I just installed a BMV-712 along with a smart solar 150v/70a mppt charge controller. The BMV is connected to my 12v house batteries, and the secondary sense port to the starter battery.
As soon as I connect a ve.direct cable between the BMV and the MPPT controller the screen on the BMV becomes very difficult to read, with all the segments of the delay partially lit so that the text is not clear.
Here's a video of it:
I can also see that in that video, when the ve.direct cable is not connected the voltage was showing just over 13v (and must be the voltage being sensed at the BMV shunt in the battery bay). When the ve.direct cable is connected, even though it's hard to read you can see that in the video the voltage displayed by the bmv drops a little to around 12.8v suggesting that there's some sort of current flowing in the ve.direct cable pulling the battery voltage down. The voltage at the MPPT controllers battery terminals is basically exactly the same as that at the BMVs sense terminal, there's good thick cables to the MPPT controller and at the time I saw this there's no load at all in the system, so any voltage drop in the cables leading to different potentials at the MPPT controller vs the BMV should be very small indeed.
Cycling through the display modes (not shown in the video, as I did this afterwards), I could also see about 0.5A flowing when the cable is connected (no other loads are on at this point) which I can only presume must have been flowing over the ve.direct cable.
Any ideas?
At the moment, my first thought is that since the BMV is being powered via the sense cable and shunt, the additional 5v feed from pin 4 of the ve.direct port on the MPPT controller is unnecessary, and that since something odd is clearly happening there, I will get some JST-PH connectors and make up a little extension that goes between the mppt controller and cable, with pin 4 disconnected and only the gnd, tx and rx wires connected. I'm thinking this is most likely going to fix it, but even so I'd like to know what's going on here.
