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Roadster battery pack specs?

Discussion in 'Roadster: Technical' started by iisjsmith, Oct 25, 2006.

  1. iisjsmith

    iisjsmith Member

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    Does anyone know the capacity of the Roadster's ESS?

    We know it is made up of 6831 18650-form-factor batteries, but there are different types and nowhere on the website does it give the specs for a single cell. I mean, an HR18650 cell can be 1300-1400mAh whereas a LR18650 can range from 1800-2200mAh.

    Also, it doesn't give any info as to how many of the 6831 cells are linked in series versus in parallel. I mean all 6831 3.7V cells can't be linked in series, otherwise you would have a 25275V pack!!

    I'd like to know the specs on a single cell...and the total capacity of the whole ESS.
     
  2. WarpedOne

    WarpedOne Mainecoon Butler

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    If you study all TM material you learn that battery pack:
    - stores about 50kWh of electric energy
    - operates at nominal 375V
    - consist of 6831 batteris that are organized into eleven separate modules with its own control electronics

    So much from top of my head. For some more, dig into the site:) It has tons of info, but it is quite widespread.
     
  3. iisjsmith

    iisjsmith Member

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    Yea, I got all that info a while back. I also know it weighs 450kg and will deliver up to 190kW of power.

    What I'd like to find out is the capacity of the entire pack, the model number and/or specs of the individual cells, and how those cells are wired together (how many serial and how many parallel). Because I keep running the numbers through and I cannot figure out how they use 6831 cells at 3.7V each and only end up with 375V. That's only 102 cells linked in series. So they either linked a whole mess of batteries in parallel or the cells they are using aren't 3.7V each.

    I'm just putting this out there, in case anyone comes across this info.
     
  4. Embassy

    Embassy Member

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    The battery pack is almost certainly 99 cells in series by 69 parallel strings - it's really the only way to make the numbers work out. If you factor 6831 you get 3 x 3 x 3 x 11 x 23, which prettty much tells you how they designed it alongside the 375 nominal volts figure. The difference between 102 cells and 99 in series is probably the difference between 3.7 volts per cell and 3.8, which is reasonable for "just off the charger" numbers.

    Here's the part I don't get. To justify a 50 kWh battery pack energy, and a 200 kW peak power, you need to have batteries rated for 4C discharge. You can calculate this for anyone's EV design by dividing the maximum power by the total energy. If the voltage is a nominal 375 for the pack the capacity of each cell has to be about 1950 mAh per cell. That's a good "real world" figure for a nominal 2000 mAh cell.

    I don't see a battery like that on the market anywhere outside of Lithium Polymer cells. The 18650 cells I can find in the 2000 mAh class are rated for no more than 1.5 C discharge.

    So what cell specifically is Tesla really using?
     
  5. danny

    danny Administrator

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    Welcome Embassy and thanks for the info.
     
  6. Embassy

    Embassy Member

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    Thanks, Greenspeed.. it's really just reverse engineering, not any actual "info" per se.
     
  7. iisjsmith

    iisjsmith Member

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    So if each cell is 2000mAh and 3.8V we get a battery pack with a 138Ah capacity and 376.2V. That works out to 51.9156kWh. Pretty close to the numbers Tesla Motors gives us.

    But what about the fact that they say the ESS is made up of 11 sectors of 621 cells each? I guess you could just arrange them that way physically and then wire them 99S69P. But that seems awfully complicated. I'd think you'd want to keep the 11 sectors distinct, so you minimize the connections going from sector to sector.

    I have seen 18650 cells with 10C discharge rates, but they have lower capacity and voltage levels. For instance, on the BiPowerUSA site (www.bipowerusa.com/products/li-ion.asp) they have a cylindrical cell called the HR18650. It's got a 1300-1400mAh capacity and 3.6V, and it can discharge up to 10C, while the normal 3.7V 2000mAh cells can only discharge up to 2C. So you are getting half the capacity at the same weight but with 5x the C rate.

    I have also seen a pack called the Flion 1200, used for RC applications. It also has a 10C discharge rate, and has a 1200mAh capacity and 3.7V.

    So if Tesla is using this kind of high discharge cell, our numbers suddenly look a lot different. Now if we have 69 parallel strings we are only looking at 82.8Ah capacity. We would need 115 parallel strings to get back to the 138Ah capacity, and that doesn't leave us enough cells to make the 375V.

    I'm going to try another plea to Tesla Motors to let us know what cell they are using. I mean, it can't be THAT secret. The value of their product is the engineering behind the whole ESS...not just what cell they use.
     
