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Charging 120V @ 32A

Discussion in 'Model 3: Battery & Charging' started by n2mb_racing, Oct 5, 2019.

  1. n2mb_racing

    n2mb_racing Active Member

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    Not sure when you'd ever do this out in the wild, but in case you are curious, it works. The highest current 120V outlet you are likely to encounter would probably be 120V @ 24A at TT-30 camper outlets. There's an adapter for those outlets that works.

    I'm curious to know if the UMC will accept 120V - 240V DC for charging... Anyone try that yet?
     

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  2. user212_nr

    user212_nr Active Member

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    Not an expert, but if a device is wired for A/C input, then it cannot be plugged in to DC. Pretty basic. The car itself can DC charge, special exception to that, but where would you get a DC input source at such voltage?
     
  3. earthwormjim

    earthwormjim Member

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    Nonsense, you can run DC power to an AC device that uses a line voltage bridge rectifier, provided the DC level is the same as the RMS AC level. What do you think a bridge rectifier does? Converts AC to DC. What happens when you pass DC through a bridge rectifier? You get DC, just with a 2 diode drop after. End result is still DC.

    The only issues you might run into are polarity and damaging contacts from arcing when plugging in, the UMC does a ground fault test, and it might not like it if the ground or negative input of your DC is not on the same terminal where the neutral normally would be. When plugging into DC, you need to be sure its a true DC outlet, to avoid arcing and damaging the contacts. You would probably need to make a dongle or pigtail adapter for the UMC, so you can use a proper DC connector, or have an on/off switch for the outlet so it is denergized when you plug in.
     
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  4. user212_nr

    user212_nr Active Member

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    You seem smart and stupid at the same time. Bridge rectifier... duh.

    Lets plug in DC to an AC device because - bridge rectifier. What is the need here?
     
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  5. earthwormjim

    earthwormjim Member

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    I honestly have no idea why you would need to charge a car with 120V DC. I do know that on some boats and docks, high voltage DC is the only power available, but I don't see needing to charge a car on a boat. I work in the lighting business, and design power supplies, so the question has come up before, what happens if you power a power supply with DC at 120V? The answer is, it works fine, about the same as powering it 120Vac. A charge controller in a car should be no different.


    What exactly seems stupid?
     
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  6. ewoodrick

    ewoodrick Well-Known Member

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    No, it won't

    And while there are situations where you may be able to run DC into a bridge rectifier, that's not what we are talking about here.

    The Tesla charger is is a switching supply that's got to boost the voltage up to charge the battery.

    And cable size gets to be an even bigger issue at DC. At the same voltage, DC, is going to create 1.4 times the heat.
     
  7. Bandit

    Bandit Member

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    I noticed the screenshot shows you're pulling 32A. If using a TT-30 remember to manually back the amperage to 24, since you don't have the one that signals it automatically.
     
  8. n2mb_racing

    n2mb_racing Active Member

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    Yes, I'm aware. I wasn't using a TT-30.

    Interesting discussion on the DC input. I was thinking the same thing. First stage input is probably a full wave rectifier to DC. The charger is likely a switching DC supply, buck boost, although almost always boost. Maybe on 277v AC with a really low charge SR pack? But yeah, the battery voltage is probably always higher than the rectified DC input.

    Anyway, might try and see. I'm guessing neutral to negative and positive to hot. And short neutral and ground (floating battery supply).
     
  9. earthwormjim

    earthwormjim Member

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    #9 earthwormjim, Oct 5, 2019
    Last edited: Oct 5, 2019
    What do you think is inside of the switching supply in the Tesla charge controller? A bridge rectifier. In fact a switch-mode power supply only works on DC.

    I have no idea where you got the idea that DC is going to create 1.4 times the heat either. 120 Vac rms load at 1 amp will cause the same wire heating as a 120Vdc load at 1 amp. A wire doesn't care if the voltage alternates up or down,, it's effectively a purely resistive loss. If the root mean square is 120V, then that is essentially the same as 120V average, or DC from a resistors perspective. In practical use, you will always have higher wire losses with AC, because you will never have a perfect power factor.

    It should even work with a power factor correcting design, the input current will just follow the input voltage, i.e. it will be DC.

    It's definitely going to be at least a two-stage design, you're not going to get a 12kw charge controller with just 1 stage. First stage will definitely be a boost, a buck-boost has higher peak current and greater losses than a boost. Since we know the charge controller can handle 277Vac, it's probably boosting the first stage to at least 430-450 Vdc, so it can handle 277+10% regulation. A boost stage will always have a higher output than the peak of it's input AC voltage (305*sqrt(2)= 431V peak). Also the most advanced power MOSFET development is all happening around the 600V level, so a buck-boost design isn't even needed, if you're going to be boosting to 450V+ anyway.

    Second stage possibly an interleaved resonant design or maybe an interleaved forward. There's no way the charge controller isn't isolated. They might have multiple separate outputs derived from this stage, so that they can deliver appropriate current to each set of batteries for proper balancing.
     
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  10. user212_nr

    user212_nr Active Member

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    A bit too aggressive and condescending. I may read through what a bridge rectifier is when I have time, but I'm not convinced.
     
  11. nwdiver

    nwdiver Well-Known Member

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    Only if it's 50 or 60Hz......
     
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  12. earthwormjim

    earthwormjim Member

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    Oh, I'm sorry.
     
  13. n2mb_racing

    n2mb_racing Active Member

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    Anyway, the answer is NO. The UMC will not accept 120V DC. It throws a red error light after startup.
     
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  14. earthwormjim

    earthwormjim Member

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    #14 earthwormjim, Oct 11, 2019
    Last edited: Oct 11, 2019
    Possibly the ground fault circuit interrupt test the UMC performs fails under this condition then. It's supposed to send a small amount of current from hot to ground, to make sure the differential current sensing is working. Have you tried swapping the + and - on your DC input? If your DC - is tied to where the UMC is expecting hot to be, when it tries to send current to ground, no current will flow, because your DC - might be at the same voltage as ground (0V).

    The car could also be looking at the peak of the rectified AC input, since 120V DC won't have a peak of ~167V, the car thinks it is being undervolted, even if the averages would be the same.
     
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  15. n2mb_racing

    n2mb_racing Active Member

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    I tied negative to neutral and ground. I connected the positive to hot. I figured it would check the connection between neutral and ground.
     
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  16. Darmie

    Darmie Supporting Member

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    What type of plug was you using?
     
  17. n2mb_racing

    n2mb_racing Active Member

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    5-20.
     
  18. earthwormjim

    earthwormjim Member

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    Car or UMC must be checking for the peak of the rectified input then, rather than the RMS level, and is concluding the input voltage is too low.

    Just curious, what did the car display show when you plugged it in? Did it even show input voltage?
     
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  19. n2mb_racing

    n2mb_racing Active Member

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    Oh, didn't try plugging it into the car since the UMC didn't start up.
     

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