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Varying Charge Speed

Discussion in 'Model 3: Battery & Charging' started by Nb1277, Oct 31, 2018.

  1. Nb1277

    Nb1277 Member

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    I always get 22 miles/hr charge on the same 30 volt level 2 charger I use at my apartment complex daily. This morning, upon waking up to an ~80% charge and realizing I had a lot of driving to do today, I initiated charging again from my phone and received only 10 miles/hr.

    The 22 mil/hr charge is very consistent but my battery usually has much less juice. Is it normal for an L2 charge rate to slow significantly when the state of charge is higher?
     
  2. Rocky_H

    Rocky_H Well-Known Member

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    I think I know why this is happening, but it has nothing to do with your state of charge. It's the temperature.
    Your charging had finished long before you restarted it, so the battery was cold, so when you told it to start, it was diverting energy to heating up the battery before it can use the whole 30A for charging. Over the long term, it will get up to temperature and the charging rate will rise, but early on, it's going to look low.
     
    • Informative x 3
    • Like x 3
  3. TexasEV

    TexasEV Well-Known Member

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    @Nb1277, level 2 charging rates don’t slow down until you’re well above 90% charge. It’s not like at superchargers where the taper starts much earlier. But your experience is a good example of why I suggest routinely setting the charge level at 90% rather than 70 or 80% as some do. The battery will be just fine.
     
  4. Big Earl

    Big Earl bnkwupt

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    Yep. It was either doing battery heating or climate control for the cabin. Those will run off of shore power, thus reducing the amount of energy going to the battery pack.
     
    • Like x 2
    • Informative x 1
  5. Nb1277

    Nb1277 Member

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    Thanks everyone for the help, much appreciated!
     
  6. VT_EE

    VT_EE Active Member

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    Maryland
    I use a wall connecter set to 40amp and have noticed some strange behavior since getting version 42.2. For the past 3 months (since I got the M3), the vehicle always charged at 40/40. Since the update (which I got on Wed), the car regularly falls to 38/38, 36/36, etc in a perfectly warm garage well into the charging. Last night it even went down to 26/26 until I stopped and restarted the charge causing it to go back to 40/40. Has anyone else noticed this?
     
  7. eprosenx

    eprosenx Active Member

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    What was the voltage reported just as the charging was beginning (but before it actually ramped the amps up) and what was it when you found it in the slower charging mode? I am wondering if you are seeing the Tesla ramp down the allowed charge speed due to it perceiving an electrical issue in the house (indicated by voltage dropping). Normally though I have only seen this trigger where it cuts the charge speed by like a quarter (so from 40a to 30a say).

    To recap what others have said though about the OP’s question:

    Could be due to the car using shore power for heating or cooling the cabin
    Could be due to a cold battery needing to be heated so power may be stolen for that
    Over 90% charge rate can start to slow due to the battery not being able to take as much current once it is nearly full

    And finally, if the car detects various electrical issues (voltage drop, hot plug, connector to car not latched, etc...) you can see the car limit charge speeds or fully refuse to charge altogether.
     
    • Like x 1
  8. VT_EE

    VT_EE Active Member

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    My run is at least 60 feet of 6AWG Copper so I expect at least a 4V drop at load, which is what I have always seen. Last night it started at 239V and immediately dropped to 235V when it ramped up to 40amp. I might lose another 1V or so after a few hours of charging as the conductors warm up. This is normal and has been the case for months. I am not noticing any change in voltage when the vehicle reduces current draw. If car drops to 38/38 or lower, I can stop charging and restart, and we are back to 40/40 for an extended period of time. Sometimes it will crank back up to 40/40 on it's own. Nothing has changed except the software version. Question: What should the reading be if the vehicle detects a line voltage issue (not that I think I have one)? I thought it would read something like 35/40 if it reduced the current due to a suspected line problem where the first number indicates the current draw and the second number is just the Wall Connectors max allowed value. Am I mistaken?
     
    • Like x 2
  9. eprosenx

    eprosenx Active Member

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    Yeah, what you are describing sounds like normal voltage loss over a properly designed install. It should not be triggering the Tesla safety features.

    I have only tripped the Tesla backoff once and that be was with a loaner Model S with a UMC gen 1 drawing 40 amps. We were intentionally “torture testing” the utility companies transformer since they installed too small a unit and we wanted to prove it to them so we ran up 63000 watts of load (including the Tesla) on their 25kVA transformer. The Tesla got pissed and backed off during the test. Here is what that looked like:

    1A999A2B-59FF-49BA-A2AB-3AC4A80696A5.jpeg

    It cut max charge from 40 to 30 amps. I am not sure why it was down to 20 amps actual though. I may have stopped the charge at the point I took the picture (as we did not want to actually destroy the transformer) and the 20a was just heating the car? I also was at 90% when I started the test so we could have been into the “taper” of the charge curve.

    You have stumbled across something interesting! I look forward to figuring out if the new behavior is intended or not?
     
  10. VT_EE

    VT_EE Active Member

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    Maryland
    I got 42.4 last night. Tonight it's charging at a constant 40/40. Bug perhaps? And just to be clear, I never got the "Charging problem" alert.
     

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