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BMV-712 suddenly reports wrong high current

I have an RV trailer with single LiFePO4 battery, solar charger and AC input. BMV-712 works fine most of the time and reports sensible numbers. Occasionally though it enters some bad state when it reports approx +17A current (as if the battery is charging). Voltage readings are still good. The only way to get it back in good state is disconnect the power. Current reading fluctuates as the actual load changes. It seems the readings are shifted up ~17A or so.

I saw the same problem earlier when I had an AGM battery and a bit different wiring. All the cables are connected to large nuts of the shunt. The battery alone to one side, all consumers to the other.

I suspect it breaks down when there is a good charging current. Last time it did that when I left my RV with solar panels plugged in. (I use MPPT 75/15 controller.) Not sure how and when exactly it happened, I found it in 17A state when I returned. The other time I plugged the AC power, the battery started charging at about 30A, that gradually went down to 17A and I figured it is stuck. On the other hand, most of the time AC charging goes without a problem. Last time it even started charging at over 50A and BMV worked fine.

BMV Battery Monitor
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4 Answers
wkirby avatar image
wkirby answered ·

This seems like a bad connection somewhere.
Please could you check that this little screws which attach the PCB to the shunt and tight and free from corrosion. Example, I can get my BMV to read ~17A if the little PCB screw on the battery - side is loose.
You don't have any wires or loads connected using those little screws? We've seen this before. It seems harmless to connect things here, but it throws the shunt measurements right off.


There could also be a bad connection or corrosion on the connectors of the cable which goes between the shunt and the BMV. Check those and re-plug them. Unplugging this cable will casue the SOC to reset, so be prepared for that.

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const avatar image const commented ·

I don't have any wires connected to little screws. I have never touched them at all. I will re-tighten them and see if that helps. Thanks for the suggestion.

Regarding the shunt-to-BMV cable. Re-plugging that does not seem to help. I tried that a few times and the current reading stayed wrong. Only detaching the battery leads actually helped. The cable seems OK, no corrosion and connection is firm.

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lehrling avatar image
lehrling answered ·

Can you use an amp clamp to measure the current throught the shunt when it is reporting "wrong high current"?


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const avatar image const commented ·

I don't have a clamp meter. Not sure if I want to get one, they are quite expensive for a single function device that I would use probably once a year :)

If I had one, I could only measure the current through the negative battery-to-shunt wire, not the shunt itself, right?

I am quite confident the current was not positive at those moments because I had disconnected all possible chargers. Highly likely, the last time, it was not large negative either because after reconnecting the wires I was able to draw almost full capacity out of the battery before charging it. Not very precise statement, but that is all I have now.

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lehrling answered ·

"I don't have a clamp meter. Not sure if I want to get one, they are quite expensive for a single function device that I would use probably once a year :)"

It doesnt have to be a Fluke!

Just for kicks, have you done the zero-current calibration for the shunt?

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const avatar image const commented ·

Not sure if I did it zero current calibration at the beginning, likely I did. But as I said the readings are sensible most of the time, including floating at 0A when loads are turned off.

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blake-bowling answered ·

I have the same thing just starting. but not 17amps... but rather 280amps. It happens when devices or some other appliance (12v roof vent) comes on.

For me, bascially, the BMV is now way over counting output and not input. My SOC has been off by 25% for the couple weeks. I have reset SOC, reset settings, unplugged nearly everything.

I think at this point its the firmware upgrade. I have no proof, but it seems to have started that day.


2 BattleBorn 24v, multiplus compact, BMV, MPPT. all working perfectly until a couple weeks ago

screenshot-20210823-075519.png


screenshot-20210823-075519.png (299.9 KiB)
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