HPWC Charging Limited to 79A

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

breser said:

Refuses to charge at 80A on my HPWC. The S85 did so just fine. I turn it up to 80A, plug it in and it turns itself right back down to 79A. Really strange because I can't fathom what's helpful about turning it down by 1A. It refuses to let me turn it up from 79A when plugged in like the pilot signal is saying it's plugged into a 79A charger. I figure service can help me by seeing if this is just something that's happening with my HPWC or something to do with the car.

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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 said:

My P85D does this on both my HPWCs, which are at two different locations. When I posted about it here, inquiring, I was basically told not to worry about it. Apparently the display truncates, and the 79 could actually be 79.9 or 79.8.

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wk057 said:

My P85D reports 79A max as well sometimes, while the P85 using the same charger will always report 80A. I've been meaning to ask Tesla about this, but keep forgetting. Please let us know how that goes.

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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.

Andyw2100 said:

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.

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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 - - -

Andyw2100 said:

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.)

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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.

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.

FlasherZ said:

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.

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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.

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.

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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.

rlang59 said:

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

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

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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.

FlasherZ said:

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.

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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).