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Can a Model-S Charge from a backup battery directly?

Discussion in 'Technical' started by Lee Tsui, Jan 29, 2015.

  1. Lee Tsui

    Lee Tsui Member

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    Hey guys as this is the technical section of the forum I have a good question for one of you.

    I'm actually a product developer in Hong Kong that specialises in ion battery packs and I've just been asked very indirectly by a wing of Tesla to investigate the idea of creating an emergency battery for Model-S.

    Rather than waste weeks for all the people to relay my spec requests I presume there is one of you guys here that knows detailed info on their minimum specs?

    I'll be designing a concept to propose to the big guys later in the month, but I need specs and it's just taking too long to get the info!

    I need to know if the Model S could potentially accept power from an external battery, if so, what spec should the output be from the battery pack to be accepted to the Model-S?

    That's my major concern as I specialise in items that output DC5V I know this car needs something wildly different.

    I've been told I can have access to the Model-S connector spec later if they like the concept so the connector is not my issue it's creating something that's output power can be accepted.

    Anyway one of you guys may know this info so any ideas anyone?!
     
  2. davewill

    davewill Member

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    #2 davewill, Jan 29, 2015
    Last edited: Jan 29, 2015
    There are three approaches that could be taken for this:

    1. Use backup battery to generate AC, and use on-board charger to charge battery. Max 20kW (10kW if only single charger car). Charging time measured in hours. Relatively easy to do. A DIYer could do it with off-the-shelf parts. Could be made to work with any EV.
    2. Use backup battery to do DC charging. 100+kW charging. Charging time measured in minutes. Need to design a power supply that is voltage and amperage agile. Current DC chargers that are powered by the grid are >$10-30K. A lower power version could be cheaper, but you lose the advantage over AC. Also big and heavy.
    3. Install the battery in the Model S, and either run it in parallel to the MS battery or switch batteries to the backup. Battery would need to be carefully designed, to match the car's existing battery. The car would need to be modified quite a bit. Also have to figure out how to charge the backup battery.

    Not sure what the goals are here. An emergency battery to carry around to allow you to limp to a charging site? A mobile truck to rescue stranded motorists? A range extending trailer?
     
  3. Lee Tsui

    Lee Tsui Member

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    The idea would be that you could put this backup battery into the frunk and if you did ever get stranded to offer up enough power back to the car (around 5 extra miles) to get to a safe recharge point.
    I think charging 5m wouldn't need minutes but essentially that's the idea.

    The unit I make must be mass production viable and basically be something the range paranoid might pick up to give them a little piece of mind.

    This concept is still in it's infancy and due to weight of extra battery may fail. It depends on how efficient it could be.
     
  4. davewill

    davewill Member

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    OK, to add 5 miles, you'd need to add a couple of kWh. At 10kW, that would take 12 minutes, at 20kW, 6 minutes. I'm thinking that with charging overhead, your battery would need to be about 3kWh. With such a small source battery, even 10kW would be a 3.3C discharge.

    BTW, I'm not sure that 5 miles cuts it. You can stretch range a LOT more than that just by slowing down when it becomes clear that you may have trouble making it to your destination.
     
  5. Lee Tsui

    Lee Tsui Member

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    Basically I've got it down to using 120 lithium ion cells at 2600mAh per cell (312A). I just need to know if the Model S will accept DC charge, and if so at what minimum voltage and what amperage? This is how I normally work. if the car can accept DC then it's familiar territory for me and then I just need to know how fast I need to make a chipset to send this power to the car.

    kWh isn't a figure I talk in so sorry about the mAh! :)
     
  6. davewill

    davewill Member

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    To do DC charging, you have to talk to the car using the Tesla's Supercharger protocol, and be ready to supply whatever voltage the car requests. The car will request a specific voltage and a max amperage. My understanding is that the Tesla battery packs are approx 350-400v. However, you may have to be ready to supply the full range of voltages the Supercharger supports (I don't know exactly what that is). I don't think that charging slower than max is any kind of problem.
     
