question

tim-w avatar image
tim-w asked

BMV-712S Showing voltage but no amps.

I just installed a BMV-712 Smart into a Jayco Melbourne 24L. Upon install, it is showing only Voltage readings. If I turn devices on, the voltage falls. If I turn them back off, it climbs backup. If I hook up to 120V, it shows 13.52v. However, it is not showing any amps. This stays at 0amps.


The system is connected to a Duralast 27DC and a Duralast 24MD-DL that are in parallel.

I tried a plethora of sources and videos, but still could not figure out exactly what the issue could be.

Should the second negative wire (chassis ground) of the 24MD-DL battery be connected to the shunt? or because they are in parallel, connected to the Negative of the 27DC?

If I do need the shunt on the other side... is it better to have the shunt closer to the ground? The two compartments are about 5 feet apart as a wire would travel.
wiring.jpg


installation
wiring.jpg (47.6 KiB)
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1 Answer
JohnC avatar image
JohnC answered ·

Hi @Tim W

What you've done here is bypassed the shunt with that Chassis Ground. It must be carrying the current you want to measure, with your loads attached to ground as well (you don't show this in your wiring).

Move it to the Load side of the shunt.

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A couple of follow ups:


The two compartments are about 5' apart.

I am trying to figure out the best place for the shunt... Guessing in the 24MD-DL compartment with Battery connected to the shunt. Would it look like this?wiring2.jpg


Would I hook the 2nd red wire to 27DC Positive and then to B2? Or could I reserve that for that starter motor?

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wiring2.jpg (36.7 KiB)
@Tim W

Technically this is a better arrangement than your first diagram, as +/- come from 'opposite' batteries.

The B1 wire to either batt if your connector cables between batts are adequate.

B2 you could use for either Temp (with added sensor) or your starter batt V (just run a small wire to the starter +ve). The starter batt must share a common ground for that to work.

Please bear in mind that you're running mismatched batts in parallel here. That's never recommended, but you may get away with it. Maybe check each by touch for Temp occasionally after a hard charge session, and if you can clearly detect a difference then you may be having imbalance issues. Don't panic, just stay aware.. :)

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Thanks John!


I thought the same about the different sizes. It was installed by the RV dealership and didnt discover until getting the monitor. Its been running that way on several trips since last fall. Also would of used a different battery altogether.

I am guessing its going to be different chassis because the ground for the starter goes to the engine and the DC goes to mid frame. Perhaps the frame is the ground for both on a Mercedes Sprinter (being that small).

If I get the temp sensor, I would be guessing that the small battery will be heating up the most?

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@Tim W

The one with the least resistance.. who knows?..

I really wouldn't let this spoil your camping, they could last for years if not pushed hard. If it were me I'd give priority to the starter V anyway.

Concentrate on getting that BMV tuned in. That's worth serious thought..

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