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how does the roadster sense max current

Discussion in 'Roadster: Technical' started by richkae, May 20, 2009.

  1. richkae

    richkae VIN587

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    Can someone please explain to me ( not an electrical engineer, but i have a basic understanding of circuit breakers, amps and volts ) how the roadster senses the max current it can draw to charge?

    Thanks
    Rich
     
  2. doug

    doug Administrator / Head Moderator

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    Funny... from the title I thought you were asking a different question. I was about to talk about how inductive loops can be used to measure current, etc.

    Anyhow, to answer your actual question, the charging station or cable has a "pilot signal" that tells the Roadster how much current is available. (The details of this signal are discussed elsewhere on the forum).

    You can also set the max current on the vehicle display system (VDS) inside the car.
     
  3. richkae

    richkae VIN587

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    How does the pilot signal know how much current is available?
    Its my understanding that if you plug the MC240 into a 30amp breaker it will limit you to 24 amps and if you plug into a 40 amp breaker it will limit you to 30 amps.
    How does it know what sized breaker you plugged it into?
    What I am asking is... is there some magic that figures out what size breaker you plugged into or do you have to enter that information through some UI on the VDS?
     
  4. Kevin Harney

    Kevin Harney Active Member

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    There is magic - you don't have to know it - but I can't tell you how it works.
     
  5. WarpedOne

    WarpedOne Supreme Premier

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    Actually, it doesn't really unless you tell it via VDS.

    On the VDS there is an option for setting the maximum current draw.
    If you set it above the breaker limit, it will tip it.
    _MG_8253.jpg
     
  6. TEG

    TEG TMC Moderator

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    Circuit breakers don't offer any signal or let anything know on the circuit how much current can be drawn before they trip. It is up to everything plugged into the circuit to not add up to more than the breaker can handle.

    So, yeah, the Roadster needs to be told how much to draw.

    Is there some GPS linked feature so once you set a charge rate for a particular charge station it will know the next time you are there so you don't have to enter it by hand each time?


    Smart Grid technology is coming, and I hope it includes provisions for devices and circuit breakers to better communicate their capabilities and current loads.
     
  7. graham

    graham Active Member

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    There is! The Roadster keeps track of where each charge occurs and what amperage you asked for last time.
     
  8. SByer

    SByer '08 #383

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    Only, if you park multiple levels up in a parking garage, it can take a few tries to get things to really stick as the GPS position wanders enough for the control system to think it's a different location sometimes.
     
  9. TEG

    TEG TMC Moderator

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    Can you ask the MC240 to try to charge at any current level the Roadster supports?
    For instance, can you ask it for 32A or 40A or does it max out at 24A?

    I am wondering if the VDS only lets you pick charge levels below what it thinks the cable is suggesting, or can you go above also?
     
  10. donauker

    donauker Member

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    You can set the VDS to any of a list of values but it will only have an affect if the value you set is less than the value indicated by the pilot signal from the charge adapter. The MC240 signal specifies 30 amps so only settings below this will have an affect. In other words Charge Rate = Minimum(Pilot Signal, VDS Setting).

    The exception to this is a charge connector without a pilot signal as the MC120. In this case default charge rate will be 12 amp and a VDS setting of 16 amps will switch to a 15 amp charge rate.
     
  11. TEG

    TEG TMC Moderator

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    #11 TEG, May 28, 2009
    Last edited: May 28, 2009
  12. graham

    graham Active Member

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    Yes, I frequently charge from the 70amp HPC by dialing down to 40 amps on the VDS. It makes the blowers blow less.
     
  13. Grin

    Grin Member

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    Graham, I was told by Stefano Paris (who heard this from Bob Sexton, Service Manager of Los Angeles store) that it is best for the batteries to charge at 24 amps. You can always dial it up if needed but 24 amps seems to be the best between charge time and longevity of batteries. Just a suggestion....
     
  14. graham

    graham Active Member

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    #14 graham, May 29, 2009
    Last edited: May 29, 2009
    Thank you! This is a great piece of information. It will be good for owners to know this.

    Edit to add: Perhaps we can either add this to an "owners" section of the FAQ wiki, or we can start a new wiki with information like this.
     

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