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manowell avatar image

BMV 712 voltage discrepancies

I have a new RV (Fifth Wheel, caravan, etc.) that came with solar prewired and a residential refrigerator on a 1000W inverter with bypass when AC is present.


I want to maximize what I can do with batteries and solar, I was recommended to Victron. I started with a BMV 712 and a Venus GX, which I've been working with off and on for about a month.

I've been struggling with the no-name cheap batteries (2 x 12vdc lead acid deep cycle) wired in parallel that came in the coach. I wanted to have my math correct before I spent any serious money on high end batteries, so I installed two brand name lead acid batteries with a stated 20 hr capacity of 90 amps each. I configured the BMV 712 to 180 Ah capacity. First, was this correct?

I've been running the inverter and fridge until the battery voltage gets to 12.1vdc per the BMV 712, which I would consider about the maximum discharge to prevent damage on this type battery. Second, does this sound correct?

The refrigerator cycles on and off, but draws an hour over hour average of 45 watts. My WiFi connection draws 5 watts, the Victron gear itself all pulls less than 5 watts together, from metering. Altogether about 55-60 watts, which the BMV agrees with.

The runtime is usually 3-4 hours, which by the math seems very light. Like 1/3 or 1/4 what I should see.

Today I've added a BlueSolar MPPT 100/30 Charge Controller (also connected to the Venus via ve.direct), during that install I was checking the battery pair and noticed the voltage didn't agree with the BMV. A good voltmeter on the battery is showing a voltage typically 25 volts or more than the BMV, so I've been watching this during testing.

I installed the BMV with the shunt, cable, and temperature sensor provided. Is there any troubleshooting or calibration to be checked?

Third, what does the "synchronize SOC" function do, when should I perform it?

Thanks,

Marc





BMV Battery Monitorbattery capacitycommunication protocol
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25V more? I'm sure this is a typo?

Yes. Missing the "."


Watching it today, it's consistently reading .21 -.23 volts less than what a Fluke digital meter shows.

3 Answers
WKirby avatar image
WKirby answered ·

In that red wire which goes from the battery + to the shunt is a 100mA fuse.
That fuse has a Voltage drop over it of about 250mV. I replaced my 100mA fuse with a 1A use which has a Voltage drop of just 400μV.
That will bring the BMV reading much closer to that of your Fluke.
A 1A fuse is still plenty adequate to protect that wire.

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Thanks, didn't expect such a difference.

I too was surprised, but I changed the fuse to 1A, and now BMV-712 is within 0.05V of my REC-BMS. Previously 0.21V difference. I don't have a Fluke, so no idea which is correct!

manowell avatar image
manowell answered ·

Following pointers to the problem from WKirby, my BMV-712 now exactly matches my Fluke.

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dtaschler answered ·

I am having a similar issue, but my BMV is measuring about 5Vdc lower than my measured battery voltage of 64.00Vdc. The BMV is reading 59.51Vdc. I checked the voltage after the fuse, and it is accurate. I even measured it at the RJ-12 connector and it reads 64Vdc. None of the settings in the BMV should affect voltage readings. Any suggestions?

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I was updating firmware while typing, and the discrepancy changed a bit. It is now reading 60.73Vdc, but still not 64Vdc.