  7. ggr

    ggr Roadster R80 537, SigS P85 29

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    You want to talk supercharger protocol to put some energy into the Model S quickly. But you don't need all that much total energy; 10 miles is about 3.5 kWh, into an empty battery, takes about 1.5 minutes at 120 kW. This is an application for a supercapacitor, not a battery! It doesn't matter if it's fairly big and heavy, because it is in the back of a service vehicle.
     
  8. freds

    freds Member

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    Actually I wouldn't worry about the upper voltages since your battery pack is for emergency situations; I would concentrate on the less then 50 rated miles or less then 20% charge range, it's at that point that the car starts limiting power, etc.

    If the driver is truly worried on a long stretch they would employ hyper mileage technics; drafting, etc.
     
  9. stopcrazypp

    stopcrazypp Well-Known Member

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    #9 stopcrazypp, Jan 29, 2015
    Last edited: Jan 29, 2015
  10. wk057

    wk057 Senior Tinkerer

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    A discharged 85kWh pack will be around 300V, and around 400V charged. A 60kWh pack will be at around 270V discharged and about 350V charged.

    If you're talking about storing an additional battery in the frunk for emergencies... forgive me, but that is a pretty dumb idea. For example, the Tesla pack modules in the 85kWh pack are about 5.3 kWh (~16 rates miles) and weigh about 80 lbs. Add in charging circuitry and you'll easily be at or over 100 lbs. (Example: an 8kW inverter I have that is capable of running a Model S charger at 8kW weighs 125 lbs by itself without batteries...) Lugging 100+ extra lbs around isn't free and you'd likely just be better off not having the thing in the first place.

    If you're talking about something a service vehicle could bring out to top off a car, then maybe that would be doable.
     
  11. Lee Tsui

    Lee Tsui Member

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    The concept is far more simple than these guys, this is to be a purchaseable accessory you can keep in your frunk or trunk to just give you a little extra if you need it. Anyways thanks everyone for the inputs I'll keep figuring out how to do this.

    I'll need Tesla to open up the DC charging protocols it seems and design and new PCB to handle the load the car requires for slower charging speeds so that the ion batteries in the pack can take the strain.

    - - - Updated - - -

    You are exactly correct and I also raised this issue to Tesla already, the reality is the extra battery capacity would have to be light enough to keep travel efficiency down as there's no point wasting 5 miles on a heavy stored battery then just not having the battery in the first place.

    Based on my calculations the pack could weigh in below 10Kg giving 5 extra miles may be ok.
    But for sure it's about as efficient as carrying a spare tyre! It's something they must consider.

    Right now it's research only which is why I'm picking everyone's brains.

    Thanks for your input, it's one of the best so far and I totally agree with you, storing a heavy backup device is totally counterproductive.
     
  12. WarpedOne

    WarpedOne Supreme Premier

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    It is just that 3.5 kWh supercapacitor bank will cost you around 40.000 USD for the super-capacitors alone.
    Add the cost of powerelectronics that is capable of converting that wildly changing voltage of a capacitor into slowly rising voltage of a charging protocol
    And it will still hold only 3.5kWh in total, practical application needs to have a higher capacity so it does not fully discharge (empty capacitor has no voltage).

    In the end of day there is a reason super-capacitors are much talked about and seldom really used.
    Modern LiIon battery is much better proposal.
     
  13. Lee Tsui

    Lee Tsui Member

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    Yeah in the end guys I've gone back to the guys who were thinking of this idea and told them it totally isn't feasible due to carrying the extra weight of the battery actually reducing overall milage achievable anyway! with all the power conversions and heavy duty output required by this kind of device it just seems like a losing horse to back right now, especially if improved ion batteries are just around the corner, this unit wont have a long lifespan anyway maybe!

