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UMC/home charge current limited: new behaviour?

Discussion in 'Model S: Battery & Charging' started by cezdoc, Jan 18, 2017.

  1. cezdoc

    cezdoc Member

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    Location:
    Aberdeen, UK
    I charge at home using the UMC connected to a 32A/240V supply. I've had my MS since March 2016 (pre-facelift S70 built ~December 2015) and the car has consistently charged overnight at an average of ~7.4 kW (32A x ~230V). On 14th Jan the average overnight rate was only 5.4 kW and I noticed the app (and car console) only indicated 24A available. This is the first time I've noted a reduced average overnight charging rate at home.

    It was below freezing (about -1 Celcius / 30F) on the 14th, for the first time in a few weeks. However I noted the charge rate behaviour on 21st November when it was slightly colder (about -3C), and in that case although it took 10 minutes to reach full power (while I presume the battery was heated up) the available current was always the full 32A (see pictures below).

    I noticed a couple of other UK owners commenting that their home charge rates had dropped in the last week or two and speculated that current supply limiting is a new feature triggered by a software update? Has anyone else seen any change in behaviour in recent weeks?


    Screenshots

    Behaviour in November, temperature ~ -3 Celcius. Charge rate ramped up from 0 to 7.4 kW over 10 minutes. Available current shown as 32A throughout. First pic 5 minutes after charging started (12A) , second pic after 10 minutes (32A).
    IMG_0683_ramp32minus2tominus4C_21-Nov.PNG IMG_0689_ramp32.PNG


    Behaviour in January, temperature ~ -1 Celcius. Available current only 24A (overnight power averaged 5.4 kW).

    IMG_1106_24A_minus1_14-Jan.PNG
     
  2. gjunky

    gjunky Waiting for the Model ☰

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    Does the car start charging at the lower rate or slow down afterwards? (I know you showed the difference after a short period. The car always ramps up charging at first). Did you try to unplug / re-plug the UMC? Could it be that you have moisture buildup in the outlet or perhaps lose connections inside or corrosion?

    The charger will slow down if it detects issues with the power source (plug). Especially since the car shows 24/24 instead of xx/32.

    Just some thoughts.
     
  3. brkaus

    brkaus Active Member

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    I'm not familiar, but what is the main source of heat over there? If it is electric, heating may be stressing the grid causing the car to reduce the rate.

    Can you set it back up manually?
     
  4. PaulusdB

    PaulusdB Mayor Gnomus Vintage Limb

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    The car currently thinks it is connected to a 24A capable charger.
    Just look at the indicated 24/24A, where it used to be /32A.
    So either the UMC malfunctions, or the pilot resistance is wrongly measured/interpreted by the car.
    Cleaning all the connectors beween car and UMC might help.
     
    • Informative x 1
  5. cezdoc

    cezdoc Member

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    Okay thanks for the thoughts, it sounds like I should double-check what the car thinks the maximum supply is once I've plugged it in (e.g. xx/24A or xx/32A) in case there is moisture etc. causing an issue. Normally I'm asleep when scheduled charging starts but I can initiate a charge after plugging in and see what the console/app is reading. It feels like this is more likely to be related to the UMC/supply than the car. The cold weather and other reports of slow charging could just be coincidence.

    Thanks for the suggestion but I don't think it's grid stress - most heating here uses gas (although we actually have a wood pellet boiler as we're not on the mains network) so overnight the electric power load is usually much lower than during the day. The house supply seems to be pretty stable. As the car was showing xx/24A it would not have been possible to set the current any higher manually.

    Usually the car ramps up to 32A within a minute or so, in the first example it took 10 minutes to get there which I think was the time it took the battery heater to get the cells warm enough to accept full charge. The other thoughts sound promising, i.e. check the connection/unplug/replug etc. I'll give it a go if I see reduced power in future.
     
  6. tga

    tga Supporting Member

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    #6 tga, Jan 18, 2017
    Last edited: Jan 18, 2017
    This is the same behavior I've seen plugging into a 30A J1772 with a cold battery in similar temps. Charge current ramped up to ~10 amps and held for 10 minutes or so, then increased to 30 but still showed a very low miles/hour rate for another ~30 min before rising to 22 mile/hour.

    This is not exactly correct. The number displayed after the / (i.e., /24 or /32) is the total current available. It is usually the value reported by the charge station/UMC on the pilot signal, but will be reduced if the car detects an electrical problem (i.e., too large a voltage drop). See this thread, especially posts #14 and #18:

    HPWC in Nashua - [email protected]

    I plugged into a J1772 advertising 32A. As the current ramps up, the voltage drops substatially (20V). The car displays the "bad wiring or extension cord used" message, and reduces charge current from 32A to 24A. Note that the display goes from "/32" to "/24" when this happens.

    So the number after the slash is the available current, after any safety-related reduction for power supply problems. At least that's how it works in North America, but I'd assume that's how it works world-wide.

    Start a charge and watch the voltage and max available current reported as the charger ramps up the current. Look for the "bad wiring" message on the instrument panel. See if it starts at /32, then drops to /24 during the ramp.
     
    • Informative x 2
  7. cezdoc

    cezdoc Member

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    [QUOTE="tga, post: 1929087, member: 23786"So the number after the slash is the available current, after any safety-related reduction for power supply problems. At least that's how it works in North America, but I'd assume that's how it works world-wide.
    [/QUOTE]
    Thanks tga, this seems to be the most likely explanation for what I observed. One of the UK owners also found it mentioned in the Owner's Manual in the Charging Instructions section (the car automatically reduces the charging current by 25% if there are unexpected fluctuations in charger power). I'll keep an eye on the charging and maybe use a data logger to monitor it if I'm tucked up in bed! It seems brkaus was right to question the grid supply!
     
  8. smilepak

    smilepak Active Member

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    I thought the UMC auto detect and would charge at the highest default possible?

    I have a dedicated 50A on NEMA 14-50. So the charge rate by default on the UMC should be 40A? I haven't pay attention since I usually charge in the middle of the night, but recently noticed it been charging at 30A/40A.

    I went on the console and noticed it auto throttle down to 30A saying 40A maximum. On the app it say 30A/40A. Should it auto go to 40A?
     
  9. tga

    tga Supporting Member

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    Once the car detects a power issue and reduces charge current, it will stay at the reduced charge rate until you manually increase it on the charging screen. So no, it won't automatically go back to 40A in your case - you have to do it manually.
     
    • Informative x 2

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