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Can a 7kWh and a 10kWh PowerWall be used together in a single installation?

Discussion in 'Tesla Energy' started by ecarfan, May 10, 2015.

  1. ecarfan

    ecarfan Well-Known Member

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    #1 ecarfan, May 10, 2015
    Last edited: May 10, 2015
    I have not seen this question addressed by Tesla or mentioned in any article I have read about the Powerwall:

    Can a 7kWh and a 10kWh Powerwall be used together in a single installation? On a single inverter? Does the Powerwall BMS support them being used together?

    I assume the answer is "yes" but I have not seen any statement from Tesla to that effect, and right now the positive response to the new products is so overwhelming, and it is months in advance of products actually being available, that I cannot get an answer fromTesla.
    If the answer is "yes" that would allow a residence to have a battery backup for occasional use and a battery for daily cycling so the residence could be off grid.

    Note: for those who are not aware of the details of the two Powerwall products and think one is simply a larger version (and I realize that many TMC members do understand the differences), see Powerwall | Tesla Home Battery
    --------------
    Models
    10 kWh $3,500
    For backup applications
    7 kWh $3,000
    For daily cycle applications
    -------------------

    The two products use different battery chemistries, have different guarantees in terms of the number of charge cycles they are rated for, and are for different applications. One is not simply a higher capacity version of the other.
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I haven't seen this addressed either, but I'd guess that it would be possible if there were two separate paths. I doubt you could hook them up in series. It would probably be less expensive to just have more of the 7 kWh modules (unless you're planning to max out the 7 kWh and then you should probably look at a PowerPack.)
     
  3. CantaMia Ron

    CantaMia Ron Member

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    Look's like you can only get this system if get the Solar City Solar pannel's
    and do not think you can get off the grid.:scared:
     
  4. Cosmacelf

    Cosmacelf Active Member

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    I think a lot of these types of questions can only be answered when the manufacturers release their Powerwall compatible inverter/chargers.
     
  5. dhanson865

    dhanson865 Active Member

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    Maybe the extra type is just like an extra "string" to the inverter

    Inverter
    Powerwall 7 kwh x3
    Powerwall 10 kWh x2
    Solar Panel String 1
    Solar Panel String 2

    So you modularly add each of the same type into one configuration then separately add one or more of the other configuration.

    Meaning so long as your inverter or cabling setup can accept another logical branch you can mix and match as you like.

    but hey I'm no electrician and I don't do Solar PV for a living either.

    I'm just thinking that it'd make sense that there ought to be some way to configure the overall system to use multiple types of batteries.
     
  6. apacheguy

    apacheguy Sig 255, VIN 320

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    I was told here on the forums that the 7 kWh version may be used as backup. It's just that the 10 kWh is restricted to backup and cannot supply on demand power otherwise. Why not just get a 7 kWh or two if necessary? I see no reason to mix and match.
     
  7. blakegallagher

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    I agree that if you are going to get the 7kWh for daily cycling it would most likely be better for both batteries to just get two 7kWh packs. That being said until all of the details on each pack is released it is kinda hard to say.
     
  8. Ampster

    Ampster Member

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    Yes, anything is possible with the right inverter logic and some relays or contactors. As Jerry33 has suggested I would buy two of the 7kWh units and program my inverter to leave more room at the bottom of the daily cycle. As others have said it will be interesting to see the new inverters that will be offered in response to this less expensive battery solution. I know at least one hybrid inverter that can be programed that way but it is a 48v model but the logic is already there in the electronics.
     
  9. ecarfan

    ecarfan Well-Known Member

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    #9 ecarfan, Jun 7, 2015
    Last edited: Jun 7, 2015
    The reason is, as I understand it, that the 10kWh version is only for backup (should only be used occasionally because of the battery chemistry) and the 7kWh version is only for daily or near daily cycling (again because of the battery chemistry). Now, that could be incorrect information. I really don't see why the 7kWh version could not be used for backup if needed. But so far there is so little detailed info available from a Tesla that it's hard to know. Here is what the Tesla website PowerWall page says:

    Models
    10 kWh $3,500
    For backup applications
    7 kWh $3,000
    For daily cycle applications

    I will be at the shareholders meeting next Tuesday and I want to ask Elon this question:

    "I was excited to watch your presentation during the Tesla Energy launch event recently. I would like to know if the 10 kWh and the 7 kWh PowerWall models can be installed in series with a single inverter as part of a residential solar system and if the battery management software is designed to manage the power draw from the two different models working together, based on the needs of the home?"
     
  10. Ampster

    Ampster Member

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    #10 Ampster, Jun 9, 2015
    Last edited: Jun 9, 2015
    That is kind of a "down in the weeds" question to be asking a CEO. He is the guy responsible for implementing a strategic vision for the company. Your question has no relevance to other shareholders who want to hear how the vision will be implemented. The people to ask are the inverter guys from Solar Edge. It is in the inverter where the decision about which pack, or conversely how much of a dual pack to use for daily cycling. The battery management system only manages the charging and cell balancng functions.
     
