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rgilwolf avatar image
rgilwolf asked

What possible reasons smart shunt reading very different voltage on aux battery than multimeter

We have the BMV712 monitoring Lithium house battery as primary and starter battery as aux. The installation was done prior to our purchase of the boat and all was fine. A few days ago the voltage measurements on our aux battery started bouncing around - eg down to 10V then up to 12 and back etc. I measured with multimeter at same time and had a steady 13V. We checked connections and they are in accordance with installation instructions. Nothing has been changed and no evidence of inadvertent disturbance. House battery monitoring seems fine. Any ideas? Is there something the Victron monitor might see that my multimeter on battery posts does not? any other ideas? It has me concerned if this is a battery issue or a smart shunt monitor issue…

SmartShunt
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Alexandra avatar image Alexandra ♦ commented ·

@Rgilwolf

Damaged cable? Poor contact or corrosion on either side.

If there are no disturbances as you say, then it is a smart shunt issue.

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rgilwolf avatar image rgilwolf Alexandra ♦ commented ·

Thanks very much for the quick responses. Yes it appears to be the connection issue. Seemed to happen around the same time of day, not sure why. Happened today again after very steady voltage reads in the morning that matched my multimeter, then once the house battery was full around 1215, voltage jumped all over the place. I removed the wire with the ferrule pin from smart shunt and positive terminal of starter battery and measured 28 ohms which is too much of course. There is a fuse and a splice (with a pretty good crimp as far as I can tell) in the line. I took the fuse holder apart, stretched the spring out to give a bit more pressure on the fuse and resistance went down to around .9 ohm and when reattached to smart shunt, measures are all good again. Why it was not consistently wrong I don't know and thus I will not assume its all good until I see a few days of consistency. But for now looks pretty good!

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JohnC avatar image
JohnC answered ·

Hi @Rgilwolf

First question to ask you here is whether your batteries are joined with a common earth. Many boats (including my own) aren't. If not, this can affect the accuracy of the Starter V. But the range you describe is unusually high, and you should perhaps investigate further. All the way back to testing at the shunt Aux terminals with your multimeter to find where the difference actually is coming from.

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pwfarnell avatar image
pwfarnell answered ·

Sometimes on boat installs the batteries and shunt are in engine compartments or other potentially damp areas and oxidation / corrosion of contacts especially the small in line glass fuse is often encountered. Check all contacts are clean and sound, especially the small in line glass fuse.

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