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Antoine avatar image
Antoine asked

BMV-712 overestimating current by ~1.5x

Hi,

Since installing an MPPT 100/30 charge controller, I've noticed there's a discrepancy between the current reported by the BMV-712 and that of the charge controller.

When getting charged, the battery monitor consistently reports a current significantly higher than what the charger reports outputting.

I've ran a test disconnecting all loads and only leaving the solar charger connected, comparing what the two systems report against a clamp-on ammeter measuring current between the shunt and the battery. I am reading those values:

  • Ammeter: 16.85A
  • MPPT 100/30: 16.90A
  • BMV-712: 25.06A

From this test, it seems the solar charger is correct and the BMV is not. While I was there, I also ran a load test, disconnecting all chargers and comparing the BMV's reported current against the ammeter's:

  • Ammeter: -5.43A
  • BMV-712: -8.19A

So it looks like the BMV-712 is consistently inflating the actual current, either negative or positive, by about ~1.5x. I don't know when this started, I believe it measured things accurately in the past. On the other hand, the reported voltage is correct when compared against a multimeter.

(I also have pictures from those tests but I can't seem to be allowed to upload them).

I'm not sure what to do here. Do I have a defective unit? Any help appreciated.

Thanks,
A.

BMV Battery Monitor
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pv-geek avatar image pv-geek commented ·
Hello @Antoine,


Good observation.


My take is that the mppt current reading is incoming current from your PV. This may be different from the current the mppt is using to charge batteries.


By design, mppt improve the efficiency of PV by increasing the incoming current from PV and reducing the incoming voltage from the PV, thereby charging batteries faster. Remember: Current charges batteries.


The BMV monitors net outgoing current from the battery & the battery voltage.


This may explain the difference you are observing.


Hope this helps.


Cheers.


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kevgermany avatar image kevgermany ♦♦ pv-geek commented ·

The MPPT should be showing input voltage and current as well as output voltage and current in Victron connect.

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Antoine avatar image Antoine pv-geek commented ·
No that's not it. I made sure to look at the battery current reading on the MPPT, and that also matched the current reading my ammeter was seeing on the cable between the shunt and the battery. In addition, the ratio between PV and battery current when charging is at least 2.5x (PV is 40V and battery is 12V), so I wouldn't have read those values if I'd been reading the PV current. And finally, the discrepancy also showed in my load test, which didn't involve the charger at all.
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1 Answer
Antoine avatar image
Antoine answered ·

I believe the correct answer is to make sure all loads are connected to the shunt M10 bolt, and *not* to the small screw linking the shunt to the PCB. I assumed that screw could be used for ring terminals that were too small for the M10 bolt, but that seems to be a wrong assumption.

The manual doesn't mention those screws at all, so I didn't know if I was right or not. It'd be great if someone from Victron could confirm this as the root cause and maybe update the installation manual to prevent people from making the same mistake.

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pwfarnell avatar image pwfarnell commented ·
Think about it, the BMV has a voltage drop of 50mV for 500A and you tested it at 20A, so the shunt was measuring 2mV drop and you were taking a power flow through the measurement connection, the tiniest voltage drop is going to affect the reading, 0.5mV gives you 25% error.
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