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HPWC Charging Limited to 79A

Discussion in 'Model S: Battery & Charging' started by breser, Feb 26, 2015.

  1. breser

    breser AutoPilot Nostradamus

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    As I mentioned in my 85D First impressions thread my car is limiting itself to 79A on an 80A HPWC setup.

    To be clear I'm not talking about the perfectly normal variability in the amperage you see for what current the car is currently receiving (what I'll refer to as instantaneous display). This instantaneous view always bounces around a bit and it's perfectly normal for it to show 79/80A while charging. To repeat, that's not what I'm talking about.

    In the charging menu there is a the selector that lets you set the maximum amperage to use. You can set this all the way up to 80A (with dual chargers) when not plugged in. When you plug in the maximum is limited to whatever the pilot current is (i.e. whatever the connector says is available to the car). In my case the car lowers the maximum amps to 79A. It then doesn't let you raise it back to 80A until you unplug. Plug back in and most of the time it lowers back to 79A. This lowering also seems to be sticky, you have to keep raising it to 80A. So far I've only ever seen it stay at 80A once. My S85 never did this.

    In the other previously mentioned thread two P85D owners showed up and said they had the same strange behavior.

    Andyw2100 made a video of it and shared it with both of us privately and we confirmed that all 3 of us were having this strange behavior. So it's possible this is something specific to D's. I have a service appointment on Monday for this and a couple other minor things on my 85D. But given that there's 3 of us with this behavior and wk057 says the P85 doesn't do the same thing with the same HPWC, I think there is something weird going on here. I'm splitting this out here under the hope that may other people have seen this and I can walk into the service center with more evidence.
     
  2. wk057

    wk057 Senior Tinkerer

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    I've noted that my energy monitoring setup also reports 79A of power draw from the P85D and 80A from the P85, so it doesn't appear to be a rounding issue.
     
  3. Andyw2100

    Andyw2100 Well-Known Member

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    #3 Andyw2100, Feb 26, 2015
    Last edited: Feb 26, 2015
    I'm guessing this affects a bunch of cars...possibly even all of the Ds. I think the reason we haven't heard more about it is that everyone experiencing it probably felt it was trivial enough not to worry about, or may have received or believed the same information I had, which we now know to be incorrect--that the display was showing 79, but that the car was actually receiving something closer to 80. And perhaps a lot of people just didn't notice. And some may not have known what to expect, but also didn't really have any place to go to ask. (When I first saw the 79 instead of 80, as a first-time Model S owner, I wondered if it was actually normal, but I came and inquired here. Someone who wasn't using the forums probably wouldn't bother to call a service center to ask about that.)

    I'd be shocked if this is limited to a small number of cars.

    By the way, below is the post I had made inquiring about this the second day I owned the car. As breser and wk057 and I have been talking about this, I've been feeling a little guilty about not having done a better job of bringing this to light when I noticed it, and wondering if I just didn't describe the issue correctly. I was a brand new Model S owner, after all. But looking at what I wrote now, I think I actually did describe the issue reasonably well, for a newb. Perhaps other P85D owners just never saw that post, or hadn't noticed the issue with their cars yet. (Or perhaps breser, wk057, and I have the only three cars doing this. I guess we'll find out soon enough.)


    Model S - HPWC - Page 86

     
  4. Puff

    Puff Member

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    ive plugged into almost all the HPWCs at the service center. 79A.
     
  5. breser

    breser AutoPilot Nostradamus

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    Yup if I hadn't had the previous car I would have thought it was normal. When I got the first car I watched charging a bit like a hawk to get some understanding of it. This time I came home, plugged it in and didn't think much about it. It took me a couple days to notice the charge limit was at 79A and then I just thought it was because maybe the service center had lowered it to that. Raised it and it took me a couple more charges to realize it was back at 79A and then I started investigating.

    - - - Updated - - -

    Don't be so hard on yourself. So far quite a few people who have seen the posts about this in other threads have concluded we were just talking about getting 79/80A on the instantaneous current display which is perfectly normal. I don't remember seeing your post but if I had I probably wouldn't have thought too much of it.
     
  6. Andyw2100

    Andyw2100 Well-Known Member

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    The video is available at the URL below, if anyone wants to watch it to see if their car is exhibiting the same behavior or not, or just to see what our cars are doing.

    P85D charging at 79 Amps instead of 80 Amps - YouTube
     
  7. RiverBrick

    RiverBrick Active Member

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    I don't have a D, but 80A and 70A charging dropped by one amp after a software update. Not 100% sure, but I think it was when going from 5.9 to 5.11.
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    We're talking about two different things...

