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Is car's charge voltage accurate for you?

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I have two 14-50 plugs, one side of the garage ~50ft run. And other side ~15ft run. Both runs are 8AWG copper.

I have a J1772 charger (with a readout of Volts, Amps, Kwh etc...) and the Tesla Mobile charger. When plugged in and not pulling any power, the readout on the J1772 and On Tesla screen agree, Voltage on the line is ~241v.

I measured the voltage at the meter and for both outlets it reads 242V. Under load (when charging) this read sometimes goes down to 239V sometimes stays at 242V.

As I start the charge and AMPs ramp up to 32 on the 15ft run the car reports as charging at 237V. When charging on 50ft run it reports as charging at 235V.

It is expected that 50ft run would have bigger voltage drop (as much as 1%). However getting 235V is between 1.5% to 2.5% drop (depending on reading at the meter). To put it in perspective a 4V drop is like powering two 60W incandescent bulbs, that heat is going somewhere. The real question is though, is there really a 4v drop or is it a reporting issue.

When I use the J1772 charger on the Tesla, the Voltages reported by the car match that of Tesla charger however the charger itself is reporting 239V on the 50ft run. This is much more in line with whats expected.

This got me thinking, either both runs have issues and additionally the J1772 readout is inaccurate. I would think all those coinciding is very unlikely. Or the Tesla's Voltage reporting is inaccurate (or maybe accurate but is reported at a different stage in the electron's journey). I eliminated voltage drop at the supply (by using multimeter at the meter), so its not a supply issue.

What has your guy's experience been with the mobile charger? The unfortunate caveat is that you could be getting 245V at the meter, then if charger is inaccurate it can still show 240V charging (at 5V drop). I guess this can be verified without opening up the panel and measuring by plugging in the car, and stopping charge and seeing what voltage is reported on the line. Perhaps this is not very scientific but better than nothing.

Thoughts?

Another thought is that J1772 reports Voltage at the outlet, while I dont know at which stage Tesla report's voltage but could it be after DC conversion and therefore some loss? Could the Tesla Brick itself be eating some voltage (it has some sort of onboard electronics) and then car's electronics eating some before its reported?

One thing for sure, both J1772 and Tesla charger are providing same amount of voltage (since Tesla reports the same number on both), but Tesla's readout is at odds with J1772's readout and also at odds of the math that dictates resistance in those runs.

Would love to hear your guy's opinions.
 
I have never heard of any problems or inaccuracies with the voltage readouts, and really, that's kind of hard to get wrong.

I think you're not considering the most likely possibility: bad wire connection issues in one of the runs is making a resistive connection point, which is causing more voltage drop in that line. Tesla instituted a software check in the car to check for this to try to catch bad connections in the circuit. It checks the voltage without load, and then compares to the voltage once it ramps up the current for charging, and if it sees a really bad voltage drop, that likely indicates a bad resistance in there somewhere, and the car will back off the current and show an error message on the screen.

A bad connection can be a few things, but two most common are either just screws not tight enough clamping a wire, or sometimes where the plastic insulation was stripped off the wire, that sometimes got pushed too far in, and some of the plastic is pinched underneath the screw, so it's not making as much connection to the metal. I've seen reports of both of these several times on this forum.

So look for stuff like that before doubting the voltage readings.

[Edit] Oh yeah, forgot to ask this:
I have two 14-50 plugs, one side of the garage ~50ft run. And other side ~15ft run. Both runs are 8AWG copper.
Was this Romex in wall or separate wires in conduit. If it's wire in conduit, 8 gauge would be OK for a 50A circuit, but if it's Romex, that's undersized.
 
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[Edit] Oh yeah, forgot to ask this:

Was this Romex in wall or separate wires in conduit. If it's wire in conduit, 8 gauge would be OK for a 50A circuit, but if it's Romex, that's undersized.

What kind of wire was used? Romex? THHN in conduit? (8AWG is kind of small for a 14-50 receptacle. So I would expect voltage drop especially on the 50 ft. run.)

