BMV-712 voltage discrepancy

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zabbaskeema asked Oct 1, '19

BMV-712 voltage discrepancy

Hi, I went through similar posts and I don't find a solution.

I have two 712 and they both behave the same: they do not display the same voltage as the multimeterS. I tested with two multimeters, both display the correct battery voltage.

I then proceeded to check the BMV + feed thinking this might be the fuse and v drop issue that was mentioned.

I disconnecting the v+ feed straight from the BMV and measured between it and the shunt I got the very same expected voltage as the + and - directly from the battery posts provide, eg no voltage drop!

That means the BMV displays a different voltage than two accurate voltmeters from exactly the same source feed/wires.

The drop is as follows:

Battery posts/BMV v+ feed voltage: 13.26v
BMV display, no bluethoot activated: 13.15v -> eg from BMV display only
BMV display WITH bluethoot activated: 13.02v -> reading from the app!

So there is an even greater drop when actually using VictronConnect and bluethoot!

That does not seem right at all!!!

Please help.

BMV Battery Monitor voltage

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wkirby answered · Oct 01, 2019 at 11:52 AM

I disconnecting the v+ feed straight from the BMV and measured between it and the shunt I got the very same expected voltage as the + and - directly from the battery posts provide, eg no voltage drop!

If you disconnect the BMV (which is a load) then you cannot expect a Voltage drop anymore because no current is flowing through the wire.

Check the fuse like NigelFXS suggests. A 1A fuse presents a much lower Voltage drop than a 100mA one.

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zabbaskeema commented · Oct 01, 2019 at 09:56 PM

Yes thanks for the free tuition here. That is obvious now that you tell me :)

So I changed them all to 1A and voltages are spot on now!

(some to 2A as I got nothing else, would a 2A current kill the bmv? No specs on that anywhere...! )

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Guy Stewart (Victron Community Manager) ♦♦ zabbaskeema commented · Oct 02, 2019 at 12:00 AM

A 2A fuse is fine.

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klim8skeptic ♦ Guy Stewart (Victron Community Manager) ♦♦ commented · Oct 02, 2019 at 12:13 AM

Hi Guy,

Oddly enough my bmv700 worked fine for six months, until I tested the relay function. Replaced the 100ma fuse and all fine.

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Answer 1 · 2019-10-01T11:08:45Z

nigelfxs answered · Oct 01, 2019 at 11:08 AM

This problem is most likely due to extra series resistance between the BMV and the Battery. This causes the increased voltage drop when Bluetooth is activated (since it uses more current).

Check the that the Fuse is rated at 1A, some units were fitted with incorrect 100mA fuses.

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