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Minimum Battery Level to Start Solar

Discussion in 'Tesla Energy' started by eml2, Oct 25, 2017.

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  1. eml2

    eml2 Member

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    During a power outage, the Backup Gateway will isolate the house from the grid, and the Powerwall will allow the solar inverter to continue to operate.

    For an extended outage, when the Powerwall is completely drained overnight, will it able to start the solar inverter the next morning when the sun is shining?
     
  2. Bigtanuki

    Bigtanuki Member

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    I see you aren't getting any answers. The scenario you described sounds like it might not be able to start up the solar until the grid came back. Obviously, that would be bad news for an extended outage. I would like to see the PW be able to stop discharge at some point (~5%). Either determined by the app or hard coded in the firmware. This would allow a system restart of the grid tied solar once the sun comes back up. Seems like it would have to be a manual process unless there was a way to trigger the PW back into discharge mode based on sunrise/sunset information from a database or just tap into the history of the solar from the app/GW to determine when the solar has actually started contributing power over the last month or week. The second option seems like it would be the most self contained and have little or no reliance on an internet connection.
    In any case, a "dark start" with no power input from either the PW or the grid would neuter most grid tied solar systems.
     
  3. fresnoboy

    fresnoboy Member

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    I think the way to support this and deal with the off grid case is pretty straightforward. You have a transfer switch that connects the house load on one side to either a genset (and/or Grid), or the solar inverters AND the PW's on the other pole. By separating the loads from the inverters and the batteries, the PW can use just a slight bit of power to generate a voltage and frequency that would be good enough for the inverters to switch on and begin charging the batteries.

    After the batteries hit a certain amount of charge, or the solar is able to generate enough surplus power, the transfer switch cuts out the genset and switches the home back to Solar and PW's.

    You could do this with the Grid too instead of a generator, or a grid/genset combination with it's own transfer switch. Given the size of the PW's max output and most solar systems, a 100A switch would probably be more than plenty, or even 50A, and they are pretty inexpensive. This would be way cheaper than a 200A or 400A whole house disconnect.

    The key point is you have to isolate both the solar and the batteries from the house loads, else you end up in the case of "I've fallen down and can't get up".

    In fact, you could build this today if the Tesla guys just opened up their control system to 3rd party integration.


    Thx
    mike
     
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  4. Parzival

    Parzival Member

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    There is already an API in the gateway to turn off the Powerwalls and drop the loads. It would be pretty trivial to build something that calls that API once the SoC drops below a defined threshold... like maybe an hour's worth of effort or less. Before I do it, I want to discharge my system and see how it behaves. It may not be necessary.
     
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  5. fresnoboy

    fresnoboy Member

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    I think what you want is something where the PW's or a "gateway" tells the transfer switch what to do, not the other way around. I guess you could have some 3rd party device that is self powered command the PW's and the transfer switch all over the local network. Can the API tell the PW to turn on regardless of it's battery discharge state?

    Can you read the current levels of the CT's in the system through that API?

    thx
    mike
     
  6. Shygar

    Shygar Member

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    Wouldn't you just have to always keep a small amount of energy in the powerwall for this scenario?
     
  7. Parzival

    Parzival Member

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    I don't think you can turn it back on once it's powered off :)

    Yes
     
  8. WannabeOwner

    WannabeOwner Active Member

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    Longshot - I have no idea what I'm talking about in this context ...

    ... we have Solar Thermal as well as PV. Solar Thermal works on dT temperature difference - if the solar collector is more than xC hotter than the tank then the circulation pump comes on. But ... there is cold water in the connecting pipe, and maybe the sun-power is not yet very strong. So the high dT causes the circulation pump to come on, a few minutes later the dT is too low, so it shuts off again (controller has separate values for "Come On" [high] and "Turn Off" [Low]). This repeats a few times until everything is hot enough to support continuous pumping ... or a cloud passes over ... or dusk falls.

    Perhaps when PowerWall charge is < x%, absent some sort of "sunshine-power-detection" sensor, then it could do "Turn on 1 minute in every 10" until it sees that battery charging exceeds consumption, and then stay on until battery > y% (and then revert to normal behaviour)
     
  9. eml2

    eml2 Member

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    That is what I am thinking too. However, you will have to manually switch off the PW via the physical switch (or with 1.9.1 the red "Stop Powerwall" button).

    But what I want to know is when PW is down to 0%, is its SoC really 0%, or does it still have some reserve where it will be able to start the solar inverter? During normal operation when the grid is up, once PW is 0%, it goes into standby mode and won't output any power. Will this behavior change when the grid is down?
     
  10. Shygar

    Shygar Member

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    I guess that's what I really mean. The powerwall can read 0% and stop sending any power out, but the software could still keep a small reservation of energy in there for this scenario. Seems like a no brainer to me to have the powerwall software do that automatically, just not sure if it does that today or not.
     
  11. fresnoboy

    fresnoboy Member

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  12. BobPaul

    BobPaul Member

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    I found this answer in the Powerwall FAQs: To resume Powerwall operation, turn off all lights and appliances in your home, then toggle the on/off switch located on the side of Powerwall. If, after an extended utility outage, Powerwall is depleted, then wait for the next sunny day before toggling the on/off switch.
     
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