Smart Shunt Placement

Horsefly

Photon Sorcerer
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I know conventional wisdom says that nothing should be connected directly to the battery, but rather everything should go through the BMS.

I have a Thornwave Labs Powermon-5s smart shunt. Although I think it is a bit quirky, this shunt has some nice features like data logging (power, voltage, current, and temperature for up to 3 years), and it can control a relay based on various conditions including timers or voltage ranges. Unlike most shunts, it connects to the positive side of the battery, but as you might expect it needs to connect to the negative side to power the smarts in the shunt. That's where it introduces a bit of a dilemma.

I can follow conventional wisdom and connect the shunt's BAT- line to the BMS rather than the battery. The problem then is that if the BMS does a discharge disconnect (like for a low cell voltage event) it will remove power from the shunt. This will cause all the data logs to be lost. It may also lose some of the relay output conditions, which in my case will be part of the battery heating logic.

On the other hand, I could connect the BAT- lead of the shunt (which is just a small 20awg wire) directly to the battery, essentially bypassing the BMS. The smart shunt would then stay powered up no matter what disconnects the BMS would do. The downside is that if something goes haywire in the shunt or if the relay line is shorted to the positive side of the battery, the BMS would be unable to save the day.

So I'm a bit conflicted. I bought this shunt for some features that would be lost if the BMS cuts the power to it.

I know several people here have the Victron Smart Shunt. I bought one, but am a bit disappointed with it. Ironically, the stuff that I wish it would do is the stuff that my current shunt would lose if the BMS cuts it off.

So, questions for the forum:
Does anyone else use the Thornwave Labs powermon-5s shunt? If so, do you bypass the BMS to power the smarts?
Does anyone here bypass the BMS to connect their Victron shunt?

Edit - I should have included the schematic from the Powermon-5s manual:
Capture.PNG
 
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Horsefly

Photon Sorcerer
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Messages
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Nobody even has an opinion? My observations up until now led me to believe that absolutely everyone has an opinion on this forum. ;)
 

Alphabavo

Solar Enthusiast
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Feb 14, 2021
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Is the shunt’s battery lead not fuse protected? The Victron has an in-line fuse on the power lead.
 

Horsefly

Photon Sorcerer
Joined
Dec 12, 2020
Messages
923
Location
Denver, mostly
Is the shunt’s battery lead not fuse protected? The Victron has an in-line fuse on the power lead.
I should have mentioned that in my post. Good point. Having had the Victron first, one of the first things I noticed was that the Thornwave shunt did NOT have an in-line fuse on the lead that goes to the negative terminal. Obviously it would be easy to add one.

Having said that: Do people believe that the fuse - the one on the Victron, as well as the one that could easily be added to the Thornwave - makes it perfectly OK to have the smart shunt connected on the battery side of the BMS? I'm good with that, but I figured maybe I was missing something.
 
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