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nickburt avatar image
nickburt asked

BMV-712 Smart reading approx 50% actual charge current

Hi, I'm having an issue with the BMV-712 Smart reading approx. 50% charge current, when compared to a (known good) clamp meter reading.

See my (very rough - sorry) schematic. The BMV is reading around 4 amps when the mains charger is on and the aux batts being charged via the Orion 12/12-30, but if I put my clamp meter on the +ve output cable between the Orion and the Aux +ve bus bar, I get around 8 amps.

I know there are couple of "unconventional" methods of connection, but, in theory, these shouldn't make a difference to the way the BMV shunt reads the -ve loads. Unless I've missed something, all -ve connections, in around about way, pass through the shunt, with one exception: my winch -ve connection is excluded from the shunt, this is done deliberately to avoid high winch currents passing through the shunt and possibly overloading it.

What am I missing? What have I done wrong. Many thanks in advance for any help.

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BMV Battery Monitor
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5 Answers
pwfarnell avatar image
pwfarnell answered ·

Do you have a ground connection from negative to the chassis, your drawing does not show it but there usually is one. If so, the chassis ground should be attached to the load side of the shunt or one of the bus bars on the load side. It should not be connected to the battery side of the shunt, nor the aux batteries nor to the aux battery bus bar. On vehicles the chassis ground is often connected wrong and some loads return via the vehicle chassis and bypass the shunt.

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nickburt avatar image nickburt commented ·

I don't use the chassis on this vehicle as ground for any of the equipment fed by the auxiliary batteries. Everything is connected back to -ve bus bars or other common connections in, for example, the fuse box, so no chassis connections. But, part of the checks today will be to make sure that nothing has any "stray earths".

Thank you.

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nickburt avatar image nickburt commented ·
Spot on with this answer - my winch negative was lso connected to anothe bus bar that had a link to the chassis. Removed the winch to chassis link and the probelm is resolved.

Thank you.

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nickburt avatar image
nickburt answered ·

Just to add, the shunt is the one that came with the BMV - 500A, 50mV and the BMV has been configured for 240Ahr batts. All other settings as shipped.

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kevgermany avatar image
kevgermany answered ·

Where are you measuring with the clamp meter?

Which battery is the Orion charging?

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nickburt avatar image nickburt commented ·

Clamp meter on the +ve output cable between the Orion and the Aux +ve bus bar.

The Orion is charging the aux batts, fed from the main batt - i.e. B2B charging.

Thank you.

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Alexandra avatar image
Alexandra answered ·

@nickburt

If the Orion output is 8A and there are no other consumers on its output, then the bmv will measure 8A on the shunt.

Do you have any if your fridges etc on?

You can check the shunt is ok, disconnect all wires from the load/charger side and see what it is reading. If it is at 0A then something else on the install is using power.

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nickburt avatar image nickburt commented ·

Everything either off, on not connected. e.g. fridges not connected.

Will disconnect one at a time today to see what is pulling the "unknown" power.

Thank you.

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seb71 avatar image
seb71 answered ·

I don't understand that wiring, but it looks like you have busbars connected to both ends of the shunt. Only the battery negative should be connected to the battery side of the shunt.

Also, if you want to check the shunt/BMV measurement with a clampmeter, put it on the wire between the battery negative and the battery side of the shunt. That is the current measured by BMV.

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nickburt avatar image nickburt commented ·
The batt side of the shunt is only connected to the bus bar that has both aux battery negatives and the winch negative connected to it (I know the winch current won't be included, but I'm happy with that). The other side of the shunt is connected to two bus bars that have the loads to be included in the measurements.

Must admit, I didn't use the clamp meter to check the BMV reading - it was getting late last night !!!!

Will do some more checking today. Thank you.

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