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Charge current offset with parallel Multiplus-II - problem with HQ2250 batch?

I have noticed some problems with parallel Multiplus-II installs with regards to the DC charge current offsets.

I have 2 sites where I have these issues and perhaps my experience can help someone else. When you have 2 (or possibly more) Mulitplus-IIs on the same phase, I have seen problems with serial numbers as little as 14 weeks apart (serials HQ2250 and HQ2312). The problem manifests where there is a constant DC charge curent offset between master and slave, even when the charge current is 0A This was measured with two identical current clamps between the phase pairs' DC cabling.

This problem was first noticed where you have AC coupled PV in the system and this constat charge current offset tends to overcharge the battery even when DVCC CCL is 0A. Eventually causing the BMS on the battery to trip.

On 1 site (2 MP2 with Fronius AC coupled PV singe phase) when the DVCC charge current was set to 0A, the AC coupled PV off, the slave would still charge at over 8A vs 0A on the master from the grid. As I say 2 MP2 in parallel with serial numbers 14 weeks apart, same firemware the works. I measured (with a milliohm meter) the AC and DC cabling showing virtually no differences between the AC and DC wire / cable pairs.

I noticed on my second site (6x MP2 5kVA - total 30kVA 3ph with Fronius 3ph AC coupled PV) the same behaviour, with the battery being overcharged on AC coupled PV. The DVCC CCL and CVL limits were commanded by the BMS but the battery kept being charged as eventually the BMS internal protection tripped.

whatsapp-image-2023-09-06-at-143043.jpegOn this 2nd site I also noticed a constant 200 to 300W being fed back on L2 ( where the 2nd MP2 serial numbers differed in year and by about 14 weeks 2023 production vs 2022 production). There seems to be a calibration problem on the voltage reference internal to the production series.

My only assumption is that the voltage reference in the 2 machines are about 50mV out. When I swapped the master and slave in VE.Config the opposite happened, where the charge (became discharge) current went negative. Causing the battery to discharge when the min SOC was reached on ESS, with a repeated grid top up being required.

Anyone else seen the same?

I did a write up

Multiplus-IIAC PV Couplingmultiplus in parallel
2 |3000

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