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PowerWall technical discussion

Discussion in 'Tesla Energy' started by lolachampcar, May 1, 2015.

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  1. lolachampcar

    lolachampcar Active Member

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    Mods: Please roll this into any other existing thread on the technical side of the Powerwall discussion...

    So, The PW is a 350V-400V DC battery. Put differently, it is a 10KW-Hr version of the MS battery. This tells me that there are inverter companies out there working with Tesla to do 400V battery supporting systems. This would also make me think that those same inverters would manage PV to PW DC charging without having to go through the AC state (DC PV inverted to AC then fed to a charger for the battery which returns it to DC).

    First, does anyone have any knowledge of, or experience with this type of inverter/battery manager?
    Second, is it not now time to use all those salvage MS batteries in combination with the above described inverter to build VERY cost effective home storage systems?
     
  2. austinEV

    austinEV Active Member

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    I just realized the powerwall probably use the exact modularization as the model S pack. 85kWh/17 modules, 5kWh each, so 2 modules in each unit. (I think it was 17 modules, working from memory).
     
  3. trils0n

    trils0n 2013 P85

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    From teslamotors.com/presskit/teslaenergy

    "SolarEdge
    SolarEdge, a leader in the global PV inverter market, and Tesla partnered for the joint development of a PV storage and backup power solution for the worldwide residential solar market. Building on SolarEdge’s proven DC optimized inverter proven and Tesla's leading automotive-grade battery technology, the solution will require only a single SolarEdge inverter to manage both PV and storage functions."

    Doesn't say if it will go direct DC from panels to battery, but shares an inverter, so one less thing to buy for PV/Battery setups. Direct DC charging would also be interesting.
     
  4. Merrill

    Merrill Active Member

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    How does the powerwall connect to an existing PV system and if you are on TOU and store power at peak times in the powerwall do you lose the peak credit that your system is generating because it is going into the storage unit?
     
  5. scaesare

    scaesare Active Member

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    The 85 has 16 ~5.3KW modules.

    However, each module is 6 series connected sub-groups of 74 parallel cells. That gives each module a nominal voltage of ~22V. So two these packs in series would only render a ~44V unit, not 350V.

    It would take around the same 96 sets of series connected groups to render the same voltage. That may be something like 9 cells in parallel per group?
     
  6. uselesslogin

    uselesslogin Enthusiast

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    So for all we know the 7kWh module is the exact same as the 10kWh except the 7kWh module keeps closer to a 50% SOC.
     
  7. Saghost

    Saghost Active Member

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    #7 Saghost, May 1, 2015
    Last edited: May 1, 2015
    The inverter is going to be the interesting part of this.

    Conventional grid tie solar string inverters will happily take power from this battery pack and make AC with it - but they all have anti-island provisions built in for safety, so when the grid goes down your power will too, even with the battery packs.

    There are solar system inverters designed to work with batteries and provide power to a critical bus when the mains are down - but all of these I know of are designed to work with 12-48 volt battery systems.

    I'm not seeing why Tesla chose to go with a high voltage string for this application - it requires more safety precautions than a lower voltage, and won't produce the desired results unless you have a new string inverter designed to work with it.

    As far as using a Model S pack this way, the variable voltage of the solar string requires a dc dc converter to match it to the variable voltage of the battery pack for best results - which is included in the power wall.

    After seeing the details, unless more posts and options are forthcoming, I'm much more interested in this product for the way it will frame pricing for competition (like Enphase's soon to be released modular battery inverter sets) than in buying it myself.

    Three years ago I decided micro inverters were a better choice overall, which naturally affects the practicality of this system for me.
    Walter
     
  8. scaesare

    scaesare Active Member

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    I suspect this is true.
     
  9. lolachampcar

    lolachampcar Active Member

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    It is precisely for the reasons you mention that I find the approach exciting. It is more efficient to gently boost/buck a PV string to charge a 350-400V battery then it is to deal with a 48V battery (legacy of SLA). You are already dealing with HV DC on the PV side so simply adding another run from the battery to the inverter is not that big of a deal (but it is a deal). What is even more exciting for me is using all the infrastructure created to support the dinky PW to manage a real battery like an 85 KW-Hr MS battery. Now we are talking useful!

    Of course, the need for 400ish Vdc of PV output does preclude using MPT per panel for maximum array yield.
     
  10. Saghost

    Saghost Active Member

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    If you're in a system that uses a DC string, then you're right it doesn't greatly reduce the safety. Not having that kind of power floating around was one reason I went micro inverter, though.

    I suppose it is true that you can always hook the battery directly to the panels, and accept the reduced output that you'll have at whatever voltage the battery sets. Though in that case you'll need some sort of mechanism to prevent overcharging - either an inverter larger than the panels' peak output so you can always dump it all to the grid, or possibly some massive resistors that you can drop 400V DC across to dissipate power.
    Walter
     
  11. Owner

    Owner Active Member

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    For those of us with 8 year old solar systems and older inverters (mine got replaced under warranty 5 years ago), this battery power pack will likely not work seamlessly?

     
  12. vdiv

    vdiv Chief Grump

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    If the expectation is for the average Joe to figure this out and integrate a system of their own then the uptake on the PowerWall would be rather small. I presume it was designed so that it can be added to an existing PV system regardless if it has micro inverters or a central one. For backup capability the system needs to include a grid disconnect contactor, similar to what a Generac installation would provide.
     
