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I thought we get 31 miles per hour with NEMA14-50
- Thread starter Francis Lau
- Start date
I find this confusing. Why wouldn't Tesla have been advertising 37mph charging back before we had EPA numbers? I thought they had been saying 31/62 for a long while before we had EPA numbers.
My math doesn't match yours. Let's see if we can bridge that:
Ideal: 85 kWh / 300 mi. = 283.33 Wh/mi.
Rated: 85 kWh / 265 mi. = 320.75 Wh/mi.
Where am I going wrong?
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It appears we don't have full access to the 85 kWh. The 300 number (I think it's slightly above -- I've been using 308) comes from two places: 1) Seeing the average WHPM in the car when rated matches projected and (with version 4.0) the location of the "rated" line in the graph.
So when I "reverse engineer" the pack capacity, I do 265 * 308 = 81620 ... and then to get the whpm required for 300 miles, 81620 / 300 = 272 whpm.
Tesla Charging | Tesla Motors
Input: 300 MILES, (whatever), NEMA 14-50 (240V/80A), (whatever)
Output: TIME 09:26, (whatever), ENERGY 84.9 kWh
300 mi. / 9.433 hr = 31.8 mph
Where does the 37 mph figure come from?
Using what I posted as you were writing this, 10000 / 272.
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I wouldn't be surprised if their calculator is including charging overhead. Their 84.9 is 4% higher than my 81.6 ...
As for their time estimator -- yeah it's the 32 number, which is definitely strange. I don't really trust their site. At this point, what I'm posting is based from owning the car for 2 months and measuring these details.
*shrug*
Another point that confuses me is that the raw numbers show the Charging page to be overpromising as well. Check me on this one as well please.
Model S Specs | Tesla Motors
From wall: 240V * 40A = 9600 W
Efficiency: 92% (best case)
To battery: 9600 W * 92% = 8,832 W
Assuming linear charging (which we know to be overly optimistic):
85,000 Wh / 8,832 W = 9.624 hr
So assuming we stay at peak efficiency for the entire duration of the charge and overly optimistically assume linear charging, we get 9.624 hr which is still longer than the "calculated" 9.433 hr.
#confused
Every way I try to slice it, the numbers end up worse than suggested by Tesla. Sad panda.
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Getting back to my original question about (b). I got the impression that people were saying that it's a bug that the Rated/Ideal setting impacts the reported "mph" of charging. I consider that an intentional feature (to be kept) rather than a bug (to be fixed).
If, OTOH, the math (but apparently not my math...) shows that rated should be > 32 mph for the Ideal setting then it indeed might be a bug if the UI is showing less for the Ideal version of the UI.
Model S Specs | Tesla Motors
From wall: 240V * 40A = 9600 W
Efficiency: 92% (best case)
To battery: 9600 W * 92% = 8,832 W
Assuming linear charging (which we know to be overly optimistic):
85,000 Wh / 8,832 W = 9.624 hr
So assuming we stay at peak efficiency for the entire duration of the charge and overly optimistically assume linear charging, we get 9.624 hr which is still longer than the "calculated" 9.433 hr.
#confused
Every way I try to slice it, the numbers end up worse than suggested by Tesla. Sad panda.
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Getting back to my original question about (b). I got the impression that people were saying that it's a bug that the Rated/Ideal setting impacts the reported "mph" of charging. I consider that an intentional feature (to be kept) rather than a bug (to be fixed).
If, OTOH, the math (but apparently not my math...) shows that rated should be > 32 mph for the Ideal setting then it indeed might be a bug if the UI is showing less for the Ideal version of the UI.
It's been noted, we don't have full access to all 85 kWh, there is still a question as to whether there is a reserve, etc. I just use 300 because it's what Tesla seems to use.
You have a unit problem (change kWh/mi to Wh/mi at the end).
Either way, 40A delivers far more than 20 mi/hr of charge regardless of whether you use 280, 300, or 320 Wh/mi.
