Are my BMV-712 shunts inaccurate?

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peter-bris asked Aug 3, '21

Are my BMV-712 shunts inaccurate?

Hi, I'm a seasoned user and seller of Victron gear.

Today, I discovered a 3.6% current discrepancy between a smart solar MPPT100-20, an IP67 charger, a DC clamp meter, a multimeter and 2x BMV-712.

12V battery charging current limit on the MPPT was set to 15A which was matched by the DC clamp meter but the 2x BMV-712s only displayed 14.45A which is 3.6% below the mark. BTW I ensured the 15A was kept constant by feeding stiff and regulated 24V to the smart solar's input with the output wired to a 12V LFP battery @ 50% SOC.

This had me curious.

I then used the IP67 charger with the current limit set to 6A and wired everything including the 2 shunts and 10A multimeter in series with the half charged LFP battery.

The result was similar: multimeter and DC current clamp were showing very close to 6A but the 2x BMVs gave a reading of about 5.78V which again, is 3.6% below the mark. And no, there's nothing else connected.

Both BMVs show zero when there's no current. Parameters 65 and 66 have the default 500/50 so it looks like the shunt resistance is out by 3.6%?

BMV Battery Monitor

shunts-small.jpg (372.4 KiB)

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klim8skeptic ♦ commented · Aug 03, 2021 at 08:40 AM

You are going to trust a clamp meter with the name of BSIDE, and a generic workzone multimeter, over 2 BMV's that read within 40mA of each other??

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Answer 1 · 2021-08-03T09:57:01Z

peter-bris answered · Aug 03, 2021 at 09:57 AM

@klim8skeptic the thing is it's not just the generic instruments which are in agreement. It's also the Victron app showing the smart solar MPPT and IP67 charger in agreement with the generics. So what are the chances of all these being out by exactly the same amount vs 2x BMV shunts being out? I'm speculating the two shunts came out of a miscalibrated batch...

I might do another test with a voltage source and a 1% tolerance resistor.

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Answer 2 · 2021-08-09T11:58:32Z

peter-bris answered · Aug 09, 2021 at 11:58 AM

FWIW, I did another test using 20 off 10R 1% resistors in parallel, 0.5R total resistance.

The 'current source' was the smart MPPT100-20 set to 2A charging a LFP battery @ 50% SOC. The yellow meter shows 1.98A, the other meter 0.995V across the 0.5R resistance whilst the BMV only displays 1.92A. Both meter readings and the smart solar MPPT are within 1%.

But the BMV reading is out by over 3% which is consistent with the earlier measurements, although the BMV-700 manual states a current accuracy of +/-0.4%.

2a-400h.jpg (22.2 KiB)

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