Sorry, the wire is 6/3 NM-B (Southwire Romex), rated for 55A and I have a 50A breaker on it.
 
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I have never heard of any problems or inaccuracies with the voltage readouts, and really, that's kind of hard to get wrong.

I think you're not considering the most likely possibility: bad wire connection issues in one of the runs is making a resistive connection point, which is causing more voltage drop in that line. Tesla instituted a software check in the car to check for this to try to catch bad connections in the circuit. It checks the voltage without load, and then compares to the voltage once it ramps up the current for charging, and if it sees a really bad voltage drop, that likely indicates a bad resistance in there somewhere, and the car will back off the current and show an error message on the screen.

A bad connection can be a few things, but two most common are either just screws not tight enough clamping a wire, or sometimes where the plastic insulation was stripped off the wire, that sometimes got pushed too far in, and some of the plastic is pinched underneath the screw, so it's not making as much connection to the metal. I've seen reports of both of these several times on this forum.

So look for stuff like that before doubting the voltage readings.

Well, I mentioned in the original post that my 1772 charger also gives voltage/amperage readings. So for there to actually exist a voltage drop, both of the runs would have to be bad in some way, plus this charger's measurements would have to be incorrect. This is why I am assuming that Tesla is not accurate, because 3 things say that voltage should be one thing, and Tesla is the only thing that doesnt agree with it.
 
You have to remember there is some resistance in the connector itself as well. 2-3 volt drop is no big deal. Imagine one of that is in the connector, one is in the 15 ft run, and one is before the meter then you realize that’s not that much heat.

if you’re concerned or want to just get nerdy, buy an IR camera and see where the heat is.

my voltage goes from 252 to 245 at 48 amps, but half of that is on the other side of the meter, and I know some is in the 18ft connector cable because that thing gets noticeably warm.
 
I get a ~2 volt drop in the 18' HPWC cable at 48 amps. Not real thrilled with that 100 watt loss. I have about the same loss across the combined run to my garage subpanel (80' of 2/0 Al), 60 amp breaker (yeah, it gets warm too), and 38' run to the HPWC (4 gauge Cu).

I verified that the voltage the car reports is correct by checking it when the current was at 2 amps (minimal drop in the wires).

BTW, I use one large loop across the HPWC when charging now, which makes the cable quite a bit cooler.
20200719_144750.jpg
 
From my experience it's normal.
Most the heating occurs at the connections.

The Gen3 HPWC gets really warm. Mine got so hot at 40 amps it shut down. Air temp 60°F. Service size temps 87°F high, EVSE side temps 111°F at the outside of the insulation. Tried to warranty it and Tesla won't actually issue an RMA. They say they will, but don't.
 
Thanks for the input guys. Yeah, I am not concerned about heat too much, I just hate wasting energy, every watt wasted makes electicity vs gas pricing skew towards gas by tiny bit, the only part that gets warm is the Tesla's own cable (between the brick and the car). @MY-Y , nice point about the loop, I was kind of doing that naturally because I dont need the entire length to reach the port. But I dont know that the loop makes it any cooler? because without the loop the excess cable is laying on concrete garage floor where draft (from under the garage door) is biggest.

@craigery havent opened it up, but just from the feel of it its got to be like 10 awg, which should be ok for 32A that is it's max. Now that I think about it. Wonder if thats where the voltage drop is coming from. The J1772 charger I have (which is also portable), has thicker cable (yet surprisingly just as flexible as the tesla one), Im guessing its 8awg. But then again, Tesla is reporting the same voltage with both chargers (caveat being that J1772 has to go through the adapter too, so maybe some loss there).
 
J1772 cables are a tradeoff between copper cross-section and ease of use. They typically have a cross section equivalent of 10 AWG or even 12 AWG, which is going to get warm and have an easily measurable voltage drop.

A rough estimate for 10 AWG is 1 ohm per thousand feet. A 25 foot cable is 50 feet round-trip, so 50 milliohms. 32 amps will result in a drop in 1.6V just for the cable. Add in the connector contact resistance and some wire thinning from flexing and you can expect a 2V drop just from that part of the circuit.