    The first is when the car shows "79/80A" which mine does: Tesla has told me that's a tolerance issue - they have to make sure there's room for the EVSE overhead and such.

    But breser is pointing out (and the video confirms) that his maximum current drops to 79A and won't go up to 80A. It seems like there might be a duty cycle sense problem somewhere, such that the car is calculating the maximum current is 79A.

    My car shows "79/80A", reporting a maximum current of 80A - but the car sometimes never draws that.
     
  9. breser

    breser AutoPilot Nostradamus

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    My guess is a software bug in calculating the max amperage from the pilot signal. Perhaps an off by one bug in the calculation (though usually I'd expect that to be consistent and this one isn't always). Possibly some sort of rounding happening in the calculation that wasn't before. Maybe the hardware is a little more or less sensitive and that's causing the off by one issue. It's hard to say but in my opinion it's a bug.

    Losing 1 amp out of 80 isn't a dramatic loss in charge rate. You go from 19.2 kW to 18.96 kW and if the voltage is lower than 240 it's even less of a difference. However, at lower charging rates it becomes more and more significant. I actually haven't tried charging at lower rates but I wonder if they aren't impacted too. I'd guess that people would have said something about this earlier though if it did impact NEMA 14-50/40A charging.
     
  10. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    The J1772 duty cycle is calculated differently for 51A+ vs. 0-50A. There's a wider margin for error below 50A... 0-48A takes up 80% of the duty cycle, then the remainder of the range (49-80A) gets to use a 16% range of duty cycle before it's considered an error, in 2.5A steps per 1% of duty cycle. 50A+ is (amps / 2.5) + 64.

    Weird, though, that 79A would be detected - I wouldn't think they'd use that granularity in their duty cycle. Wonder if it's a different approach in the firmware to the 79/80A situation I see.
     
  11. wk057

    wk057 Senior Tinkerer

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    I have two HPWCs and both give the same result. 79A max on the P85D (9 times out of 10) and 80A on the P85.

    The first thing I considered was the duty cycle being misread, but I haven't been able to test that.
     
  12. breser

    breser AutoPilot Nostradamus

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    I'm going to be lazy and ask if you know where the docs are on that. I'm hoping there's a source I don't have to pay some absurd fee to see the spec.

    Could be but it's annoying since it makes the 79A sticky. Once it detects 79A max when you unplug the location is still 79A. You have to turn it up every time to get 80A. Most of the time it reverts back to 79A but I did have it stay at 80A last night. So it's definitely intermittent.
     
  13. rlang59

    rlang59 Member

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    You can find some of the info over on the OpenEVSE wiki:

    https://code.google.com/p/open-evse/wiki/J1772Basics
     
  14. breser

    breser AutoPilot Nostradamus

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    Thanks that helps me visualize the potential problem. Looks like the duty cycle for 80A would be 96% and 79A would be 95.6%. Can't really fathom why they'd round that down to 79A in software. Seems more likely this is a hardware issue that's causing a misread of the duty cycle.
     
  15. andrewket

    andrewket 2014 S P85DL, 2016 X P90DL (soon 100)

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    Sounds like a software bug.
     
  16. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    So those of you with the P85D have never seen 80A as the pilot-current max on any chargers?

    I'm just guessing here that they may have simply changed the code in the newer cars to show 79/79 instead of 79/80, all to achieve the same means.

    I'll ask a few folks I know.
     
  17. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I have never seen the 79/79 display on my P85D with multiple firmware versions and two different HPWC's. I consistently see 80/80 or occasionally 79/80.

    In an earlier version of firmware on the P85D, I saw 70/0 displays while charging on two different 70A Clipper Creek J1772's. I just treated this as a curiosity because while the max was displayed as 0 Amps I got the 70 Amps delivered to the car.
     
  18. Andyw2100

    Andyw2100 Well-Known Member

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    Well there goes my theory that it affects all the Ds.

    Was yours a fairly early P85D, Cottonwood? I know WK057s and mine were built at around the same time, because at one point I thought they might have been on the same truck. (In retrospect they almost certainly weren't, but they arrived at their respective SCs on the same day.)

    So I'm wondering if there could be some small hardware change.
     
  19. breser

    breser AutoPilot Nostradamus

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    I've seen 80/80 and 79/80 and 79/79. Based on wk057's post I believe he has as well (note his comment about 9 times out of 10).
     
  20. mknox

    mknox Well-Known Member

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    FWIW I see a similar situation on 100 amp Clipper Creek / Sun Country Highway J1772 chargers. In theory, they should deliver 80 amps.

    When I plug in with my car initially set to 80 amps, it will drop to 79 amps and once charging, you can't bump it back up to 80. While charging, I will see 79/79 amps but more typically 78/79 amps.
     

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