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Inspired by Tesla Powerwall, began work on how to make the Earth Greener

Discussion in 'Tesla Energy' started by ScanTech, Feb 3, 2016.

  1. ScanTech

    ScanTech Member

    Feb 3, 2016
    Norrkoping, Sweden
    Hi guys,

    This is my first thread on Tesla Motor Club, so enjoy and Elon, I really hope you will find this thread, I probably will notice and if not if will try other channels.
    This since I would like to present my idea in person and Tesla is the first company I would like to give a chance to "bite".

    I´m a Swede, so don't get stuck on grammar...

    Once Tesla released the Model S, I got up my eyes and said to myself "One day, I will have that car" and since then I have kept an eye on Tesla
    When the Powerwall was released last year, my interest got even bigger for Tesla and thought, "yes they are on a good way...but will we get there in time?" (The Green Earth)
    Struggling in a business where authorities are daily obstacles, time flied and it wasn't until the early of January 2016, when a client putted me in a bad position, that I took the time to start work on my ideas.

    So why is Energy Storage interest me? Well, my view on Power Storage Industry today could be explained by below comparison or parable.
    Imagine for a while that the Power Storage Industry (PSI) was a limited Car Industry, were Volvo made cars that only could be fueled up in Exxon, Ford could only be fueled up in Texaco and Dodge could only get fueled up in BP. That is almost how I see on PSI, limited, squarish and in lack of a standard or someone who want to take charge to push the industry to an open "standard".
    Without that, how will we make it in time....?

    So the lack of an open standard is my fuel and have created my great idea, but on the way there, the facts journey began of course with a Google search of "Tesla Power Wall" and the straight in to Wikipedia, where I started to absorb like a magnet.

    Really liked what I read under Technology "the chairman, CEO and product architect of the Tesla company, promised not to start patent infringement lawsuits against anyone who, in good faith, used Tesla's technology for Powerwalls as he had promised with Tesla cars". "in good faith", fit me like a glove, I thought and began with some basic questions to myself.

    1. How does Tesla manage to give 10 years warranty with a 60 % capacity guarantee after those 10 years, based on the 1 000 - 2 000 cycles that the NCA cells has.
    2. How can I design a Power Storage Unit that is better, more flexible, more adaptable and with a much greater capacity.
    3. How can I provide even better warranty terms.
    4. How can I get it to be more cheap to consumers, so that they actually can save money within 10 years or less.
    5. How can I make it more maintenance and service friendly
    6. How can I help the Industry to set a new standard for Power Storage.

    I will share how I answer myself on question 1, based on how I twisted and worked with the numbers, based on the behavior and characteristics of Lithium-Ion Batteries/Cells.

    First the basic technical info about an average Lithium-Ion 18650 (NCA) Cell:
    Size: 18mm x 65mm, Weight: ~45g ± 2.5 %, Nominal capacity: ~1 000 – 3 400mA,
    Nominal voltage: ~3.7V, lifespan: 1 000 – 2 000 theoretical cycles (2.7 – 5.5 years), when charged with 4.2V (charging voltage is +13.5% higher than the nominal voltage, to reach 100 % of the cells capacity).

    Above gave me the idea to calculate the following way, since I could find out online that there is 888pcs (2pairs with 6 groups) cells in the product and the system is using 48V plus a DC to DC conversion.:

    Tesla is using 888pcs that probably are the 3 000mA (3.0Ah) Panasonic NCA cells (as is used in Model-S) for the Powerwall with 1 000 – 2 000 nominal charging cycles.
    These cells are divided in 2 pairs (serial), each with 6 groups (serial). One group is a giant 3.7V battery with some 74 cells in parallel.
    The nominal capacity is 888 cells á 3.0Ah= 2 664Ah x 3.7V= 9 857W. (The 10kW Powerpack?)
    The nominal battery voltage for the system is: 2 (pair) x 6 (groups) x 3.7V= 44.4V.

    So how then, can Tesla give a 10 years warranty (5 000 cycles) with a 60 % capacity guarantee on their Powerwall?

