I have a SolarEdge 3-phase 5kW inverter, combined with an LG Chem RESU10 (9.8kWh) and 6kWp of PV panels. I purchased this system in the summer of 2021. Despite multiple complaints to my installer and contacts with SolarEdge, I continue to struggle with the same battery efficiency problem. Over its 14 months in operation, the system charged the battery for 1321 kWh, and discharged for 757 kWh. This would imply an efficiency of only 57%.

 

I can also showcase this with a specific example from 28-29 september 2022. During daytime charging, my battery behaves as expected. When the system charged 8 kWh of energy into my battery in 4h45min, then 77.7% or 7.6 kWh of energy was added to my battery. That is an efficiency of 95.1%, which is in line with the data sheet value of 94.5%. However, my battery does not behave as expected at all during overnight discharge. Overnight (covering 14hours in total), the system withdrew 2.54kWh from my battery, which caused the energy in my battery drop by 49% or 4.8 kWh. That's an efficiency of 52.9%.

 

A few other points:

- When I disable my inverter for 9 hours, I see the charge dropping by about 1%. I have also seen my battery consuming 1.1 kWh on a series of 4.7 days (with the inverter on but with barely any sun) when the battery was fully discharged. This all seems pretty normal to me.

- The standby consumption of my house is about 80W in summertime (when heating is not on) and 200W in wintertime when heating is on. I presume some of this comes from the inverter and battery, but it does mean that this should be less than 40W. That seems pretty normal and expected to me but is nowhere near enough to explain my efficiency problems (over 14h, this would maximally drain 0.56kWh).

- I understand the inverter efficiency is reduced at low power consumption, but this is insufficient to explain the efficiency losses. My inverter still has 92% efficiency at 250W (based on graph published by SolarEdge themselves).

- My battery and inverter are placed in our garage, which is part of our house. This room is not heated, but tends to be 10-15°C. These should be decent conditions for the battery and inverter.

- I don’t have the sense that the battery gets excessively hot. In fact, it often feels cold and lukewarm instead of hot.

 

I have already read several possible problems on the internet:

- Terminals connected incorrectly: this has already been checked extra once by the installer, and appears to be correct.

- Software updates: the firmware has already been updated at least once since the installation, but without effect.

- The state of charge of the battery is set incorrectly. I can't check this myself, but if that were the problem then discharging and charging should give the same low efficiency values (which they don't).

 

Any idea what the problem could be and how to fix it?

It could be a simple top balancing need. 48 volt battery are made up of 15 or 16  three volt cells with some cells  getting to full capacity faster than others.

My 9,8kw dyness needed to top balance at least once a week. It wasn't an issue in summer where there was enough pv to fully charge the battery by midday. 

The issue was obvious in winter when I often charged the battery from the grid. The dyness protects itself by forcing a grid charge to 50% everytime the cells drop below healthy levels. 

If your battery is forcing grid charging then it could be that it is not getting charged enough to top balance.

Its just a measurement issue , if they were truly that inefficient then the batteries would probably be very hot indeed.  My Victron round trip efficiency is around 70% , the actual battery round trip loss is around 3%  , charging losses around 15 % and inverting losses the rest.