physicsfita
Member
Actually, that was a pretty darn good layman's explanation -- the only thing to add is that even the nuclear bomb isn't 100% efficient. The 2nd Law of Thermodynamics guarantees that there will always be losses.
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Nuclear bombs aren't very efficient (around 25%). The closest you can generally get to 100% efficiency is converting gravitational potential energy to kinetic energy (dropping stuff).
Thank you kindly.
Charging efficiency is also related to charge power. Charging at 1.4 kW via a 120v outlet will be quite a bit less efficient than charging at 9-10 kW via a 40A 240V wall charging station both because it is pushing more current per kW at 120V but especially because it takes a lot longer to charge and there is a fixed overhead for the car's electronics to operate and monitor the charging process.
Someone claimed 95% charging efficiency but that is at the outer reaches of charging efficiency. Most cars are closer to 10-15% charging loss at 16-40A 240V and may be as bad as 25% loss at 12A at 120V.
I'm not completely certain about Tesla, but almost all EVs report kWh usage and Wh/mile "from the battery" without including the charging overhead on the display inside the car. Meanwhile, EPA estimates include charging overhead so you can't directly compare the car's console Wh/mile display with the EPA rated Wh/mile. The only car I've seen which accounts for charging overhead is the 2016+ Chevrolet Volt and it only accounts for charging overhead in the actual driven calculated MPGe it reports.
Someone claimed 95% charging efficiency but that is at the outer reaches of charging efficiency. Most cars are closer to 10-15% charging loss at 16-40A 240V and may be as bad as 25% loss at 12A at 120V.
I'm not completely certain about Tesla, but almost all EVs report kWh usage and Wh/mile "from the battery" without including the charging overhead on the display inside the car. Meanwhile, EPA estimates include charging overhead so you can't directly compare the car's console Wh/mile display with the EPA rated Wh/mile. The only car I've seen which accounts for charging overhead is the 2016+ Chevrolet Volt and it only accounts for charging overhead in the actual driven calculated MPGe it reports.
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The meters in the car that have to do with range are merely wild assed guesses. This is because the car has 7000 small batteries. Range used or range remaining is simply a guess, all of the time. You can hurt yourself if you overthink this.
The trip graph is pretty accurate and becomes more accurate the further you are on the current leg of your trip. The rated range is just a number, but you soon figure out if you do better or worse. Before the trip graph you could compare the rated range to the projected range and as long as the projected range was greater than the rated range, the rated range could be used as a reasonable estimate provided there was no big change in either topography or weather. I used that method for a couple of years before the advent of the trip graph--now it's mostly obsolete.
I just detailed my car today and I had the AC running for about two hours straight with the car unplugged. I lost about 15-20 rated miles but the kWh used since last charge did not increase. This confirms that power used to cool is not accounted for in the calculation, as I suspected.
Using your formula, I get: 173 (projected miles) / 174 (rated miles) * 295 (Wh/mi) = 293 Wh/rated-mile from a snapshot of the 30 mile average.
Cross check: 240 rated-miles * 293 Wh/rated-mile = 70.4 kWh this seems a bit high for a 70D.
If we "check in reverse", (70kWh - 6 kWh[reserve])/240 rated miles = 266 Wh/mi.
So with this methodology, the rated Wh/mi is somewhere between 295 and 275. However, I took a long steady, slow, flat trip with AC off and achieved 248 Wh/mi average over 60 miles and yet the rated range decreased by more than 60 miles from the start of the drive to the end.
Consider this hypothesis: The center console knows the car is a 70D, and it displays rated-range = 240 * SOC, where SOC is fraction of full charge (which would agree with my comparisons between %SOC and rated miles). Also suppose it displays projected range as SOC / (Wh/mi) * Rated Capacity. Now combine this with the BMS system having an internal configuration as software limited to 60 kWh capacity. This would explain why the descrepancies I keep seeing.
Another alternative is that the unaccounted energy consumption during driving is ~ 2.9 kW. If that's true I think there is something very wrong.
