LG Chem battery efficiency

[edk_fan]

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. The attached image at shows that my inverter still has 92% efficiency at 250W (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?

[Mary B]

From the battery description

"The compact and lightweight battery unit produces a peak discharge of 7 kW and 5 kW at nominal discharge. "

Sounds like it is working right

[SparWeb]

I'd start by looking for more information.
A few strategically located ammeters, independent of the inverter, would tell you where the current is flowing, especially in the overnight time that seems to concern you the most.

The "Peukert effect" will affect your system.  How much?  Hard to say without more measurements but I think you want to get familiar with that.  That would require some research.  I have seen a spreadsheet calculator that may help you.  I can look for it if you want to.

So far I don't see anything wrong in the information you have provided about your installation.  Peukert effect is probably temperature dependent, although how much and whether cold is good or bad, I can't say. 

The efficiency of your inverter when inverting is different from its efficiency when it is charging.  I would expect there to be two different curves. SolarEdge might share that with you.  For what it's worth, the efficiency in the plot you posted is very good.

Just my starting point.  Troubleshooting functional problems is hard enough.  Troubleshooting performance issues is harder.  I see a methodical approach in what you are doing - keep it up because you'll need it to get a good answer.

[joestue]

This is a lithium pack, should not be any energy loss.

I would speculate the batteries are fine and the system is misrepresenting the energy somehow.

[DamonHD]

I may be misinterpretting you but there will be *some* roundtrip losses, as heat in the electronics and the cells utlimately.

For a very good system that may be << 5%.

Rgds

Damon

[joestue]

I may be misinterpretting you but there will be *some* roundtrip losses, as heat in the electronics and the cells utlimately.

For a very good system that may be << 5%.

Rgds

Of course, but he is reporting 50% losses.

Damon

[edk_fan]

I was aware of the Peukert effect, although I did not know its name. Now knowing its name, I did some research in the scientific literature. It seems that the Peukert law does not really apply to Lithium-ion batteries, because high discharge rates increase temperature, which in turn makes the batteries more efficient (and counteracts the Peukert law). This does not mean discharge rate does not affect the apparent capacity, but the effect is certainly minor for Lithium-ion batteries. The Peukert law seems more relevant for lead-acid batteries.

LG has just responded that the state of health of my battery is 99%, so that's clearly not the problem here. I hope they can now dig deeper.

I don't have an ammeter, so I can't do those measurements. I'll see whether I can get a hold of one though, but this may take some time.

[SparWeb]

Just in case the obstacle is awareness:
https://www.google.ca/search?q=current+clamp+ammeter

Just get a cheap one.  You aren't looking for the last decimal place, and the currents you are measuring are simple pulsed DC, and AC sine.