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Amp Limits on UMC

Discussion in 'Model S: Battery & Charging' started by Alexander, Jan 25, 2014.

  1. Alexander

    Alexander P# 8,878

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    Does anyone know the maximum number of Amps you can get through the UMC? When I was at the service center yesterday, one of the reps said I could set the car to charge at 40 Amps through the UMC. But I'm curious what you guys/gals think?

    If you could set it to 40 Amps, should you? is that a good idea? My NEMA 14-50 outlet is a 50 Amp outlet.
     
  2. Discoducky

    Discoducky Active Member

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    40 is limit as the 50 amp circuit is for overhead
     
  3. ZBB

    ZBB Emperor

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    40... You can only have a continuous load at 80% of the breaker rating... For a 50 amp breaker that is 40 amps...
     
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    50 amps is the rating of the circuit breaker used. Because charging is a continuous load the limit is reduced by 20% or to 40 amps.
     
  5. Alexander

    Alexander P# 8,878

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    So if you had a 60 amp circuit, could you set it to charge at 50 amps? or is 40 amps the absolute limit for the UMC?

    Sorry if this is dumb question...
     
  6. howardc64

    howardc64 Member

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    I don't believe the car will let you dial the rate above 40A using the UMC?

    I set mine at 30A because the short cord from the adapter the UMC main body is significantly cooler than charging at 40A rate. Only lose 4mph charge rate (29-25).
     
  7. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    That's correct. I usually set mine at 32 amps.
     
  8. Alexander

    Alexander P# 8,878

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    Learning has occurred... thank you!
     
  9. TexasEV

    TexasEV Active Member

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    Actually you don't have to set it at all. When you use the UMC with the 14-50 adapter the car sets the charging at 40A(unless it's reduced because of sensing a voltage drop). If you were to use the 5-15 adapter to plug into a standard household outlet the car would set itself at 12A, etc. You don't need to think about it, unless you have a problem charging it which case you should try dialing down the amps.
     
  10. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Dialing the amps down, as long as you don't need the speed, is easier on the grid and may help the pack remain in balance.
     
  11. caddieo

    caddieo Member

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    Exactly. I have the car programmed to start charging at midnight, and for routine daily errands, I need no more than 50 miles. I've tried everything from 15 to 40 amps, and regardless of the amp level, I've got my 50 miles by the time I'm ready to go. I've settled on 20 amps most of the time - less if I have a smaller deficit.
     
  12. youlikeadajuice

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    As for pack balance, I am skeptical that lower is better. I charged for months at 240 volts 15 amps and consistently finished charging at 168-170 rated (90% charge in a 60). The last few days I've charged at the full 40 amps, and have seen 173 rated every time when charging finishes.
     
  13. Larry Chanin

    Larry Chanin Model S Perf Sig 1055

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    You may be correct to be skeptical about lower amperage being better for balancing, but in your example you didn't do a maximum range charge. I understand that balancing occurs when the battery pack is fully "topped off".

    Larry
     
  14. howardc64

    howardc64 Member

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    #14 howardc64, Jan 25, 2014
    Last edited: Jan 25, 2014
    I believe a recent firmware update within in last couple of months (can't remember which one) dialed the default 14-50 charging rate to 35A (probably for higher safety precaution). But as soon as you dial it to your desired setting (up to 40A), the car remembers the charge rate per location (and probably per plug type as well? Haven't tried this). Not sure what the default 14-50 setting is for latest firmware (5.8.4) with auto charge current reduction feature.
     
  15. youlikeadajuice

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    I've read this many places as well, I wonder what accounts for my increased range at 90% when charging at higher amperage?
     
  16. Atebit

    Atebit Member

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    The UMC is limited to 40A output. If I understand correctly a car with a single on-board charger will draw 40A max (except for DC charging). To get above that you need twin on-board chargers.

    The info about the default with the new updates is interesting...it's sounds like there's some but not a huge benefit to upgrading my 40A EVSE-RS+ circuit to 50A & a 14-50 receptacle.
     
  17. Rainbow

    Rainbow Member

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    Sometimes when batteries are charged for a short period or very fast they will sometimes appear to have more of a charge than they really have. This phenomenon is often called a "superficial charge".

    Generally it is better for battery life and safety to charge at the lowest practical rate. What is the lowest practical rate depends on your driving habits.
     
  18. Chris TX

    Chris TX Active Member

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    Is your 40A EVSE being limited to 32A? If so, I would upgrade it to a 50A circuit to get the full 40A that the car can pull. Even if you limited your car to draw 20A through that 50A circuit, you're getting the conductive efficiencies of a fatter pipe.

    I'd rather pull 20A through a 50A line than 20A through a 40A line. Small efficiency gains, especially if you're going to switch from 8-3 to 6-3 copper over a long distance.

    - - - Updated - - -

    The 18650A batteries used in the Model S do not suffer from that phenomenon. While they are the same form factor as the ones used in your laptop, flashlight, and nicotine vaporizer, Panasonic has developed some notable differences in them for Automotive use.
     
  19. MSullivan

    MSullivan Member

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    I set my UMC at 32 amps, it is on a 40 amp circuit. I usually come back to find that the car is charging at 30 amps with a max of 30 amps. I think it is because it is in the same panel as my geothermal unit. When it turns on the lights blink so I guess it is causing a voltage drop.
    Does anyone know if after throttling down the UMC will go back up when it thinks the voltage is adequate? Hard to tell because sometimes when I go out it is at 32. Did it go back up or never drop down? Short of sitting in the car the whole time I can't really tell.
     
  20. TexasEV

    TexasEV Active Member

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    No when it drops to 30A for a possible safety issue it stays at 30A unless you manually turn it up to 40A in the car.
     
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