  13. Saghost

    Saghost Active Member

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    Too many things I don't know. However, this system appears to be designed to be hooked to the high voltage DC side of a traditional string system like you probably have. My guess would be it'll work great with your current system and inverter - as long as there's grid power.

    You'd have to make significant changes, likely including replacing the inverter, to use it as a backup battery for when the grid is down.
    Walter
     
  14. CapitalistOppressor

    CapitalistOppressor Active Member

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    I'm pretty much agnostic on this issue, but while it's certainly possible and one way to get the claimed performance I don't think that having the same number of cells makes enough sense.

    First, Tesla is charging less for the 7kWh system. Why would they do this when the main cost driver is the number of cells?

    Second, presumably the 7kWh unit has more expensive/capable/durable power electronics and cooling to handle the much more intense cycling. Why would Tesla charge less for this, especially when they are either taking a loss on the cells for the 7kWh version? (or is it that they have low margins on the 7kWh version and absurdly high margins on the 10kWh version?)

    Alternatively, the systems are identical and the performance differences are software based. But then, why have two systems, and why charge less for the 7kWh system? Seems cheaper for Tesla to just have a single product and install a switch.

    None of this makes much sense to me. The price/kWh for the 10kWh version is $350 and the more capable 7kWh version is ~$430/kWh. The simplest reason is that the 7kWh version has much more expensive/capable/durable power electronics and cooling in order to get the claimed performance.

    But the dark horse alternative is that there is also a chemistry difference between the cells in the two systems. The claimed performance points to ~500 charge cycles being under warranty for the 10kWh version and ~2,500 cycles being warrantied on the 7kWh version. This leaves open the simple possibility that the cells themselves use a different chemistry in addition to the likely changes in the power electronics and other components. I don't like this option because it slightly complicates production and potentially reduces your savings from producing cells at scale. But its still a possibility.
     
  15. mdemetri

    mdemetri Member

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    Does anyone know whether it will work with micro-inverters, such as Enphase?
     
  16. Saghost

    Saghost Active Member

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    Lots of things we don't know, and this might be the most important one. There was nothing in the presentations I saw which suggested it would work well with micro inverters. I think to work with them, it'd have to have a bi-directional inverter/charger module - possibly integrated into the transfer switch to put the critical loads off of the grid when the grid dies.

    Nothing was said about such an inverter - but nothing was said about the transfer switch you have to have, either, and it might all be one box.

    I know some existing inverter/chargers (design for lead-acid systems) will "AC couple" with micro inverters - they sit between the micro inverters and the grid, provide the reference sine wave the inverters have to see before they deliver power, and if they need to stop the solar production, they knock the micro inverters offline by forcing the frequency of the reference outside of the range programmed into the inverters.
    Walter
     
  17. lolachampcar

    lolachampcar Active Member

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    "I suppose it is true that you can always hook the battery directly to the panels, and accept the reduced output that you'll have at whatever voltage the battery sets. Though in that case you'll need some sort of mechanism to prevent overcharging - either an inverter larger than the panels' peak output so you can always dump it all to the grid, or possibly some massive resistors that you can drop 400V DC across to dissipate power.
    Walter"



    Actually, a small buck/boost circuit controlled by the inverter. These circuits will decrease or increase a DC voltage and are reasonably small/efficient and cost effective in the types of current levels we are looking at here. The smart inverter would take, for example, the instantaneous MPT output of your string PV system of 22KW and apportion 7 KW of it to power your house (ACs blasting away in the middle of the day) with the remainder going to the battery. A cloud cruises by and the inverter sips a bit off the battery. The sun comes back and power goes back into the battery. As PV falls off in the afternoon into evening, the battery supplies the house. If you are using a MS battery, you have some reasonable reserve although you would need more than one or an active home wide power management capability if you wanted to survive a rainy day without tapping into the grid. If it gets too rainy for too long, charge your battery off the grid at night and power your house during the day.

    At least this is my ideal scenario.
     
  18. mhan00

    mhan00 Member

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    I don't know much about this stuff, but I do know that Batteries tend to degrade faster the deeper the cycles you put them through. Could the 7kwh and 10kwh battery be the same battery, just with different software to keep the "daily cycle" from using all 10 kWh, thus letting it withstand more discharge cycles?

    edit - I see that others have already speculated this. Late to the party, I guess.
     
  19. tga

    tga Active Member

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    I'm also in the "how will this work with my microinverters" camp. I have 36 250W panels with Enphase microinverters (9kW DC, 8.1kW AC).

    I don't have net metering (I'm on a municipal power company, so they are exempt from the net metering laws), or TOU (also not offered). I buy power for $0.143/kWh, and sell for $0.053/kWh (sort of a reverse TOU, since I'm incented to use my own power during the day).

    I think I'd want some sort of charger/inverter that can measure my net inflow/outflow, and dynamically scale up/down charging or inverting depending on flow into/out of the house. Others may want to add some sort of scheduling to prioritize based on TOU rates.
     
  20. iridium

    iridium Member

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    Did you notice the operating temp is up to 110F? That kind of rules it out for Arizona summers.. (and probably many other places in the southwest)
     

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