Tim Rohrer avatar image
Tim Rohrer asked

Is my system defying the laws of physics? :-)

I have a BMV-702 installed in my truck camper with two lithium 100 Ah batteries. The primary bank monitor is connected to lithium house batteries. I went ahead (because I could) and connected the feed coming from the truck as the second bank for monitoring. The batteries in the truck are isolated from the house batteries using a Blue Sea Systems isolator.

What is weird is that I've seen the BMV percentage of charge *increase* over night while boon docking. Once again this morning (after an overcast day yesterday with rain), the BMV was showing a fully charged system even though I saw it down around 90% yesterday.

The video is representative of what I've experienced The monitor may indicate ~90% charge when I go to bed, but be back up at 100% in the morning.

What could be causing this?

BMV Battery Monitorbmv power consumption
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1 Answer
Alexandra avatar image
Alexandra answered ·

@Tim Rohrer

Can you post a picture of your setting

It seems to be syncing to 100% when it shouldn't.

I am assuming you have it installed physically the correct way around. Loads and chargers on one side the the battery on the side marked battery.

Does the percentage go down in normal use when there are no charge sources and a load attached?

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Tim Rohrer avatar image Tim Rohrer commented ·

I believe I have it all correct. This is my third battery upgrade project and my second install with solar. That doesn't mean I am incapable of a mistake though.

I assume you wanted pictures of my setup?


I just turned off the solar charging and the battery bank (by volt meter) was 13.47V while the monitor was reporting 13.41V. I'd call that reasonably close. Also with the solar off, I have a negative draw on the bank. It is currently (pun intended) still showing 100% charge, but I have seen it drop as low as 90%. We've got 200Ah of batteries and no inverter so most of our usage goes to fans, charging electronics, and lights. I wasn't surprised to see the charge drop only to 90%, but yesterday with the rain I was expecting to be lower this morning. Instead I was back to 100%.

In the few minutes that I had the solar off, the percentage on the BMV dropped to 99.7%

Oddly, if I press Setup on the BMV display, nothing seems to be happening.

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Alexandra avatar image Alexandra ♦ Tim Rohrer commented ·

@Tim Rohrer

So then it is having comms interrupted with the display somehow. Check the connection there. All the clever bits are in the display like settings and tracking. And it is powered up through the little 'network' cable.

So if starts synchronised is checked in the victron connect it will start at 100% if it is interrupted somehow, even briefly.

You have verified the shunt is correct, so it will possibly be a bit of a settings issue mixed with that cable issue.

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Tim Rohrer avatar image Tim Rohrer Alexandra ♦ commented ·
Had to drive for a while.

It turns out I was wrong about the Setup button. Apparently I needed to press and hold it for a bit to activate it. Way too many options :-)

Basically, I believe I left everything at the defaults. I am thinking about buying a dongle so I can use the bluetooth software for this.

Are you suggesting I turn Start Synchronized to off? I guess I'm trying to understand *how* the power would be interrupted? Perhaps the cable is bad? I believe it is a regular ethernet cable so I could buy a new (and shorter) replacement to help rule that out.

Tonight I'll turn off the solar charging and make sure the batteries drain down a bit before I turn it back on.

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Alexandra avatar image Alexandra ♦ Tim Rohrer commented ·

The dongle makes it easier with victron connect.

If you havent programmed it, it will never be correct.

In the manual, just mark the ones that need to be changed and leave the rest alone.

Start synchronised should be off. On first connect if they are fully charged it is fine. I usually dosable it afterwards as it can cause issues if power gets interrupted. It is better to assume a flat battery than a full one.

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Tim Rohrer avatar image Tim Rohrer Alexandra ♦ commented ·
I turned the solar charging off last night.

When I went to bed last night, SOC: ~89%

Upon awakening, SOC: ~91%

Not sure of the acceptable error with the BMV 702, but I’m intrigued by the mystery.

I should be able to install a smart dongle today and will do more programming although I’m perplexed as to why any setting could be responsible for the charge percentage increasing over night unless there is some interaction between the house batteries and the truck batteries?

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