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Smart Shunt Placement

Horsefly

Solar Wizard
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Dec 12, 2020
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Denver, mostly
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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Nobody even has an opinion? My observations up until now led me to believe that absolutely everyone has an opinion on this forum. ;)
 
Is the shunt’s battery lead not fuse protected? The Victron has an in-line fuse on the power lead.
 
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.
 
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:
View attachment 49949
I have just made an almost identical setup with the powermon 5s but dont have the SPST relay installed yet. What have you been using and would you recommend it?
 
I have just made an almost identical setup with the powermon 5s but dont have the SPST relay installed yet. What have you been using and would you recommend it?
Wow. You are replying to a post I did about 18 months ago, so I had to re-read it a few times to get myself back to what I was doing then.

The Thornwave shunt had a few problems that I couldn't overcome (subject for another thread, probably), so I removed it from the system. Instead, I purchased a relatively inexpensive Victron Smart Battery Protect. This replaced both the Powermon 5s and the SSR that I was using. The Victron SBP can be programmed via the Victron Connect App to turn off power at whatever voltage you set. So I hooked up my heating system to the output of the Victron SBP (but still requiring the thermostats), and if the battery gets too low the SBP turns off, which prevents any power going to the heating pads.

I still use the Victron Smart Shunt, as it seems to do better at reporting SoC of the battery. The Thornwave Powermon 5S is back in its box, waiting for me to either find another use for it or sell it.
 
@Horsefly please share your experience with the thornwave product.
Sure.

Some of my early experience was in this post and some of the subsequent comments by me and others.

Then later I did this post. I was messing the the Junctek Battery Monitor, and compared some of what I was seeing from the Thronwave Powermon-5S (with updated firmware) and the Victron Smart Shunt, as well as my JBD 8S BMS.

To summarize: The Powermon-5S (or at least mine) never seemed to report SoC correctly. The Junctek Battery Monitor, Victron Smart Shunt, and the JBD BMS all reported very nearly the same SoC throughout the charge / discharge cycle, but the Thornwave seemed to always assume some larger self-discharge, as it would go down while nothing was charging or discharging. The data logging was originally seriously broken (it didn't matter what sample frequency you picked, it would assume it was every second), but that seemed to get fixed with the firmware update and new app.

The Powermon-5S had a load of things you could set for relay control, turning power on/off based on conditions. That is cool. The JuncTek apparently has some of the same relay control features, but I haven't looked into it much.
 
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