  11. ecarfan

    ecarfan Well-Known Member

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    Yes it is a rather specific question, but I learned a lot today about the PowerWall from Elon and JB's response to my question. Elon welcomes derailed questions about Tesla products and I think he was glad to have the opportunity today to discuss and clarify the difference between the two PowerWall products. It can be difficult to get anything new out of him when he is asked a "strategic vision" question. He is very consistent in his answers to such questions: today for seemingly the hundredth time I heard him repeat the Tesla mission statement about "accelerating the advent of sustainable transport". Great vision for the company, and we have known that for years.
     
  12. nwdiver

    nwdiver Active Member

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    Perhaps.... but perfectly appropriate for the 'Product Architect' :wink:
     
  13. Ampster

    Ampster Member

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    ....and the answer is?
     
  14. Cosmacelf

    Cosmacelf Active Member

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    Frankly, I don't think Elon really knew the answer. He said "no", but then he backed off that a bit. I think the capability is still being developed right now, and part of the engineering is being done at the inverter manufacturers. It is the inverter that tells the Powerwall that a grid failure has occurred.

    Just thinking out loud here. Assuming a SolarEdge inverter and architecture. The backup Powerwall battery is full as it must be all the time for it to be a backup battery. The grid goes down. The automatic transfer switch trips isolating the house electrical system from the grid. The inverter notices the grid has gone down, and must now somehow match power loads from the DC side to the AC side. Ie. The AC side needs 5kw of power and the inverter tries to grab 5kw of power from the DC side. Since the Powerwall and the solar panels are on the same power bus, somehow the inverter must try to grab the 5kw just from the solar panels if they are producing enough power, but then grab the rest of the power from the Powerwall should a cloud pass by. This might be as easy as telling the Powerwall that it is now in power distribution mode and for the Powerwall to supplement power as needed to the DC bus.

    Anyways, in such a system, I don't see any difficulty in having mixed daily cycle batteries and backup batteries, except that the inverter software needs to know which batteries are which and be able to communicate individually with them. Perhaps their architecture isn't robust enough for individual battery addressing. If so, that's an unfortunate limitation. Or it could be that v1.0 of the inverter software for the Powerwall just doesn't support mixed battery types.

    We will just have to wait and read the actual product technical manuals when the products are finished...
     
  15. Cosmacelf

    Cosmacelf Active Member

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    Here's Elon's answer about combining the 7 kwh and 10 kwh Powerwalls:

    "Well, combining them actually wouldn’t be super, super great. It’s going to be -- I mean actually we could make the software do that but it doesn’t currently do that. It would treat them both -- it’s simply going to treat it, if you had a 7 and a 10 like a 17 kilowatt hour system and depending upon what your draw is, it would currently I believe draw equally from both. Maybe JB should join me on the stage and answer some of these questions."
     
  16. ecarfan

    ecarfan Well-Known Member

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    #16 ecarfan, Jun 10, 2015
    Last edited: Jun 10, 2015
    It was as if I was posing a use case that Elon had not considered. He was assuming that a customer would either be on a fairly flat rate and so would only want a backup PowerWall, or someone was on a time of use schedule for electricity costs and then would only want a daily cycle PowerWall.

    But if you are on a time of use rate plan and want a backup battery, why not have both PowerWalls, one for backup and one daily cycle to power the house in the late afternoon and evening when rates are at peak?

    He and JB seemed to say that having both PowerWall versions in one installation was possible. But if the firmware in the inverter had to be set up to manage both types of PowerWalls in a single installation, then I will have to wait to see which inverter manufacturers says "yes our inverter can do that".

    JB made a point of stating that the 10KwH PowerWall could be cycled over 1,000 times during its useful life, but no way would I use it for a daily cycle function.
     
  17. electracity

    electracity Active Member

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    "Can a 7kWh and a 10kWh PowerWall be used together in a single installation?"
    You would probably need separate inverters. The powerwalls are designed to be connected in parallel. All the powerwalls in a circuit need to be on or off. You probably couldn't have the 10kWh being optional in that circuit.
    How a group of powerwalls will balance the batteries is an interesting question. Its the balancing that is the tricky part. Musk knows this of course. But his engineering brain was flashing to how it could work.
     
  18. ecarfan

    ecarfan Well-Known Member

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    In Elon's and JB's response to my question they never mentioned a dual PowerWall (10kWh and 7kWh) setup would require two inverters. I do not think you are correct in assuming that two inverters would be needed.
     
  19. schueppert

    schueppert Member

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    I don't think any of those three statements is true. The Powerwalls work very much like the SolarEdge optimizers: they connect in series, but the total voltage of the string is constant. And so it's perfectly possible to control the voltage and power delivered by each unit. My interpretation of Musk's comments is that he is saying it is possible, can be done with software, but isn't a use case they have considered and so is not currently supported.

    I have to say I do not understand the use case for combining the units. The 10 kWh supports backup uses, the 7 kWh supports backup and time-of-day shifting use cases. If you need/want to support both use cases, buy the 7 kWh units.
     
  20. electracity

    electracity Active Member

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    How could powerwalls be connected in series?
     

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