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Inner workings and engineering behind the Model S

Discussion in 'Model S' started by emir-t, Aug 17, 2015.

  1. emir-t

    emir-t Member

    Joined:
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    Hey everyone;

    I’m not an engineer but the Model S astonishes me in every way so I want to learn all about it. I’ve been following news regarding Tesla and EVs very closely for the past 2-3 years now and I’ve done lots of readings over time regarding electronics, Model S and Tesla. However there is no one unified source that tells in detail how the Model S works so I’ll try and write what I know so far, I’ll inevitably have some questions while doing so and please correct me if I’m wrong with any point. I’d appreciate any links to good readings about these.


    Although I kind of regret selecting a business major after discovering such a “hobby” of electronics, please acknowledge I’m not an engineer and I’ll be talking as basic as I can so curious people like me can also read and learn from this thread. Here goes;


    AC Induction Motor: This is the engine. It makes the Model S move and regen. It is a 3 phase induction motor that runs on AC and it can be used to convert energy to mechanical energy (moving the car) and it can use mechanical movement and turn it into electrical energy. (ReGen braking)


    It consists of a rotor and a stator. With the 3 phase AC a spinning magnetic field is created around the stator. (look up sine waves) Rotor is attracted by that field so if the magnetic field is spinning faster than the rotor it creates movement. If the field is slower than the rotor it ReGens.


    Battery Pack: The battery pack consists of Panasonic’s 18650 (cylindrical cells that have a radius of 18mm and height of 650mm) battery cells that are modified by Tesla. I am not entirely sure on what modifications were done with the cells but these cells are heavily modified and not available to other customers of Panasonic. I remember reading a detailed article about the modifications but I can’t find it now. I think they were about removing some of Panasonic's safety & cooling features from individual cells and moving it to pack level.


    Anyway, these cells are around 3.6V when discharged and around 4-4.1V fully charged. They are bricked below 2.6V I think so they shouldn’t be left around without a power source for a long time especially with a low state of charge. Here’s someone who did;
    http://www.energytrendsinsider.com/2012/02/26/how-to-turn-a-tesla-battery-pack-into-a-40000-brick/


    For the 85kWh pack 74 of these cells are hooked in parallel. That becomes a group. 6 groups in series form a module and there are 16 modules hooked in series. So 74*6*16 = 7104 cells. To take a nominal voltage of 3.9V * 6 * 16 = 374,4V is the pack’s voltage. 3,1Ah * 74 = 229Ah. To convert that into watts, 229*374,4 = 85,737Wh = 85kWh.


    12V Battery: Although we are used to seeing these batteries in regular ICE vehicles. The Model S has them too. It is used to power all the other electronics in the car from speakers to infotainment to wiper blades because they all use 12V, not the pack’s 375V. A DC/DC convertor constantly tops up the 12V battery from the main pack and Tesla has had some problems regarding that system. There’s been some amount of stranded Model S when the 12V battery died, or contractors from the main battery snapped etc. I think only the air conditioning uses the main pack other than the drive train because it requires more power. Just like in an ICE car where the A/C can’t be used without the engine running.


    Charger: I’m not very clear on this. There’s a 40A version and you can hook up a second one to get up to 80A. Is this different from a convertor? All I know is that AC that comes from the wall is converted to DC to be put into the battery and the charger does this? With DC charging such as Chademo or Superchargers, this is bypassed.


    DC/AC Convertor: This converts pack’s stored energy to AC for it to be used by the electric motor. I think it sits right next to the motor in a bundle called “Power Electronics”. Among those power electronics lies a single speed gearbox and there has to be a digital differential lock somewhere.