    If the cells charging voltage is lowered to 4.1V (11% higher than the nominal voltage) the upside is that cycles increase to 2 000 – 4 000 theoretical cycles (5.5 – 11.0 years) but the downside is that the cell only reach 84 % of its capacity. If the charging voltage is lowered to 4.0V (8% higher than the nominal voltage) the cycles increase to 4 000 – 8 000 theoretical cycles (11.0 – 22.0 years) but then the cell only reach 72 % of its capacity.

    The 888 cells in the Powerwall in two pairs, each with 6 groups of 74 cells, creates a battery with a nominal voltage of 44.4V and since the system is a 48V system it gives a charging voltage that is 8 % higher than the nominal voltage, ending up with only 72 % capacity.
    So the 888 cells has a nominal capacity of 9 857W x 72 %= 7 097W (7kW) as is what Tesla specify.

    (If the battery only have an average of 75 % depth of discharge, the charge cycles increases with another 1 – 2 times and if the battery have an average of 50 % depth of discharge, the charge cycles increases with 2 – 3 times.)

    So with above estimated solution on the life cycles, it wasn't that big of an issue any more to solve how to supply a +10years warranty, first problem solved.

    One question still bugged me and the brain started to do numbers again today during my ride back to home, after an investments meetings with a client/business angel in Stockholm.

    Why the 888pcs of cells in the Powerpack, as both are a repdigit and a strobogrammatic set of number. Was the designer a gamer?
    Well, I ended up with the conclusion that it would have been really freaking funny if the Powerpack would have been loaded with the Samsung ICR18650, 3.75V 3 200mA cells instead.

    Here is why:
    1 group of 74pcs cells á 3.75V, 3 200mA, gives: 74 x 3.75V x 3.2A= 888W, bite on that.

    12 groups á 888W would have been 10 656W in nominal capacity, as would have been on the right side of the 10kW specified.

    Well, I hope someone enjoyed above and my own estimated answer on question 1 as is a wild guess, since I never have seen the product live, let alone opened it.
    My own answers on question 2-6, I hope will change this industry and accelerate the journey to a greener world and it would be awesome to present it in person for Tesla management.
    If not, I am quite sure that the less innovative, but still smart friends in China will love it.
  2. strood

    strood Member

    Feb 4, 2016
    To clarify:

    Powerwall uses NMC not NCA which is a manganese heavy variant of lithium ion batteries that gives a deeper depth of discharge and more frequently cycling.

    The warranty on the 7kWh daily cycler is 3650 cycles (once per day) but battery is rated to 5000 cycles 13.7 years.

    At the moment unless users have power costs in excess of .40cents per kWh it isn't likely to break even in 10 years.
  3. ScanTech

    ScanTech Member

    Feb 3, 2016
    Norrkoping, Sweden
    Strood, you are 100 % correct, it was my type error in the late midnight.
    Powerwall uses NMC and Powerpack is using NCA, according to the source. Also well aware about the differences between LCO, LMO, NMC, NCA, LFP and LTO :)

    Sites I have visited says there is a 5 000 cycle warranty, I really don´t find any official info about cycle warranties on (haven´t looked that hard).
    But warrant for 5 000 cycles wouldn't be an issue if they have solved it as I think they have done...

    Well, those 0.4cents per kW we can cut in half, adapting my thinking.
  4. ScanTech

    ScanTech Member

    Feb 3, 2016
    Norrkoping, Sweden

    I forgot to say that NMC cells defiantly are not rated to 5 000 cycles based on the nominal voltage and capacity.
    This is how it works, NMC chemistry is rated 1 000 - 2 000 cycles when charged with 4.2V, if we lover the charge voltage to 4.1V, you will increase the charge cycles to 2 000 - 4 000 and if we lover even to 4.0V (Tesla Voltage for Powerwall, if I haven´t calculated wrong), we would get some 4 000 - 8 000 cycles.
    NCA starts with 500 cycles when charged with 4.2V, 4.1V gives ~1 000 cycles and if charged with 4.0V it would give ~2 000 cycles.
    LTO on the other hand is rated to 3 000 - 7 000 cycles, but sucks on capacity (70-80Wh/kg compared to the NMCs 150-220Wh/kg or NCA 200-260Wh/kg)

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