Boatguy
Active Member
Run this a few more times and see if there is a strong consensus number; it's definitely a constant. Rated Wh/mi is pretty fine grained whereas the RM and PM are not so you will see some swing plus or minus by 3-4 Wh/mi. You can also try manipulating your driving (e.g., over 5mi) to put the Average Wh/mi right on the dotted line on the graph, that is when RM = PM and Avg Wh/mi = Rated Wh/mi.
If your average Wh/mi was less than the Rated Wh/mi then the change in RM should have been less than the distance traveled. More specifically, your RM should have changed by 248 / 293 * 60 = 51.
I can't explain the results you observed.
Your hypothesis assumes that maximum rated range is a constant at 240, which is not the case. Rated Wh/mi is a constant and battery capacity declines over time, thus maximum rated range also declines. Is the 240RM the observed value when you charged to 100%?
What is the battery capacity you purchased, the unlocked capacity?
Here's another data point:193 projected miles/163 rated-miles * 252 Wh/mi = 298 Wh/rated mile, a bit higher than before.
Neither can I, except to say that generally speaking rated miles consumed is much more than actual miles unless I am achieving very, very low (compared to any "normal" driing) Wh/mi. Of course there are hyper-miler's out there going even lower, but then they are also presumably go way farther than rated range in the process.
i haven't been looking at the projected range before now, it's pretty funny, because it keeps showing that I'm seriously beating rated range, and yet as I continue to beat it (projected range - rated range going up), the rated range usually still drops faster than my actual miles traveled. Here is a screen shot (both screens) showing a slightly better case. In this case change in rated miles was 188 - 163 = 25 rated miles since the charge (2 hours before the end of this trip). The actual trip miles was 27.1 in two segments parked for ~ 1.5 hours turned off in-between and I achieved 247 Wh/mi for the trip. i ended the charging 5 minutes before it finished (didn't reach target of 80%), so the starting range was 188 instead the typical 191/192 when I complete an 80% charge.
This is actually encouraging because i've previously achieved lower Wh/mi and used more than rated miles rather than less. Still this was done mostly at steady 55 mph with AC off.
My hypothesis didn't assume a constant maximum rated range. The battery degradation could be accounted for by the BMS system reporting that a full charge is less than 100% for instance.
I have never charged beyond 80% yet. My car is a refreshed 70D and I did not pay for the 75 upgrade, so my purchased capacity is 70 kWh. 240 miles range is my EPA rated range from the window sticker.(i think the website when I ordered said 243, but there is no longer a 70D on Tesla's website).
My biggest concern is that for every trip I've checked against EV trip planner, the kWh used is spot on, but the change in rated miles is way more than what EV trip planner predicts. It's to the point that for actual long trips where EV trip planner says I would arrive at a super charger with 32 miles rated range remaining, scaling my short trip experiences suggest I will hit 0 before arriving at the super charger if I follow the suggesting charging from EV trip planner.
I really do wonder if my car's BMS isn't configured for 60 kWh even though the rest of the car says 70.
I have had my 70D for just over a year now. Based on everything you have said here, I don't believe your car is misconfigured as a 60kWh battery. I track many parameters for my car, and my rated Wh/mi computes to around 260, and if I achieve this while driving, then I will get actual miles equal to the rated miles, which I can do when driving on freeways, but rarely achievable with local driving. But my calculated 100% range shows around 230 miles, not 240.
Thank you! I've been hoping for other 70D comparison points. Your car is not a refresh while mine is so that may explain why I need to achieve < 250 Wh/mi for actual miles to equal rated miles. But it's good to see another Wh/mi point at least close to my actuals. Your car being a year old means it has an actual 70 kWh battery, not a software limited 75 kWh battery right?
Is your 100% range the display range from a range charge, or do you compute it some other way?
What method did you use to compute 260 Wh/mi for rated range?
Have you compared any trips against EV trip planner estimates? If so do you usually see significant differences in rated miles consumed vs predicted?
