Tesla Motors Club
I just completed testing my proof of concept Backup Runtime Extender for my Powerwall 2.0 system. It worked pretty much as expected. I was able to inject about 500W into a normal 120V household outlet and the Tesla App registered reduced household usage and reduced Powerwall output in Self Powered Mode with no grid draw. The energy came from the 12V system of my RAV4 EV that uses a Tesla DC-DC converter similar to the Model S to maintain the 12V battery from the traction battery.
After several discussions here on TMC about generator support (and the lack thereof) and the difficulty restarting Powerwalls after low SOC shutdown, I decided that I wanted a way to make sure that the Powerwalls would not shut down during a prolonged power outage during the winter when my solar production is very low.
In order extend the Powerwall's backup runtime, you must use an inverter like a solar grid tied inverter that synchronizes to the existing power waveform. Most mobile power inverters don't do this - they generate their own free running waveform, much like a generator does. So, I started looking for an inverter that I could use. I came across this line of inverters that is listed for use with 12V solar panels or 24V battery. My first thought was to use a 24V DC power supply running from a mobile power inverter, into this grid tie inverter, then into a 240V transformer, then into an extension cord, to the Tesla Backup Gateway and through the solar CTs. I originally wanted this power injection to look like solar generation. However, I concluded after a short time planning the system that most of that was unnecessary. I found a DC/DC step up converter with a wattage higher than the grid tied inverter, so that eliminated a DC-AC-DC round trip. The bonus is that I don’t need the 240V transformer and I can use the existing outlet that’s next to the car’s normal parking space in the garage.
Strengths:
Key Parts List
600W Grid Tie Inverter
https://www.amazon.com/gp/product/B071F5XJMD/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
UXCell DC/DC 720W Step Up Converter 12VDC to 24VDC
https://www.amazon.com/gp/product/B01M0IEYZJ/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
DC Power and Energy Meter with Current Shunt
https://www.amazon.com/gp/product/B013PKYILS/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
AC Power and Energy Meter with CT
https://www.amazon.com/gp/product/B00YY1KOHA/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
4 Pin Waterproof Connectors
https://www.amazon.com/gp/product/B01A6M8CD8/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
100 Amp 12VDC Circuit Breaker
https://www.amazon.com/gp/product/B076FYCRJ5/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
Hyclat 2-4 Gage Battery Disconnect (Anderson type)
https://www.amazon.com/gp/product/B01KHQR0K4/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
After several discussions here on TMC about generator support (and the lack thereof) and the difficulty restarting Powerwalls after low SOC shutdown, I decided that I wanted a way to make sure that the Powerwalls would not shut down during a prolonged power outage during the winter when my solar production is very low.
In order extend the Powerwall's backup runtime, you must use an inverter like a solar grid tied inverter that synchronizes to the existing power waveform. Most mobile power inverters don't do this - they generate their own free running waveform, much like a generator does. So, I started looking for an inverter that I could use. I came across this line of inverters that is listed for use with 12V solar panels or 24V battery. My first thought was to use a 24V DC power supply running from a mobile power inverter, into this grid tie inverter, then into a 240V transformer, then into an extension cord, to the Tesla Backup Gateway and through the solar CTs. I originally wanted this power injection to look like solar generation. However, I concluded after a short time planning the system that most of that was unnecessary. I found a DC/DC step up converter with a wattage higher than the grid tied inverter, so that eliminated a DC-AC-DC round trip. The bonus is that I don’t need the 240V transformer and I can use the existing outlet that’s next to the car’s normal parking space in the garage.
Strengths:
- DC/DC Step Up Converter is high efficiency and runs cool
- Powerwalls can handle significant split in charge/load on opposite phases (needs to be verified with grid disconnected)
- This grid tie inverter gets hot within a few minutes and seems to reduce output with temperature until the internal fan turns on. It is only about 78% efficient (AC out / DC in)
- Power output of the chosen components is barely strong enough to be useful (higher power versions are available or can duplicated on the opposite phase for better balance)
- Significant conversion losses stack up from the EV traction battery to the Powerwall battery.
Key Parts List
600W Grid Tie Inverter
https://www.amazon.com/gp/product/B071F5XJMD/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
UXCell DC/DC 720W Step Up Converter 12VDC to 24VDC
https://www.amazon.com/gp/product/B01M0IEYZJ/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
DC Power and Energy Meter with Current Shunt
https://www.amazon.com/gp/product/B013PKYILS/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
AC Power and Energy Meter with CT
https://www.amazon.com/gp/product/B00YY1KOHA/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
4 Pin Waterproof Connectors
https://www.amazon.com/gp/product/B01A6M8CD8/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
100 Amp 12VDC Circuit Breaker
https://www.amazon.com/gp/product/B076FYCRJ5/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
Hyclat 2-4 Gage Battery Disconnect (Anderson type)
https://www.amazon.com/gp/product/B01KHQR0K4/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1