  8. Embassy

    Embassy Member

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    Well, I have a hypothesis that seems to fit all the facts we know about .. try this on:

    The application is a 4C discharge application, we know the pack is about 375 volts and we know it's 6831 cells. Clearly that's 99 series by 69 parallel. But the 10C cells you can get in the 18650 format have too little energy, and the 2C cells have too little discharge rate.

    I think they're using both kinds in parallel. Specifically, I think there are 47 strings of 99 2300 mah cells that have 2C discharge (such as the Sanyo 18650F), and 22 strings of 99 1500 mah cells that have 10C discharge characteristics such as the Sanyo 18650W. That stack gives you 536 Amps at maximum, about 52 kWh of total energy, 141.9 amp hours of capacity. I'm guessing each cell is probably two or three bucks, giving a total of 20K worth of cells in the battery pack.

    Building a mixed-cell battery pack like this would be a significant challenge but the benefit is the ability to tailor capacity and discharge rate to the requirements accurately, and to use commodity cells. I guess if it were me I'd approach the problem with a current limiter on each string - a 4.6 amp limiter on the low rate strings and a 15 amp limiter on the high rate strings. That way, you can always draw the pack down at 1C or 2C.. but when you step on the amps you get an extra surge from the high rate strings.

    If this is what Tesla is doing, then they have a right to be very proud of a tricky bit of engineering. Keeping a pack like this balanced and charging it safely is a very impressive achievement.

    If that's actually what they're doing. I'm not sure I see another way to go about it, frankly.
     
  9. malcolm

    malcolm Active Member

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    I think we have a problem in assuming that the cells are charged to their nominal voltage values.  Take a look at the bar chart/graph just over half way down this page:-

    http://www.mpoweruk.com/bms.htm

    Typical Hybrids only charge to 80% in order to leave enough headroom for Regen braking.

    Under-running the batteries on pure EVs may have further benefits in terms of heat extraction and charge balancing.

    The trouble is, we have no idea how much headroom (if any) Tesla have built into their battery system. 
     
  10. Embassy

    Embassy Member

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    Malcolm, I don't think a pure EV needs regen headroom the same way a hybrid does. The regen energy comes out of the battery pack in the first place rather than from the gas tank as is the case with a hybrid. And once it comes out of a charged battery pack then it has plenty of headroom to go back in, given the inevitable losses. I suppose there's a "charge up at the top of a big hill" scenario that might change matters but if that were the case I would just make sure NOT to regen and switch to wheel brakes for speed control on the downhill stretch - you can always set the inverter to freewheel the motor, after all.

    Also, I think that mid range charge cycling is more of a NiMH thing than an Li-ion thing. I have a Prius and I'm fairly sure that it cycles through the middle 200-300 Wh in a 1200 Wh battery for lifetime management issues.
     
  11. malcolm

    malcolm Active Member

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    Thanks Embassy.  That's a useful insight.  Regen braking is set to mimic conventional engine braking, but if it is under software control then it could be varied.  One possible scenario would be the car leaving a house on a full charge and having to go straight down a steep hill.  Unlike brushless motors, induction motors do not have to act as generators.  (It's one of those phase/timing things)    Regen braking would be off so it would just feel like freewheeling with the clutch in.  Mechanical braking would have to be used.

    Does this put us back to a Tesla battery pack containing mixed battery types?  Astonishing achievement if true.
     
  12. iisjsmith

    iisjsmith Member

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    So Embassy, are you suggesting that the two packs are separate? That in the ESS there is one sub-pack with a low capacity but high discharge rate, and one sub-pack with a high capacity and low discharge rate, and there are no connections between the two? And the inverter is switched back and forth between the two depending on the driving load?

    Wow, if they did that then I agree they deserve all the kudos they are getting. I don't know why they are so hush-hush about it...that is one hell of an accomplishment.
     
  13. Embassy

    Embassy Member

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    Iisjsmith, I think it might be a little simpler than two separate packs and a switching inverter, but still clever all the same. And if I were designing such a thing I would want to scatter my high current lanes in and around my low current lanes so that the heat disappation would be distributed. A hot spot inside the pack would be bad.

    Imagine a pack with 99 series by 47 parallel 2C cells and 99 series by 22 parallel 10C cells. Each cell has a current limiter so that the 2C cells never pass more than 4.6 amps each (assuming 2300 mah capacity) and the 10C cells never pass more than 15 amps each (assuming 1500 mah capacity). You probably want to throw another current limiter on each series stack as a whole, too.

    Now when the inverter wants amps, it'll get them from the battery pack. And each "lane" if you like will discharge at the correct rate. But as soon as you as for more than 2C, you'll hit the limiters on the 2C cells leaving the 10C cells to pick up the current demand. I'd have to look at the individual cell specs but I expect that the current limiter would also protect the cells against overcharge during regen braking, too.. and the 10C cells would regenerate quicker than the 2C cells, most likely.

