driven it home for me. With my basic electrical knowledge, I am used to thinking of everything Neg. terminating at the bock, not flowing from the block to the Battery. I knew I was missing something, but just couldn't quite get my mind around it. I assumed this magical "shut" would somehow take care of this issue through osmosis. It doesn't!
The other thing the post pointed out to me is how substantial (and apparently simple) the Microlog shunt is compared to the others in the post. Maybe the simplicity is due to the fact the Microlog doesn't do the Amp Hour projection function, and/or maybe the Microlog unit does more of the work. In any case, the Microlog shunt appears larger (more metal) than the ones shown in the other post and the connections more straight forward. Here is a picture of my shunt (slightly out of foucus, fingers included for scale).
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Last edited by L124C; 09-24-2012 at 04:05 PM.
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post #37 of 38 Old 10-03-2012 Thread Starter
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Re: How do Shunts work?

Bought a butt splice kit and a 3 foot section of 2AWG from Genuinedealz.com. Extended the Ground cable to the left side of the shunt. I'll find out how it works this weekend...Blue Angels AND AC World Series on the SF Bay (sorry...off topic, but I'm jazzed!)
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Re: How do Shunts work?

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Originally Posted by fairbank56 View Post
The way this Microlog DMM-1 monitor is wired, with it's dual shunt, seems rather unusual to me. Even when the alternator is charging, it's going to monitor load current through the left side of the dual shunt as shown in the drawings. If the battery is charging at 10 amps and the load on your DC panel is 10 amps, then there's going to be 20 amps going through the right half of the shunt and 10 amps through the left half. The monitor is going to have to subtract this 10 amp load current to be able to give an accurate battery charging current. What happens when you select discharge current mode while the alternator is running? The current through the left half of the shunt is no longer "discharge current" because the battery is not discharging. It is now load current supplied by the alternator. Will the display show it when "discharge current" is selected? This is not the "standard" method of installing battery monitors and shunts. Very odd.Eric
As I understand it (and I think I do now!) The left half of the shunt is never related to "discharge". It only measures charge (in my case, shore power charger (connected directly) and Alt. (via ground cable)), with the exception of the brief Starter discharge (also via ground cable). The right side is dedicated to discharge, and center completes the circuit via the Neg. battery terminals.
Both Pos. battery terminals are connected directly to the instrument.
So if I have it right...in your example, I would have 10 amps (not 20) entering (and being monitored on the left side of the shunt, and 10 amps leaving (and also being monitored) on the right side of the shunt. Battery state is monitored directly and has nothing to do with the shunt (with the exception of the center completing the circuit).
I do wonder what I will see on the charge setting when I'm cranking the starter!?

Last edited by L124C; 10-03-2012 at 04:15 PM.
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