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how electrically efficient is the Mobile Connector?

Discussion in 'Model S: Battery & Charging' started by syswei, Jun 23, 2016.

  1. syswei

    syswei Member

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    My Mobile Connector gets moderately warm. I'm on a NEMA 14-50, 50A circuit at around 230V, and the car defaults to 40A. I assume the warmness reflects resistance and somewhat less than 100% efficiency. Is the number known? And would the efficiency loss be markedly less if I set the car to charge at 30A or 20A at home?
     
  2. bkp_duke

    bkp_duke Member

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    It's just as efficient as the wall connector for your purposes.

    UL safety specifications dictate that you cannot draw more than 80% of the max for a circuit. So 0.8 x 50A = 40A.

    There is, of course, some resistance loss along the cable and this manifests in terms of the heat you feel. I really don't know if dropping to 30 or 20A would do very much. You could possibly run some detailed tests and find out, but not sure if just the monitoring on the car itself would be enough to see any differences.
     
  3. mikeash

    mikeash Active Member

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    Just a few watts will make something that size quite warm. You're putting nearly 10kW through it, so if it's not bursting into flames the efficiency will be better than 99%.

    It's basically a smart extension cord. There is no conversion happening in the UMC. The only losses would be resistance in the (fairly large) wires, and a little bit of energy to run the electronics.
     
  4. Polly Wog

    Polly Wog Member

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    @syswei, regardless of the efficiency of the UMC, Tesla previously used to provide a calculator on their website which showed that the on-board charger was more efficient at higher power levels, so you don't want to dial it down. That would end up costing you more money to charge.
     
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  5. bkp_duke

    bkp_duke Member

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    Good to know, thanks for the efficiency info.
     
  6. supratachophobia

    supratachophobia Active Member

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    I think it's actually less efficient at lower voltage and amperages as well. I think there is something like 20% waste on a 5-15
     
  7. syswei

    syswei Member

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    Thanks for the info, I'll definitely use 40A then. Makes me wonder if I should have ponied up for a wall charger, but too late now.
     
  8. syswei

    syswei Member

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    I'm guessing the charging process...the car's charger (and does the battery need cooling when charging?)...might be putting out significant waste heat? My home office is in the room over the garage so in the summer months we'll set the timer to charge when I'm asleep.
     
  9. mikeash

    mikeash Active Member

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    There is definite waste heat generated during charging. Between the charger (which is in the car for this, the UMC is just a connector) and the battery, the process is around 93% efficient. That means that if you're charging at 10kW, there's about 700W of waste heat being generated, which is like a small space heater. I'm not sure if this is enough for the battery to need active cooling, it probably depends on the ambient temperature. It can be really noticeable when Supercharging, as the waste heat from 120kW of charging is huge. The system can get amazingly loud if it's hot out.
     
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  10. Rocky_H

    Rocky_H Member

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    Yes, I think that is about right--maybe 400W or so just running the charging circuit.

    Well, I'll slightly disagree. Yes, the principle is true comparing tiny power levels, like the 120V X 12A = 1.4kW of a regular wall outlet. The 400W or so is a significant portion of that, so increasing the power is all gravy and goes straight to miles added. But it's diminishing returns. Once you're up around 5, 6, 8kW, the 400W of loss is relatively not much, so you aren't really gaining much efficiency in that range. So there is this one remaining benefit to turning down the current a bit. The UMC does get pretty warm at 40A. Heat is a degradation factor of shortening the life of electronics. Just by turning it down to about 32-34A, the UMC does stay noticeably cooler, which should be less hard on it from the temperature cycles of heating/cooling/heating/cooling and make it last longer.
     
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  11. Polly Wog

    Polly Wog Member

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    I just checked the Tesla website, and the charging calculator is still there, but with greatly reduced choices. (Tesla Charging | Tesla Motors)
    You can still compare 240V/40A charging with 110V/12A charging, though, and at $0.12 per KWh, you save nearly $3 per charge when charging for 200 miles of range. Obviously, the savings would be significantly less if you were comparing 40A charging to 30A charging at 240V, but it would be a savings just the same. For me, I pay $0.38 per KWh, so my savings are significantly higher than average by charging at the higher rate.
     

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  12. Rocky_H

    Rocky_H Member

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    *gulp* Whoa, I am a bit spoiled with our rate of 7 to 8 cents.
     
  13. DCGOO

    DCGOO Member

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    It doesn't matter. The UMC is included with every car. The wall connector is always extra. I bought one for two reasons: I can charge at over 40 if I want; and I can leave the UMC in the car at all times.
     
  14. AWDtsla

    AWDtsla Active Member

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    HPWC is more efficient than a UMC even for 40A. I don't have an exact measurement, but a ballpark guesstimate of 3-5 additional volts lost through the UMC vs HPWC. So around 160 watts of loss at 40A.
     

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