Enphase Backup Storage - DIY Self-Installer Hurdles

[neilgaede]

This is my solution to avoid over producing PV when off-grid.

Building a PV Shedding Controller for AC Coupled Solar

Back Ground I have an Enphase system with 35*350 watt panels. It uses the IQ8+ micro inverters at 295 watts each. So the total possible output is 10,325 watts. The actual maximum output is about 9000 watts. I have 7 panels that face east and 28 that face west. The slope of my roof is about...

diysolarforum.com

The IQ8 are supposed to be able to respond fast enough to prevent this. Is this not the case?

 

[wheisenburg]

Do you still need to buy the $2500 IQ System Controller 2?

No, but you do need something to:

1. Disconnect your microgrid from the grid.
2. Act as the grid forming inverter.
3. Capture the excess power coming from the micro inverters and send it to the batteries.
4. Perform frequency shifting to lower the output of the micros to prevent over charging.

A hybrid inverter will do all of these things. My dual Schneiders are working well for me. You do need to pay attention to the PV to inverter power ratio and the pv to battery amperage ratios.

 

[wheisenburg]

The IQ8 are supposed to be able to respond fast enough to prevent this. Is this not the case?

I was having issues with the battery voltage going to high and causing the micros to turn off. I probably should have 6 batteries rather than 3 with close to 10 KW of PV. Thats 190 charging amps. The system can handle this at 90% SOC. At 97% SOC the high voltage shuts theinverters off completely. So 60 seconds on and 5 minutes off doesn't really get the batteries recharged.

With the load shedding, the PV inverters charge the batteries at about 0.5 - 0.6 C up to 92% SOC. The 3rd string then disconnects. Then the charging rate drops to around 0.25 - 0.30. Once the second string sheds, the system is pretty near balanced. It generates a little more power than I need. Eventually, the batteries get to 100% and the frequency goes up until the last string quits. One string is always connected and the AC coupling seems to manage the remaining 3 KW with no issues.

So, I guess I would conclude that if you have 5000 KWHs of batteries for each 1 KW of AC coupled PV, your system should work fine. At 5000 KWHs for 3 KW of AC coupled PV you will likely have problems. I have not tested enough to know exactly where the cut off is.