Smartshunt Voltage Issues

Q&A and troubleshooting
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1

Hi all,

New to the group here although I’ve read plenty online without joining in the past. I also frequently Facebook community as well. Thanks to everyone for their expertise!

I have a question for the group as I’m trying to troubleshoot something that just started occurring.

First, I’ve had this set up for about 2 1/2 years in a sprinter van that I’ve been living in full-time. Other than a bad BMS in one of my battery banks that is now recently replaced, everything has been working as expected. Until this week.

I’ve attached images of “normal” readings with my smart shunt - while parked with no solar, shore power, or DC to DC charging active. Also attached one of my BMS screenshots.

As you can see, everything looks great.

On two occasions this week, while driving and operating the DC to DC , the smart shunt was showing voltage above 14v while the batteries were showing 13.3 As a result, the smart shunt throttled back my DC to DC charging to about 10A.

On one of the two occasions that this occurred, once I turned the van off, I saw voltage drops below 13 on the smart shunt - while the batteries were holding steady at about 13.3

I did a reset by cutting all power and the system went back to normal.

Today’s episode showed the same voltage over 14 while driving, but upon shutdown of the van there was no bizarre below 13 V readings on the smart shunt. It’s settled to normal. Then I started the van up again a little later in the day and the same behaviour returned with high voltage and then lower than expected voltage after shutdown of the van. I’ve attached that screenshot now because I have one to share. Again battery voltage’s were still in the 13.3 area according to each individual BMS.

Just so you know it’s difficult to work on the system in this particular van build. I did not build it myself. The couple I purchased it from have a Murphy bed and everything is kind of behind and under the Murphy bed. That said, all connections are tight and I even unseated and re-seated the victron cable to the Cerbo with no impact. The only solution is to kill the house batteries via the rotary off on switch, and then turn them back on. once that happens, everything returns to normal.

I suspect you may need more information potentially and I apologize for not including it because I’m not sure what that would be. I also can’t be sure that this is limited to the DC to DC charger which is a “Battery Doctor” unit that was in the van when I purchased it. I have attached the link below if anybody is unfamiliar with this device.

Given the lower voltage readings upon shut down during one of these episodes , I’m not leaning toward the Battery Doctor DC to DC isolator at the moment as a cause. But I could be convinced otherwise.

Any idea on why the smart shunt would indicate over 14 V when both batteries are showing 13.3 on their BMS, with the subsequent throttling of charging from the DC to DC charging unit?

Where do I start troubleshooting this when it occurs again?

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Again thank you so much for all of the things I’ve learned by lurking here. And I look forward to your input and happy to provide any other information I’ve missed!

2

Sounds like a high resistance between shunt and batteries, or battery. You said “on one of my battery banks” how many are there? Do the individual batteries all agree to a common voltage when being charged? So theres not an odd one that sees only 13.3V while the rest are at 14.0V?

If the bank as a whole is at 13.3V and the shunt at 14.0V then those 0.7V are getting lost somewhere inbetween. You checked connections already, but also reseated fuses, checked crimps for heat buildup? Best would be to search for the 0.7V loss with a multimeter.

I dont think the DC/DC isolator is the cause of this, but depends on how the electrics are setup. If they used busbars then every device should be seperately connected to those, then the source of the charge current doesnt really matter much. If the busbars where left out then the cables were likely daisychained from one device to another, which is suboptimal, as any connection in the chain could cause this

3

Thanks so much for your response.

First, I misspoke in my message using the word “banks”. I have two, 280 amp LIFPO4 batteries daisy chained for a total of 560 amp hours. The wiring from the shunt goes to the battery with the new BMS which means there had been some playing with that wire when removing the battery for service.

Upon further inspection this morning, there’s some play in it at the crimp where the wire connects to the shunt itself. I’m going to take it off and try to redo the crimp.

Interesting that this could also potentially cause the extraordinarily low voltage after taking the charge source away, and that a power reset by cutting everything off restores proper voltage instantly. I can’t seem to understand how a bad wire could cause that behavior.

Any thoughts on that would be appreciated. I will also report back once I remove and repair that wire.

4

OK so an update if anybody wants to take a stab at this.

The system has been stable for the last couple of days.

today I installed a new negative wire from the smart shunt to the battery. I purchased this new, I did not make it myself.

Tonight as I was using the inverter and drawing about 80amls for my air fryer, the entire electrical system kicked off completely, (lights, inverter, et …) turned itself back on, then turn itself back off, then turn itself back on. A complete loss of power twice that eventually restored itself

now the new thing in the equation is obviously the new wire so I suppose it’s possible I got a bad one. Or, does this indicate even more that the smart shunt is failing?

I asked because when I checked the shunt voltage after everything was turned back on, the smart shunt was about .30 V below what both individual battery BMS were showing. It may have been lower than that before I checked I don’t know but it was a significant difference.

i’m not really sure how to proceed other than putting the old wire back on and seeing if this happens again. Or replacing the smart shunt

Thoughts?