Battery monitor vs. Victron 12/3000/120-50

My 500A Battery Monitor With Shunt reads .5V lower than the Victron. Currently I am charging at 90A, 13.83V at 20C so sayeth Victron. While the Shunt is reporting 13.3V at 75A. Whiskey Tango Foxtrot, over?

Of course the Shunt is directly connected to the battery bank, I have to open the battery compartment to access the read-out. The Victron is ~10'-15' away on 2/0 cable. Class A diesel motorhome setup with the inverter/charger in the middle storage bay while the batteries are behind the rear tires, next to the engine bay. Batteries connect to the vehicle electrical control, then another set of cables run from electrical control to the Victron.

edit: I am currently on 50A shore power

You are going to have to do some readings with a DVM to determine which is correct.

Please independently verify these numbers with your dvom at the same charge current.
Both at the battery terminals and at the "victron" terminals.
Voltage drop is a factor of resistance and current.

If I read across the battery terminals the DC V reading agrees with the Battery Monitor.

I am old, fat and frail, it will take an unknown time for me to access the Victron. I shall return.

thank you both.

Denver said:

If I read across the battery terminals the DC V reading agrees with the Battery Monitor.

I am old, fat and frail, it will take an unknown time for me to access the Victron. I shall return.

thank you both.

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Remember to measure with 75 amps of charge current because current is a factor.

I will provide 4 measurements at the same charging voltage and amperage as set by Victron and reported by VictronConnect. Victron V and A as seen by VictronConnect. V and A as measured by VOM at the Victron terminals. V and A as seen by the Battery Monitor and V and A at the battery terminals as measured by by VOM.

I have to empty the storage bay. One backbreaking box, followed by resting, after another. I shall return.

Ok.
VictronConnect app reports bulk charge at 13.96V at 90A.
Victron Battery connections to 2/0 cable report 13.85V
2/0 cable connect battery terminals reports 13.65V
Battery monitor reports 13.35V at 75A.

Thank you for your support.

Denver said:

Ok.
VictronConnect app reports bulk charge at 13.96V at 90A.
Victron Battery connections to 2/0 cable report 13.85V
2/0 cable connect battery terminals reports 13.65V
Battery monitor reports 13.35V at 75A.

Thank you for your support.

Click to expand...

Using this calculator

200 milli volts drop over 15 feet of 2/0 awg at 90 amps is 10 milli volts better than expected.
Considering there is joinery between the 2 endpoints I suspect your dvom reads a bit low.
I can't tell what application you are getting data from via Victron Connect so I will ignore that for now.
The Battery monitor appears to be reporting significantly low.
What is the make and model of the battery monitor?
Where is the positive lead for the battery monitor terminated?

I suggest you see how many of these devices can be calibrated and make them equal to a reference at 0 current.
Just to set your expectations even perfectly calibrated equipment will read different voltages when significant current is present due to their different positions along that 15 feet from the inverter/charger to the battery.

Also 2/0 wire is far too small for a 3000 watt inverter at any length.
You should be using 2x 2/0 awg for that distance to get voltage drop under 3%.

The Victron Connect app is the app. Ethernet from Victron to ethernet adapter to USB on PC.
the Battery monitor is the 500A unit sold by Renogy.

The positive lead for the monitor is direct connect to the nearest battery.

Note, now that we're getting close to float at 14.6V @ <10 amps reported by Victron Connect. The monitor is showing 14.4V @ 2A.

The 2/0 is what I went to war with. this is a 21 year old Monaco Dynasty which came equipped with 4 6V batteries, a 95 watt solar panel and a 2000 watt Trace Engineering modified sine 2x120V inverter charger. I still have the Trace inverter/charger and it's remote if anyone wants it. It will run 2 120V legs.

funny thing is, the voltage is lowest at the batteries and reads highest at the Victron (or is that funny?).

thanks for the reply.

Denver said:

The Victron Connect app is the app.

Click to expand...

I know that, I have one.
Mine shows the voltage from the point of view of my only victron smart device with is a smart battery protect.

Denver said:

The 2/0 is what I went to war with.

Click to expand...

Well, you are concerned about voltage drop and you running cables of less than half the prescribed circumference.

Denver said:

funny thing is, the voltage is lowest at the batteries and reads highest at the Victron (or is that funny?).

Click to expand...

Its absolutely expected.
The Victron inverter/charger is the charge source in this scenario.
In order to get 90 amps to flow to the battery you need at least 200 milli volts of positive potential difference to push that current.

no other smart device. the batteries are 4 170ah renogy's.

ok, so the charge is originating at the inverter charger and drop, drop dropping on it's way to the batteries. Then no harm may occur while charging these batteries to an absorption voltage of 14.6V as seen from the Victron. However if I understand you the batteries will never be charged to 14.6 V either?

Normally I wouldn't see any of this as I am on shore power and would usually be continuously charging. But due to cold weather I have disconnected the solar arrays and set the Victron on passthru. I've been charging the batteries weekly when the temps are comfortable. It was only recently that I thought to compare the Victron Connect readings to the battery monitor. the result caused me to join up and axe my questions.

smoothJoey said:

In order to get 90 amps to flow to the battery you need at least 200 milli volts of positive potential difference to push that current.

Click to expand...

which means absolute bupkis to me.

thanks for the help.

Denver said:

no other smart device. the batteries are 4 170ah renogy's.

Then no harm may occur while charging these batteries to an absorption voltage of 14.6V as seen from the Victron. However if I understand you the batteries will never be charged to 14.6 V either?

Click to expand...

The voltage will get very near to 14.6 at the end of the absorption phase.
Its somewhere between possible and very likely that your bms is tripping with each charge.

Also floating at 14.6 accelerates calendar aging.
Its a bad idea.

But its your battery and you can do as you wish.
I'll be on my way.

there is no float as the charger is turned off after float is reached. However the tech sheet/manual from Renogy says absorb to 14.6. In the warmer weather I was absorbing to 14.2 and floating as the charger wasn't turned off and I was either on shore power or solar (set to 14.2 as well).