Battery Discharging when it should be charging

Hello all,

I am trying to simulate a SolarEdge PV system with DC optimizers and the Tesla Powerwall in Australia, and my battery SoC seems to be doing very strange things. I have set my battery dispatch to manual so that it should only discharge at night, but it is discharging at around noon. There seems to be plenty of leftover PV power to charge the battery after meeting the load, but the battery never reaches 100% charge. I have the charge rate set at c0.5, and the discharge rate set at C0.3.

The battery model does not seem to be working correctly to me, although it could very easily be something I missed.

I have confirmed that I have the most up to date version of SAM.

Regards,
David

EDIT:

I have added a screen shot of the battery discharge on automatic discharge looking 1 day ahead. As you can see, the battery reaches about 80% SoC by 11 AM, but then drops back down to 65% without having discharged to the load at all!

Paul,

Thanks for your response.

1. Duly noted.

2. Yes, I am seeing something different. Please see my new image attached to my original post. The battery state of charge drops dramatically after charging, without having discharged to the load. The image is from the time series hourly data sets.

3. Let's just try to tackle the manual discharge for now. See point 2.

4. How do I attach an image into a reply / comment? The only way I know how to add a file is to edit my original post.

Regards,
David

Hi David,

Please change your desired bank voltage. You currently have it set at 3.3V. The voltage range for the Tesla Powerwall is listed at 350-450V. By setting such a low voltage, the model requires very large current throughput to achieve the power rating desired. There is a limitation in our software occurring at this large current level which we'll try to correct.

Also, note that 6.4 kWh is the fully advertised cycle capacity of the Powerwall. When modeling in SAM, you'll either want to change the minimum and maximum states of charge to allow that full cycle capacity, or upscale the system. Realistically, the Powerwall has greater capacity and is only letting users cycle 6.4 kWh in order to maintain battery lifetime. For instance, if the battery is cycled 6.4 kWh from 100% to 20% state-of-charge, this would imply a full battery capacity of 8 kWh. Such details are somewhat important for truly evaluating performance. Please let us know if you run into additional anomalies or need more help getting setup for your analysis. I would encourage you to check out the analysis case studies here: www.nrel.gov/docs/fy16osti/64987.pdf , which illustrate some of the considerations of setting up a PV-battery analysis.

Best regards,
Nick DiOrio

Nick,

Thank you very much for your thorough response. I changed the bank voltage, and now the battery is behaving as expected. I have looked through some of your battery analysis, and they were very helpful. Thank you for the good work you guys do!

Regards,
David