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Why does NEMA 6-20 (240v 20a) get 4x the charge of 5-20 (120v 20a)?

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user212_nr

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Aug 26, 2019
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On Tesla's website, it states that NEMA 6-20 gets 15 mph for M3, and 5-20 gets 4 mph. That is exactly 3.75 times as many mph. I understand that watts = amps * voltage, so that equals 240x20=4800 and 120x20=2400. That would be twice the watts, exactly.

What am I missing?
 
I think it's actually more like 5mph at 20amp.

You're also missing the charging system over head.

But using their math let's says it's 4mph for 2400w and 15mph for 4800

That would mean the additional 2400w directly to the battery gives you 11mph more per hour. So 1 mph per 218w. (2400w / 11)

So to get the original 4 mph on the 2400 from the 120v at 20amp. That would be 4x218w =872w.


Meaning the charging system is using 2400-872=1528w

120v x 20amp= 2400w
1528w for charging system
872w to battery for 4mph

240v x 20amp = 4800w
1528w for charging system
3272 directly to battery for 15mph

Or it's late at night and I'm crazy...

Edit: Just realized the watt numbers are off because you're also not pulling 2400w or 4800w because it caps at 80% of the amperage. But again it's late and I'm not reworking the numbers. You get the overhead idea.
 
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The Tesla charts are wrong, it’s actually ~5 mph (8 kph) on 12A 120V for an SR+.

Overhead seems to be more in the ~250 W range +/- if you are charging with the car locked and user not present, HVAC off.

Just being in the car with the screen on pushes you up closer to an overhead of 350 W +/-.

As the charge power goes up, this overhead diminishes as a percentage.

I posted some charts in a 120V vs 240V efficiency thread if you search.
 
On Tesla's website, it states that NEMA 6-20 gets 15 mph for M3, and 5-20 gets 4 mph. That is exactly 3.75 times as many mph. I understand that watts = amps * voltage, so that equals 240x20=4800 and 120x20=2400. That would be twice the watts, exactly.
What am I missing?

The numbers on the Teslas website are a little optimistic, but that is only part of the explanation. Charging at higher rates is more efficient (the 'overhead' @darth_vad3r referred to) in all cases.

Real world example:

When I was charging (at home) from a NEMA 5-15 (120V, 15A) I was getting 4.2 MPH, with an efficiency usually around 75%. After switching to a NEMA 6-15 (240V, 15A) I see 9.4 MPH on average with efficiency around 90%. Tesla says I should get 3 and 11 MPH, respectively.
 
If you keep the amperage the same and double the voltage, your charge rate will double. It's a fact.

The AC input power will double. I suppose the disagreement (which does seem a bit excessive) was that the charge rate will more than double, due to fixed overhead, and to a lesser extent, efficiency changes in the AC-DC converter with a lower input power.

Charging 120v vs 240v efficiency
 
How can any informed individual disagree with my statement? If you keep the amperage the same and double the voltage, your charge rate will double. It's a fact.

This:

The AC input power will double. I suppose the disagreement (which does seem a bit excessive) was that the charge rate will more than double, due to fixed overhead, and to a lesser extent, efficiency changes in the AC-DC converter with a lower input power.

Charging 120v vs 240v efficiency

Thanks for digging out the link to my chart, I should probably bookmark some of this stuff :)

That was the chart I mentioned upthread folks.
 
Nope. You'll never see any efficiency differences due to 120V 20A vs 240V 20A. Any minuscule variation from a 1-to-2 ratio will be lost in the integer mph rating. Tesla's 4 vs 15 mph charge rate is simply laughable and isn't even close to the realm of reality.

The car carries significant charging overhead that you seem to be ignoring. My AC charging on TeslaFI averages 83% efficiency over almost a hundred charging sessions and ~1500 kWh added to the pack (kWh added over kWh used.)

But my 120V charging sessions are all around 70% efficient - despite the fact that Sentry mode was disabled for all of them and is mostly enabled for the rest.

Granted, I don't have actual sessions at 120V @20A and 240V @20A to compare directly, but there's no question in my mind or in the data that slower charging is inherently less efficient, because some of the power goes to keeping computers and coolant pumps awake and it is a larger fraction on slower sessions.
 
Nope. You'll never see any efficiency differences due to 120V 20A vs 240V 20A. Any minuscule variation from a 1-to-2 ratio will be lost in the integer mph rating. Tesla's 4 vs 15 mph charge rate is simply laughable and isn't even close to the realm of reality.

Did you even look at the plot? I will not disagree with your post...but...at least look.

No one here is saying that Tesla’s chart is accurate, to be clear. It is obviously wrong, an furthermore it does not even make a distinction between different types of Model 3s! (Which have up to about 10% difference in kWh/mi rates.)
 
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Nope. You'll never see any efficiency differences due to 120V 20A vs 240V 20A. Any minuscule variation from a 1-to-2 ratio will be lost in the integer mph rating.

I must emphatically disagree with this.

Assuming 20 A circuit with 16 A continuous charging ..

Aprox 3 A at 120 V (360 W) will be directed towards keeping the car awake, leaving only 13 A for charging the battery.
13 A x 120 V = 1.6 kW

At 240 V, aprox 1.5 A will be required for the car, leaving 14.5 A for charging the battery.
14.5 A x 240 V = 3.5 kW

That’s 2.2x faster.

Double would be 3.2 kW. 3.5 kW is 300 W more, which is over 2 km/hr or over 1 mi/hr in “charge rate” terms, so it most definitely should be noticeable and NOT “lost in the integer mph rating”, especially not lost in km/hr.

AWD should show 10-11 km/hr (~10.5) and 23 km/hr (more than double).
In mph this would show 6-7 mph charging vs 14 mph. 6 vs 14 is clearly more than double, however 7 vs 14 is in fact hidden by the integer display.

On an SR+ it would be 7 mph (7.3) vs 16 mph (15.98), clearly more than double.

To be clear, I do agree with this:
Tesla's 4 vs 15 mph charge rate is simply laughable and isn't even close to the realm of reality.
 
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Hmm. Is the 3 charger different than the S? I noticed higher efficiency charging at 240v in my old S. Even high amperage 120v 30A campground charging wasn't as fast as you'd expect from the wattage.

I don't think you can charge higher than 16A at 120v even if the breaker is rated higher than 20A (do you remember what the screen read?). I've charged my Model 3 @ 120v 12A, 120v 16A and 240v 16A and they usually charge at 5, 7 and 15 mph respectively.
 
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