    Of course, as soon as you do this, you'll have your two types of lane at slightly different voltages, which would be problematic. Maybe there's some trickle recharge of the 10C lanes. Or maybe both types of lane are stepped down to a voltage they can both always meet before they go to the inverter.
     
  14. iisjsmith

    iisjsmith Member

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    That was my concern. If the two packs are discharged at different rates, their voltages and capacity will eventually be different. And if the two groups can "see" each other, then the one will start to charge the other, resulting in a loss of power and therefore range.

    Keeping them isolated stops this issue. It would be like having an ICE vehicle with two gas tanks...one with regular fuel for normal driving and one with jet fuel that gives you a power boost for high accelerations.

    But that brings up a host of other concerns, though. If you do a lot of high power driving you could "drain" the jet fuel tank sooner, leaving you with no power for high accelerations and only normal fuel for normal driving. Not good if you suddenly find yourself needing to accelerate up an freeway entrance ramp.

    I would love it if the first people who got their hands on their Roadsters just rip the thing apart to see how it works! ;) What's $100k to these multi-millionaires?
     
  15. Embassy

    Embassy Member

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    Well I'm not taking apart mine! Not that I'll see it this side of next September

    But does this make sense? Suppose the inverter wants, say, 360V. Though physically in the same container, the high-rate and low-rate strings could probably always manage 360V. And I think you're right.. they'd have to be electrically isolated to prevent them from cross-charging. Maybe each individual series stack is isolated - why would you want inevitable battery variation to permit cross charging even in the 2C stacks?

    Something like this with a mixed-cell pack seems to be the cheapest explanation that fits all the facts we know so far. Unless they're using a honking big capacitor, a NiMH boost pack, or some other trick. I don't get the impression that they are.
     
  16. malcolm

    malcolm Active Member

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    New information on the Tesla battery pack in this week's blog (30th November):-

    http://www.teslamotors.com/blog1/

    Seems max charge available is between 4.15 and 3V.  Maybe this is the clue to Tony's recent post about 1/3 of the battery pack being used.
     
  17. tonybelding

    tonybelding Active Member

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    I think that comment about using 1/3 of the capacity must have been garbled in translation somewhere along the way. The new blog entry really clears up the subject for me. It's good stuff.
     
  18. iisjsmith

    iisjsmith Member

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    :-\ Eh, I wasn't too impressed with the recent blog entry. Some new stuff, like the user-programmable charging levels and the fact that they limit the discharge rate, but nothing really new. Nothing about how the batteries are wired, what the specs of each cell are, etc.

    In fact, this brings up even more questions. We have already determined that the cells need to be high discharge capable...on the order of 4C. But if they suggest that discharge should be limited to C/2, and that they have designed the pack "such that even at maximum discharge rate, the current required from each cell is not excessive", then how do they get enough power out of the battery?

    Every time these guys talk, it just brings up more questions. Don't get me wrong...I think the fact that they regularly talk to the community is fantastic. I just wish they'd get a little more technical than 'our battery is good!'
     
  19. malcolm

    malcolm Active Member

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    Another piece to the puzzle:-

    http://powerelectronics.com/mag/602PET06.pdf

    18650 form factor Li Ions rated at 10C for the power tool market, still with 1500 mAh capacity.

    I agree however that the C/2 rate mentioned in the article doesn't seem to make sense.
     
  20. iisjsmith

    iisjsmith Member

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    Yea, I saw those powerelectronics cells. My post on page 1 has a link to similar cells on the BipowerUSA site.

    The problem is 1500mAh and 3.7V discharge isn't enough. If we continue with the assumption that the 6831 cells are 99 series by 69 parallel, then 1.5Ah x 69 = 103.5Ah total capacity. With a total voltage of 3.7V x 99 = 366.3V. So 366.3V x 103.5Ah = 37912Wh or 37.9kWh....far less than the 50kWh number cited by Tesla Motors.

    Now if you look at the table on page 2 of that PDF, there is the Sony 26650VT which definately fits the bill. It has an average capacity of 2400mAh at 10C discharge, with a discharge voltage of 3.75V. This is pretty sweet, considering it has an average capacity of 2500mAh at less than 1C discharge. This constant capacity at a wide range of discharge levels is freakin sweet...and is exactly what the Roadster could use.

    2.5Ah x 69 = 172.5Ah total capacity. 3.75V x 99 = 371.25V total voltage. 371.25V x 172.5Ah = 64041Wh or 64kWh. A bit higher than the Tesla's number...but I'm sure that number will come down in reality. Looks like these cells fit the bill.

    Now we just need confirmation of the 99S69P wiring.
     

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