question

tmac avatar image
tmac asked

Negative charge current from alternator.

Hi there!!!

Recently installed a BMV-712 smart with CTEK 250SE and smartpass 120, two 100ah batteries, and 100w solar panel.


On testing initially everything was just fine but now when there is charge coming from vehicle alternator it is showing as a negative current -1.2 ish amps and therefore the BMV is seeing that as a load and depleting the soc % accordingly.

What have I done wrong?!?


Some info that might assist:


The only thing out of the shunt is the negative side of the battery bank, all loads on correct side of shunt.

The negative side of the battery bank is grounded to the vehicle.


When the vehicle is off and it’s just the solar, this is registering as a positive value input and therefore charging the battery bank through the dc/dc charger (ctek d250se)


Some pictures also attached of wiring and what is shown on app when vehicle is running.


Doing my head in trying to work it out! Thanks in advance for any responses!


BMV Battery Monitor
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3 Answers
JohnC avatar image
JohnC answered ·

Hi @Tmac

"The negative side of the battery bank is grounded to the vehicle."

Change the vehicle ground to the 'Load' side of the shunt..

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tmac avatar image tmac commented ·

Thanks for your response John,


Just so I understand fully, so instead of the vehicle ground going to the negative side of the battery it will go the load side of the shunt. This will leave connected at the negative side of the battery bank, the second battery connection and the non load side of the shunt.

This is a pic of the negative side of the battery bank at the moment showing connected the vehicle chassis ground, the non load side of the shunt, and the joining negative to the other battery.

Can you please explain for my understanding what connecting the vehicle ground to the load side of the shunt will do?

Also, could you offer some advice if the system is actually charging despite the reading on the BMV with the existing wiring. I’m on the road at the moment and can not adjust the setup.

Really appreciate the responses!

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JohnC avatar image JohnC ♦ tmac commented ·

Kevin seems to have covered your questions admirably.

I tend to look upon the shunt as just a part of the -ve wiring, a 'calibrated' part which won't affect normal operations. But must be in the correct place to give useful readings of current flow.

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Kevin Windrem avatar image
Kevin Windrem answered ·

In order for the BMV to accurately calculate house battery SOC it must account for all current in and out of the battery bank. Connecting anything else to the battery size of the stunt means there could be current that is not included in the charge/discharge.

Connecting the vehicle electrical system and/or chassis ground to the battery side of the shunt means that current in or out of the battery isn't counted.

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tmac avatar image tmac commented ·

Makes perfect sense Kevin, thank you for taking the time to explain.

This would also explain why the solar charge IS being counted by the BMV because it is running through the load side of the shunt.

My CTEK 250se (dc to dc charger) is showing that when the engine is running that it is receiving charge from the alternator so I think that the battery bank is being charged when the vehicle is running. It is just the fact that the chassis earth is not running through the load side of the shunt that the BMV is not recognising this as an input from the vehicle alternator and therefore not using this to calculate overall state of charge. Would you agree?

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Kevin Windrem avatar image Kevin Windrem tmac commented ·

Yes, this is all explained by having the vehicle electrical system/chassis connected to the battery side of the shunt.

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tmac avatar image
tmac answered ·

Great, really appreciate the feedback and will update the feed when I’ve made the changes for others to see.

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