    I’d love to know much more and I regret not being an electronics engineer as I delve deeper into this. My questions;



    • When we press the accelerator pedal, do we vary the amperage that gets out of the battery to the motor? Is it a fixed voltage? We know that pack’s voltage varies with SoC, temperature so is it transformed to a fixed voltage while being converted form DC to AC? Also, we know that Ludicrous mode was enabled by putting in a fuse that allowed a 1500Amp draw. How can a 229Ah battery drain 1500Amps at once? Could someone explain this?
    • I’ve read somewhere that the Gigafactory will produce 20700 cells instead of 18650. What’s the reason for this? Also, when 74 cells are in parallel, if one of them dies, the entire group dies. That means a cell’s death in one group = 3,1Ah * 375V = ~1,2kWh of capacity lost. How is Tesla controlling and balancing the cells? This must be very, very difficult considering all the variables including temperature, SoC for individual cells, charge cycles etc.
    • Why is a battery bricked below a certain voltage? Can’t it just be charged back up again to an operable voltage? A friend of mine was involved in an accident with his Model S, immediately after the 12V battery died. Then, the car laid in a parking lot for 3 months waiting for the regulatory issues to be resolved. (We’re in Turkey where there’s no service centres) I think it is safe to assume the battery pack is bricked with that logic but why? Say the pack’s cells’ voltage is 2.1V, can’t we just charge it up till it hits 3.6-4.1 interval again?
    • How's the charging controlled? The current and voltage coming in to the battery can vary and harm the battery. What would the result of a power outage whilst charging, or sudden increases in voltage with unearthed outlets per se be for the pack?
    • What's the point of hooking groups into modules and then connecting modules in series? Why not 74 cells in parallel and 96 groups in series?


    Can anyone suggest further readings? If you guys want I can edit this main post as more detailed information about how the car works comes in so we can have the "unified source" I was talking about.

    Thanks in advance.

    Cheers;
     
  2. No2DinosaurFuel

    Joined:
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    #2 No2DinosaurFuel, Aug 17, 2015
    Last edited: Aug 17, 2015
    I am, by not means, a guru engineer on tesla cars. But I will try my best to answer to the best of my knowledge. I am sure if I am incorrect, someone will chimm in and correct me.


    I guess would be it is doing 1 of 2 things. 1: controlling the duty cycle (the percent it is high in 1 cycle) of the PWM (Pulse width Modulation) to the motor. This is essentially controlling how much current the battery gets. Lower duty cycle means lower AVERAGE current hence less power. The motor will see whatever voltage the battery is currently at. Though the higher the current you draw, the more "sag" you have on the batterie's voltage. The AC you are referring to is the AC generated by the controller which is essentially generating 3 sinusoid 120 degrees out of phase with each other through a network of electrical components to smooth the signals from the PWM to a sine wave. Max Voltage is determined by the battery voltage. Since sine waves have highs and lows, the voltage will vary accordingly. Search for a youtube video on how an AC motor works. 2: controlling power. Power = Current * Voltage. Tesla might be clever and control the power by monitoring the voltage and current such that they will get the right power level base on the current control. I suspect this is more complicated than tesla needs to design their car hence I doubt this is done. Though it might be a simple software fix if they wanted to it this way.


    The 20700 is suppose to allow for better efficiency in terms of packing. With a large cell format, you can pack more energy instead of wasting empty space. As for the 74 in series, if one cells dies, it will be "automatically" disconnected due to the fuse embedded in them. This link will be removed. This will prevent major capacity loss if one cell dies. Tesla is controlling and balancing the cells through their proprietary BMS (Battery Management System). This is quite simple. You just monitor each cells within a group and balance and charge accordingly. Simple Divide and Conquer.

    Bricked means that the cells are never the same again. Theoretically, they can be restored. I have done it plenty of times with other Lithium chemistries and have read plenty of cases for the tesla cells. I suspect what teslas does is the fault of the BMS. The BMS prevents recharging and such when voltage drops below a certain voltage. If you want to charge the cell's voltage up again you would have to bypass the BMS and charge and connect again and hopefully it would work then, but no guarantees. Tesla might just prevent you from reactivating it until they get a hold of battery. As for the cell's conditions after it goes below the voltage, it just means there are some crystal formation and other permanent damage to the battery. It may or maybe not work the same as before because the damage is already done.

