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Wind Gen diversion loads and smart battery monitors

Apr 1, 2019
19
Freedom 38 Sunset Spy Boston, MA
I'm about to install a Balmar SG200 Battery Monitor (which will be replacing my Balmar SmartGauge), and I'm grappling with a question that may exceed my understanding and expertise. Like most shunt-style battery monitors, the manual for the SG200 says that you should put the "SmartShunt" between ALL LOADS and the battery, i.e., no negative cables should be connected directly to the battery. I already have everything going to a large negative bus just before the battery, so I should be all set.

However, I realized that there's one part of my system potentially should be wired directly to the negative battery terminal: the diversion load for my wind generator. It's currently connected on the positive end to the wind gen's charge controller. When the batteries have reached a set voltage, the controller diverts the current from the generator to the diversion load, which is just two ceramic resisters. At the moment, the negative cable from the diversion load is connected to the negative bus, which means it would be "counted" by the SG200 shunt.

Since the current going from the wind generator to the diversion load isn't really doing anything (not charging or discharging the batteries), why should it be counted? Couldn't that potentially confuse the SG200 and throw off its calculations? I've done some preliminary googling and I'm at a loss. Please set me straight!
 
Sep 30, 2008
1,429
Catalina 310 Quincy, MA
I'm confused here. Back in the day, when I worked with power shunts, the shunt was installed in the positive side of the circuit, between the battery and the load. Ideally, that would be between the battery post and a positive bus.
The diversion load should remain as it is. The power from the wind gen will be diverted to ground (negative), and not seen by the shunt.
 
May 17, 2004
1,938
Beneteau Oceanis 37 LE Havre de Grace
So, it is made in England, then?
Shunts on the negative side are pretty common. It allows them to be connected to electronics that run on low internal voltages, like 5V, without having to deal with some input signal way up at 12V.

I don't know the specifics of the OP's wind gen, but intuitively I agree that the diversion load shouldn't be counted by the shunt, as it's neither adding nor taking from the batteries.
 
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May 17, 2004
1,938
Beneteau Oceanis 37 LE Havre de Grace
Given Maine Sail's response I went back and thought about this more to figure out where my intuition was wrong. The key misconception is this statement -
At the moment, the negative cable from the diversion load is connected to the negative bus, which means it would be "counted" by the SG200 shunt.
The only current that will be counted by the shunt is that which goes over it on its way back to the battery. The dump load just sits essentially between the positive and around lines of the generator, so any current through it just goes in that loop, never back to the battery. So what you want is a path for that current to go back to the generator without going through the shunt. The way to do that, as Maine says, is to put it on the load side of the shunt, along with the negative side of the generator.
 
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Apr 1, 2019
19
Freedom 38 Sunset Spy Boston, MA
Before I saw the responses to my post, I found this passage in the installation instructions for my charge controller:

“a) Connect the Divert Load's negative (-) wire to the negative (-) terminal on the battery or the system's negative battery bus. If you are using a smart battery fuel meter that measurers total Input to Output Amp/Hours, it will usually have a shunt in the (-) connection to the battery. Connect the (-) wire from the Divert Load to the battery (-) not to the shunt as shown in the meter’s manual. Reason: The DIVERT load is operating from excess (free) power generated by the alternative charging system. No energy is flowing into, or out of the batteries to the divert load when the controller is in DIVERT.”

I know that I should trust the experts on stuff that I don’t fully understand like this, but can someone explain to me why the above instruction is wrong?
 

Maine Sail

Moderator
Feb 6, 1998
11,034
Canadian Sailcraft 36T Casco Bay, ME
“a) Connect the Divert Load's negative (-) wire to the negative (-) terminal on the battery or the system's negative battery bus. If you are using a smart battery fuel meter that measurers total Input to Output Amp/Hours, it will usually have a shunt in the (-) connection to the battery. Connect the (-) wire from the Divert Load to the battery (-) not to the shunt as shown in the meter’s manual. Reason: The DIVERT load is operating from excess (free) power generated by the alternative charging system. No energy is flowing into, or out of the batteries to the divert load when the controller is in DIVERT.”
The divert - lead needs to be on the load side of the shunt with the SG200. The SG200 is NOT a typical "smart battery fuel meter"...... The better question is why the engineers of that unit contradict themselves and don't appear to even understand why or what it is they are saying.The load side of the shunt is your "system's negative battery bus."

What diversion load controller is this....?
 
May 17, 2004
1,938
Beneteau Oceanis 37 LE Havre de Grace
Here's a (admittedly very rough) picture I used to prove it to myself:
Screenshot_20190403-115710.png


If you put the lead from the dump load straight on the bus, on the load side of the shunt, current follows the green loop. That's what you want - to have it flow without traversing the shunt. If you put the dump load to the battery terminal current follows the purple loop, which traverses the shunt, and then the shunt thinks your battery is charging when it's not.
 
Apr 1, 2019
19
Freedom 38 Sunset Spy Boston, MA
The better question is why the engineers of that unit contradict themselves and don't appear to even understand why or what it is they are saying.The load side of the shunt is your "system's negative battery bus."
I think it's written to be understood as follows, with my additions in bold italic:

"Connect the Divert Load's negative (-) wire to the negative (-) terminal on the battery or the system's negative battery bus. HOWEVER, if you are using a smart battery fuel meter that measurers total Input to Output Amp/Hours, it will usually have a shunt in the (-) connection to the battery. IN THAT CASE, connect the (-) wire from the Divert Load to the battery (-) TERMINAL DIRECTLY, not to the shunt as shown in the meter’s manual."​

That's how I read it, at least.
 

Maine Sail

Moderator
Feb 6, 1998
11,034
Canadian Sailcraft 36T Casco Bay, ME
I don't know how to be any clearer on this point.

The SG200 MUST HAVE the diversion load negative on the LOAD SIDE OF THE SHUNT.
 
Apr 1, 2019
19
Freedom 38 Sunset Spy Boston, MA
I get it, loud and clear. I will leave the diversion load negative connected to the negative bus.

I just wonder why the manual for my charge controller would say the exact opposite. I guess they just didn't anticipate battery monitors like the SG200.