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I'm not going to continue to overanalyze the math because it will make me cross-eyed this late at night and we don't have all the necessary variables, but I have come to three conclusions that I can make:
1. Showing 20 miles per hour of charge is clearly wrong, regardless of which math you use or mode (rated vs. ideal) you're in. That was the OP's primary argument. In fact, you only begin to approach 20 mi / hr of charge when you get to 208V @ 30A, a far cry from the OP's 235V @ 40A. Could this perhaps be a leftover of the "projected" math?
2. The ideal miles per hour of charge should be higher. I really don't care about ideal miles, because I'm currently averaging 400 Wh/mi anyway. That's because I drive like an idiot: a smiling, happy, gleeful idiot. But for those people who like to drive the model S like the average Prius driver (slow with no go), ideal should show a higher ideal miles per hour of charge.
3. Based on my experience, charging my car @ 245V, 40A yields ~31-32 rated miles per hour of charge. While I don't know how Tesla derived that number in the first place, it happens to be correct.
Fedderman
Original: 2012 Model S (85); Current: 2024 Model S
This what I have noticed when charging off the 14-50 in my garage: When first connected, the voltage reads around 240-243 Volts at 0 Amps. As the charging current ramps up 40 Amps, the voltage drops to around 230-233 Volts (~10 V drop). Is this typical?
I don't have my MS yet ;-(. Seems like it ramps up, peaks and then ramps down as battery is reaching max charge????
Yes, higher currents cause higher voltage drops across wiring (since wiring has resistance). The voltage drop is dependent upon the type of wire used (Cu/Al), the length of the circuit, the wiring gauge, and in some cases the load on your home service. 6% is considered the maximum recommended drop, and you're within that. Anything more, and I'd go looking for the reason.
Not unheard of ... might be a little on the abnormal side but probably nothing to worry about unless you have heat building up somewhere. Let it charge for an hour and check all your connection points (plug, breaker, etc.)
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He's talking about observed voltage sag during charging -- nothing to do with tapering off.
The Model S does taper the amperage as it reaches 100% but that's ONLY in a range-mode charge when talking about NEMA14-50 charging. It doesn't taper during normal standard mode charging (from a 50-amp, 40-amp-draw circuit like a NEMA14-50)
If the drop is voltage is under 10%, you really don't have to worry about heat. At that current, you'd have a serious voltage drop if something were generating heat. A 10V drop is only a 250 milliohm resistance at 40A. #6 copper has about 500 milliohm resistance per 1,000 ft, so 250 mohm would point to a total (round-trip) circuit size of 500 ft, or 250 feet (one-way) from the panel.
The only other contributing factor could be a heavily-loaded transformer, or near-capacity on service wiring. To troubleshoot further, you'd want to measure voltage at the breaker panel main while the car is charging. If your voltage drops to 230V across the buss when the car is charging, it would point to more of a service / transformer load issue than circuit length.
Either way, 10v is nothing to sneeze at. It's within the 6% typically considered to be "normal".
I've been charging for 5 hours with 22 MPH displayed - 14/50, 228, 40 - similar to everyone else. I've charged a bit over 150 miles in the 5 hours, so it does seem to be a 4.0 display bug - thanks for alerting Tesla.
Today was my first charge using a Nema 6-50. Had 80 miles left and started the charge. Within a few minutes I was getting 31mph charge with amperage holding at 40 and 237-240V.
colinb
Member
Plugged into a NEMA 14-50 at home I noticed this (mind you this was near the end of the charge):
I would expect at 31 mph charge would work out to 50 km/hr. I never have bothered to watch too closely but with a 300km trip coming up I'll be sure to pay a lot more attention.
(while we are at it if Americans would kindly sign the petition to convert to the metric system it would be easier for us Canucks to compare without a calculator on hand.)
I would expect at 31 mph charge would work out to 50 km/hr. I never have bothered to watch too closely but with a 300km trip coming up I'll be sure to pay a lot more attention.
(while we are at it if Americans would kindly sign the petition to convert to the metric system it would be easier for us Canucks to compare without a calculator on hand.
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