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Commercially-Installed HPWCs Limited to 8kW (16kW with dual chargers)

Discussion in 'Model S: Battery & Charging' started by gavine, Sep 5, 2015.

  1. gavine

    gavine Petrol Head turned EV Enthusiast

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    I understand the math behind it, but I don't understand the limitation. The Model S onboard charger supports up to 10kW which is about 29 MPH charge rate. On my 240V UMC at home, I achieve that rate. I don't charge publicly very often and the first time I charged at a Tesla store, I was disappointed to only get 24 MPH.

    I understand the math behind it. At home, 240V x 40A = 10,000W (+/-). Commercial buildings are 208V so 208V x 40A = 8,000W (+/-). 80% of 29MPH = 24MPH.

    What I Don't understand is the limitation. Why can't an 80A/20kW charger, charge my car at 29 MPH like my 40A UMC can at home? It certainly has the power to do so. There should be a way to mitigate that limitation. Any ideas?

    if the onboard charger is limited by AMPs, why do they call it a 10kW charger and not a 40A charger? The limit is abviously in AMPs, not Watts.

    So, anyone who will be charging for the first time at a commercial UMC, be prepared to charge 20% slower than you can at home.
     
  2. wk057

    wk057 Senior Tinkerer

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    208V is a single phase voltage derived from 3-phase power, generally available in commercial buildings and many times multi-unit residential buildings (condos). Residential power uses a different transformer for 240/120V that are generally less powerful than commercial 3-phase power.

    The charger actually isn't limited to 10kW, either. With my off-grid system here I have control of my home's AC voltage and I can feed it say, 255V/40A and get 10.2kW. I believe the max for the charger is ~277V or something close to that, which theoretically would be 11kW @ 40A (I haven't tried it. but I've tried 255V).

    At 40A normal household 240V will be 9.6kW and 208V will be 8.3kW.

    That's the thing. If you have 40A available at 208VAC you *don't* have the power to get 10kW. You'll get ~8.3kW under decent conditions, likely less after voltage drop (I've seen some 208V HPWC @ 80A drop as low as ~190V). The car can't pull more amperage than it's allowed because the wiring sizing is based on maximum amperage, not voltage. (Voltage increases require more insulation, not bigger wire, generally speaking).

    The "29 MPH" number is just the car trying to give you a user friendly number for the charge rate. All it's doing is taking the power in watts to the battery (after charger efficiency losses), dividing by whatever your car version's rated miles are worth (I think 280Wh/mi for a 60?) and putting that number up. I actually have my car set to display kW instead of mi/hr, since I find that more informative.
     
  3. randompersonx

    randompersonx Member

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    I think what he's asking though is "Why can't my single-charger car pull 48 Amps from a 208 Volt HPWC".

    I'd guess the answer is because the wiring to the charger is only 40A, but don't really know.
     
  4. gavine

    gavine Petrol Head turned EV Enthusiast

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    Basically, yes.........
     
  5. wk057

    wk057 Senior Tinkerer

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    Yeah, the charger can only go up to 40A on the AC side. That's not just a wiring limitation but a limit of the electronics in the charger. If it were a 48A charger that would mean that technically we could charge at ~11.5kW at home, and we'd just be having the same discussion on why we can't do the same at 208V...
     
  6. FlasherZ

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

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    Components and conductors scale according to current (amps) not power (volts x amps). More current requires higher-rated components and thicker connections between them, and that's the primary gating factor. There is a limit to the voltage, though, as wk057 noted - higher voltage makes it easier for the potential to jump across insulated components and so components must be designed to provide that level of separation based on voltage. Most AC components tend to be rated in steps of 300V or so... 300V, 600V, 900V, 1200V, etc.

    Supercharger cabinets use the same chargers at 277V, so we know it will scale. However, Ingineer has reported that above a certain voltage level, the car-mode charger firmware will scale back maximum current (amps) to maintain a max 10 kW power level.
     
  7. wk057

    wk057 Senior Tinkerer

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    Yeah, you worded this better than I, for sure. :)

    The car side throttling might be due to the output amperage to the battery. Those conductors aren't particularly large. At 255-256V (highest I've tested it so as not to blow up anything else in my house... comes out to 128V to neutral) the car was still showing 80A and 20kW, TED showed about 20.5kW draw on the HPWC.
     
  8. FlasherZ

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

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    Ingineer also noted in another thread that his analysis of the UMC / HPWC was that while the car's charger could accommodate higher voltages, the UMC/HPWC was at risk of destruction at the higher voltages. So aside from having an EVSE that could handle the higher input voltages, I wouldn't pump it up either.
     
  9. StaceyS

    StaceyS Member

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    #9 StaceyS, Sep 5, 2015
    Last edited: Sep 5, 2015
    Just to clarify: 208v is found in commercial buildings, but commercial buildings are in no way specifically 208v. Don't assume that every commercially installed HPWC or UMC is only 208v.

    Whether a commercial UMC or HPWC is actually 208v or 240v depends entirely on the installation and what the building has available. For commercial buildings with 3 phase power (often with elevators and large HVAC systems), parking garage lights and exterior site lighting can be run at 208v, which can be more energy efficient than 120v lighting in the case of long wire runs. If this is the case, it might be easiest to simply tap one of the 208v branch circuits for the UMC or HPWC rather than run a new 240v circuit. If a building owner has the choice of tapping a 208v lighting circuit for less money than installing a whole new 240v circuit and wire set, they might just go that route instead.

    There's a country club near me that has an exterior HPWC installed. My twin charger P85 can pull a full 80 amps at 240v from it.
     

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