    Charging is just AC to DC and adheres to the CC-CV (Constant Current, Constant Voltage) method of charging. Tesla has an onboard AC/DC converter which converts the 240V AC from your house to whatever battery voltage DC it needs. Then it charges at CC until it reaches near the end voltage and then switches to CV to top off the cells. The BMS will balance the cells at this stage. At a supercharger or CHadMeo, you will by pass the on-board AC/DC charger.


    There is really no point IMO. They could've done it any other configuration. There are trade offs with each configuration though. If you put them in series, the if one dies, then that one chains dies and you lose some capacity. But if it's parallel, then that pack will be limited by its weakest cells. The weakest cell will drag the rest of the other cells down. I am sure there are other pros and cons with paralleling them first and then series or vice versa. I am too lazy now to think of all of them.


    Hope this helps. Most of the engineering design tesla used are not earth shattering. They have been in use and proven for a long time now. Tesla just took them and put it in a car. IMO, they are a master at manufacturing. I mean imagine a new production line from nothing to build something completely new in a short period of time and such. That is where they shine.
     
  3. Saghost

    Saghost Active Member

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    Actually, it's 18 mm *diameter* by 65.0 mm long - a little over 2 inches, not 20+ inches long. The modifications are mostly in the form of what's missing, I believe - Lithium cells normally have a over current protection fuse and vent cap that Tesla has left out because they manage those aspects during pack assembly instead, making the assembly of the cell less expensive.

    Yes, the car has a charger module under one of the rear seats hooked in to the cooling system and high voltage lines. This rectifies the A/C power into DC at the required voltage for the battery, and manages the current flow into the battery - in conjunction with the BMS it decides how much current the battery should receive and delivers that amount (up to the capacity of the EVSE or charger, of course.) A second basically identical slave module can be added under the other rear seat.



    It's a bit more sophisticated than that. The Power Electronics/Inverter synthesizes 3 phase AC power at a variable frequency, phase, and voltage based on the current motor RPM and desired level of acceleration - this is the essential bit of magic that made the modern EV possible, and allows far better efficiency and power/weight than classic DC motors.



    The first half is all about the power electronics - it takes DC power at pack voltage and pulls as much of it as needed and converts it to 3 phase AC to drive the motor. The voltage coming from the battery is basically fixed - it varies with state of charge and a little with power draw. The voltage to the motor is not fixed at all. 1500 A from a 229 Ah battery is no problem - it means about 9 minutes to drain the pack at ~6.5C. That's aggressive for most batteries, but an actively cooled lithium pack will take it just fine.

    As another poster mentioned, the current approach is actually much more resilient than the approach Nissan and GM have taken. If one cell fails with a short, the individual cell fuse pops, and you lose ~1.4% of the pack capacity (because you stop charging/discharging when the weakest group reaches a given voltage.) If a Leaf or Volt loses a cell, they lose a third of the capacity assuming it fails open, or a complete cells triplet if it fails short (this really hasn't happened or we would have heard about it on the GM-Volt forums - but that's how it would happen.)



    Lithium batteries experience permanent damage to the cell when the voltage drops below a certain value - the chemicals in the battery react in ways they aren't designed to, and the process is irreversible. Hence bricking. However... The battery you're describing is likely fine. Lithium cells seldom have much tendency to self-discharge, and it sounds like the pack has been isolated from everything else since the accident. It'd need to be checked, but sitting for 3 months without draws shouldn't be a killer.


    As I mentioned above, the charger modules are in charge of all of this - the battery is never connected directly to external power, and the charger will stop converting AC to DC when the battery reaches full or if there's a power failure. A surge would either be stopped by the charger or would damage the charger electronics, but likely not get through to the pack itself.

    The modules are useful for packaging, handling, and repair. They let Tesla assemble the pack out of units that are less fragile than individual cells but still have voltage low enough to be safe for handling, and let Tesla replace only one piece of the battery when it gets damaged. There's no functional difference between what you're suggesting and what the car has - but by grouping it into 16 modules it becomes much more producible and repairable.

     
  4. mmh

    mmh Member

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