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Charger, how does it know?

Discussion in 'Technical' started by brucet999, Mar 19, 2015.

  1. brucet999

    brucet999 Active Member

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    Model S comes with remote charging cable and a 240V 40A Nema 14-50 adapter capable of charging at 9.6KW, or 29 mi range per hour. Also available are adapters for the two extant pin patterns of 30A 240V dryer outlets, from which source it charges at only 5.67KW (240V 23.6A) or 17 mi range per hour.

    So, when the onboard charger sees 240V power from the portable charging cable, how does it know not to pull too much amperage, if it happens that the source is a 30A outlet, so as not to overload the house wiring and trip its 30A circuit breaker? Is there a chip in the adapter that tells the charger what capacity outlet it is connected to?
     
  2. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Yes, there is a chip (it's a resistor that differs between the adapters) in the adapter that limits the current draw.
     
  3. brucet999

    brucet999 Active Member

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    Wow! That was quick.

    Thanks
     
  4. mrElbe

    mrElbe Member

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    The charger / car is even more intelligent. Had to charge last week from an AC outlet and had to build a cable to the 14-50 plug adapter. The charger expected 50 Amps but gave me a message on the dash about a charging problem and had to reduce current but charged at 30 Amps which was the capacity of the AC outlet. Didn't even have to manually adjust the charging Amps on the screen.
     
  5. apacheguy

    apacheguy Sig 255, VIN 320

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    There should also be a pilot current.
     
  6. deonb

    deonb Active Member

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    Current is drawn, not pushed. There is no such thing as a capacity of an A/C outlet, only a point at which either the wiring would melt or the circuit breaker would trip.

    You got 'lucky' in that the weak wiring most likely caused a voltage problem - akin to an arc fault (which can be detected), so the Model S went into its safe mode and only charged at 30A. 9 times out of 10 this won't work.
     
  7. tga

    tga Active Member

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    Actually, since EV charging is considered a continuous load, and therefore has to be reduced 80% from the circuit rating, you were still dangerously overloading the circuit by charging at 30A on a 30A circuit.

    On a 50A circuit, you can only draw a 40A charge load. On a 30A circuit, you are limited to 24A.

    Don't rely on the car "figuring it out". You got lucky that you didn't start a fire. You should have used the correct UMC adapter, or, at a minimum, dialed back the charge current to 80% of the circuit rating.
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    The car will reduce current by 25% if it detects a higher resistance than expected (i.e., undersized wiring, misbehaving appliances, etc.). It's really not that smart - and if you used your adapter to connect it to a standard NEMA 5-15 outlet, you'd still find the car trying to draw 30A across it.

    As noted just above, circuit ratings and capacity are required to be 125% of the charging current used. As a result, a 32A charging load requires a 40A circuit; a 24A charging load requires a 30A circuit.

    Non-Tesla adapter cords are a danger, for a number of reasons - see my FAQ from my signature, below.
     
  9. brucet999

    brucet999 Active Member

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    I'm not sure what you mean by "build a cable" Did you make an extension cord with male/female 14-50 connections so you could reach the stove plug in the house? If you did, and you used smaller than #6 AWG wire, there could have been significant voltage drop that caused the MS charger to lower its rate of draw.
     
  10. TEG

    TEG TMC Moderator

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    Pilot signal is a separate wire on J1772 and Tesla EVSEs. It tells the car the capability of the charging station. When you plug into an (RV) NEMA14-50 plug, Dryer outlet, etc, there is no "pilot signal" from the wall to the car. The MC (portable EVSE) generates a pilot signal to send to the car. So it varies this pilot signal based on the type of plug-end you installed on the mobile connector. So, yeah, there can be cases where old wiring, under-fusing, improper wiring, etc, doesn't make it safe to charge at the "normal" rate for that plug-type. Sometimes the car will detect voltage sag when it starts charging and will "back off" for safety reasons, but that is not always guaranteed. If you know the location should be used at a lower rate, then you can manually dial down the charge rate and have it remember this location (based on GPS) so that next time you plug in there it knows not to try the full possible current draw.
     

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