    Thanks for the chat on it, I'm gonna go scurry away to develop another smarter item instead for them! :)
     
  14. Supercharged

    Supercharged Banned

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    The simplest method is to buy a temporary portable Supercharger, like they just installed in Kingston, Ontario, and put it in a trailer behind you wherever you go.
    Problem solved.
     
  15. wk057

    wk057 Senior Tinkerer

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    I think a way to charge an EV with a portable charger on a service vehicle would make perfect sense. Kind of like AAA bringing someone a gallon of gasoline, but bringing a few kWh instead.
    .
     
  16. bwa

    bwa Member

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    It might make sense to rent out a pack from some nearby store and use it on a stranded vehicle to get it to a more stable location. This would only be useful where a service vehicle/tow was less attractive, and I'm not sure that's a very common scenereo. But gas stations do sell 2.5 gallon gas cans that we see people walk to their vehicles with to gas up. This seems like more of a psychological embarrassment trait, individualism trait, or cost savings trait often from the very poor who can't afford service vehicles. That last one's not even Model S's current target market.
     
  17. Danal

    Danal electricmotorglider.com

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    Would carrying something in the Frunk that could charge the car help people who run their cars all the way down to the point the car stops, thus stranding them?

    There is a math side to this question:​

    What is the energy density of the storage device that is proposed to be carried the Frunk, calculated while including the weight of the actual storage plus all packaging, electronics, cables, etc? Knowing that form of the energy density, it is then possible to estimate the net gain/loss of carrying such a device. Essentially, what is the loss in range from carrying the devices weight, vs. the number of miles/kilometers it can add? Is that positive, or negative?

    I offer that detailed math is not required... because there is nothing shipping in volume today (at less than exotic prices) that is significantly more energy dense than the Model S's traction pack. As such, it is easy to see that the Frunk Modules separate packaging/electronics/cabling weight will make anything carried turn out to be a negative as vs. just having that same capacity in the main pack.

    If someone knows of something shipping at reasonable cost per kWh, and substantially (let's just say > 20%, to have a number) higher kWh per Kilo than a Panasonic 18650 variant, please provide a link. Not to a news report or lab research, a link to an order page.

    There is a human behavior side to this question:​

    The car gives multiple warnings of energy depletion before it stops. Of course, there are people who will push past those warnings until the car stops. If such a person had the Frunk device, they use it and add a few miles to the car. What happens next? Drive directly to a charger?

    Why did they not drive direct to a charger several warnings ago? From a human behavior angle, is stopping, plugging in the booster, and driving that last 5 miles, is that just another form of warning?

    There is a marketing side to this question:​

    It may be very possible to sell such a device, based on FUD. If so, more power to you! (Pun Intended). I won't be buying one. :wink:
     
  18. Mark Petersen

    Mark Petersen Model S EU P71

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    as I see it you have 2 options (easy or difficult)

    the easy solution is just to use X deep cycle 12v batteries and a 120v or230v DC-AC inverter and charger, and then just connect the UMC and set the max amp to what the inverter support
    this can be done with off the shelf products today, but the Kwh/kg is low but so is the price

    the hard way is to use the tesla DC charging protocol (SuperCharger/ChaDaMo/CCS) , and for that to work you need to build you own controller with a programmable DC-DC converter
    that can supply the correct voltage 300-500v and signal the max amperage to the car
    this solutions also give you the opportunity to use more advance batteries, but is difficult to make, but will provide the most Kwh/kg
    also you will need a more intelligent recharging solution to extend the life of the battery

    but for both cases the user will need to charge this extra battery every week, because they will slowly discharge :-/
     
  19. arg

    arg Member

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    There are already solutions on the market, eg:

    http://www.andromedapower.com/ORCA_Inceptive.php

    CHAdeMO based, so could probably be used to charge a Tesla via the adapter, but probably not Tesla as the 'tanker' vehicle.
     
  20. Matias

    Matias Active Member

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    http://powerequipment.honda.com/generators/models/eu1000i

    ;)
     

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