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Powerwall 2.0 Backup Runtime Extender

Discussion in 'Tesla Energy' started by miimura, Aug 20, 2018.

  1. miimura

    miimura Active Member

    Joined:
    Aug 21, 2013
    Messages:
    3,078
    Location:
    Los Altos, CA
    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.

    Power Export Connection.jpg

    Powerwall Runtime Extender Test.jpg

    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)
    Weaknesses:
    • 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
     
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  2. strangely

    strangely Member

    Joined:
    Mar 29, 2017
    Messages:
    57
    Location:
    SoCal
    I was thinking of doing something like this using our Nissan Leaf and a couple of enphase micro inverters, since they can output spilt phase 240V. I went as far as buying some spare M215s for this purpose off of eBay and had them running off of some 36V Lithium Ion battery packs a few times as a test, however I ended up tripping the inverters and putting them a faulted state and gave up for the time being.

    So that said, curious why you haven't considered some enphase inverters yourself? All you would need to do is choose 12-36V DC DC converter instead and you should be good.

    Those cheap 120V inverters don't seem to last too long.
     
  3. miimura

    miimura Active Member

    Joined:
    Aug 21, 2013
    Messages:
    3,078
    Location:
    Los Altos, CA
    Since I have Enphase on my roof, Enphase was my first thought for this project. However, on a $/W basis, it just wasn't going to happen. I even looked at used ones on eBay but I came to the conclusion that if they were taken down they were likely bad and I didn't want to do component level SMD repair on it too. Since this is an emergency backup kind of situation, I wasn't going to invest the big money in something that would last 15 years 24/7.

    Also, part of the reason for this particular test was to show that you don't have to put the power in at 240VAC, the Powerwalls can easily handle 500W of imbalance on one side of the neutral. In any case, there are other units available that are larger, output 240V, and have adjustable battery parameters like adjustable Watt output.
     
  4. strangely

    strangely Member

    Joined:
    Mar 29, 2017
    Messages:
    57
    Location:
    SoCal
    Ok well if you do want to give this a try, then there is a company called renvu near you that I bought most of my PV system system components from, and they sell the M215s for $57 which is pretty respectable price wise.
     

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