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victrowok asked

Strange DVCC issue, need to disable and re-enable daily to get full MPPT harvest.

Hi guys, wondering if anybody can point me in the right direction on a little MPPT VE.Direct problem.

I have a small Multiplus 24/5000 grid-tied system running great with BMV-700, Venus GX and off-brand DC coupled PV charger with 200Ah of Li-Ion. I've recently added another PV string on an MPPT 100/50 Rev2. Everything has latest firmware as of this post.

The MPPT is working great, except that every day its output is down by a few hundred watts. Heres the wierd part - It comes good when I disable DVCC then imediately re-enable it again. It remains at full output until the next day. Here's a video showing this:

In this video you can observe the output is down from what it should be. After disabling DVCC, the MPPT settles on a much higher output, with most going to the battery (huh? already 100% soc) and a little extra going to the grid. When DVCC is then re-enabled, the MPPT continues to produce full power, and the excess goes correctly to the grid. That is, until the next day.

The utility connection is a 10A circuit, so export is limited by the Multiplus to 2400W. When there is excess solar and the grid circuit hits 10A export, DVCC correctly backs off the MPPT as intended, so the battery is not overcharged past 100% SOC and the 10A grid circuit limit is observed - great.

My suspicion is that there is a bug somewhere in this code which is somehow causing the MPPT to stay backed off even when it should be instructed to return to full power output. When this condition is in effect, the charge voltage target in the MPPT is observed at 27.3V (while battery remains at 27.1V, as observed in the MPPT's menu on the venus GX). The mppt *should* be going flat out, but somehow remains backed off.

No battery current limits are ever being hit, the battery temp is mid 20C at all times, wire gauges are all correctly sized and nothing is getting hot.

Any ideas?

MPPT ControllersESSmultiplus ve.busDVCC
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1 Answer
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victrowok answered ·

Small update: This issue only seems to be triggered when getting close to that max grid AC connection limit (within a few hundred watts). I have a theory that ESS is using the wrong battery voltage for one of the equations (ie possibly from the multiplus instead of the BMV700), and some sort of undocumented offset is being applied after this is triggered, which persists even after conditions return to normal.

I'll test this theory by installing the voltage sense wires into the multiplus. I didnt think they were required as the multiplus is meant to use the BMV700's sensed voltage. Correct me if i'm wrong here.

Thanks to anyone that has put thought into this.

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