In old Tesla (classic) like mine, Rated was I believe 300Wh/mile in the US, and 320Wh/mile in EU and JP. When dual motor cars are introduced, D cars seemed to have 290Wh/mile number but very similar power consumption in normal driving to single motor cars. Thus, in D cars, I speculate that it is more difficult to achieve RM.
RM is just a constant, that is set by Tesla. I would recommend to use RM as a ballpark number.
That said, if you drive like 250Wh/mile but still can't beat RM, I can't explain that. 250Wh/mile is Ideal miles I think. In power consumption calculations usually AC and battery heater/coolers are not included, and these typically spend a lot of energy in the beginning of driving. Do you use AC (e enough a bit) and happened to set the AC temp to be higher than ambient? If so, it's going to be very difficult to spend less than RM.
RM is just a constant, that is set by Tesla. I would recommend to use RM as a ballpark number.
That said, if you drive like 250Wh/mile but still can't beat RM, I can't explain that. 250Wh/mile is Ideal miles I think. In power consumption calculations usually AC and battery heater/coolers are not included, and these typically spend a lot of energy in the beginning of driving. Do you use AC (e enough a bit) and happened to set the AC temp to be higher than ambient? If so, it's going to be very difficult to spend less than RM.
Boatguy
Active Member
@hiroshiy makes an excellent point. There is a setting in Units/Format that controls whether the dash displays Rated Miles or Ideal Miles. Unfortunately there is no indication on the dash which you are seeing and I'm not sure if that setting ripples through the other displays on the CID (Trip, Energy, etc.). If it does not, it would explain the crazy discrepancies you are seeing.
Verify that the Units/Format settings is set to display "Rated" and not "Ideal".
I have never done a full range charge. I usually charge to 80% every night. At the end of the day I take rated miles remaining divided by % rated miles remaining from the green battery. Of course, you can do the same thing at the start of the day before you drive anywhere. I think one time I got close to a range charge and it was consistent with my calculations.
To calculate my 260 Wh/mi rated value, at the end of the day I take energy used divided by rated miles used. This number can show a lot of variation from day to day, so you need to average over a period of time to get a good number. I have been tracking this over 6 months now, and the trendline is very flat over that time.
I have never used trip planner so I am not familiar with it. However, on the energy graph, there is a dashed line around 292 Wh/mi and when my actual driving Wh/mi matches this, it shows projected miles equal to rated miles. I think that is an error on the energy graph, because that would mean you are using 70kWh of battery(.292 x 240) to get your full range. And I know that I need 260 Wh/mi to achieve rated miles as I have described above. If trip planner uses this value, then what you are describing makes perfect sense, since it wouldn't be accurate.
Boatguy
Active Member
I previously made an erroneous reference to the dashed line on the energy display showing the rated Wh/mi. It's actually the solid line which shows the Rated Wh/mi, the dashed line is the average Wh/mi for the selected period.
If you manipulate your driving (pretty easy to do over 5mi) so that the dashed line is on top of the solid line, the Average Wh/mi number displayed is the Rated Wh/mi being used for RM calculations.
If you manipulate your driving (pretty easy to do over 5mi) so that the dashed line is on top of the solid line, the Average Wh/mi number displayed is the Rated Wh/mi being used for RM calculations.
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The trip graph takes into account hills and speed limits, but not weather or traffic. I've found it to be pretty good when used to get to the next stop. As you drive, it then shows if you are doing better or worse than the estimate and you can adjust your speed accordingly.
The data I've reported here is mostly with AC turned off (and heated seats off etc), because I've been trying to achieve actual miles traveled > rated miles consumed. I am definitely displaying rated miles, not ideal miles. I'm in Maryland and the temps have been fairly mild lately in the low 80's (F) so it's hot not cold, so there is no way I would turn any heat on.
I can understand how AC, heat etc while parked can be excluded from kWh trip measurement (because it doesn't count any energy towards a trip while parked or off), but I'm skeptical that accessory energy isn't counted while the car is driving. For one thing, when I do turn the AC on while driving, I always see higher average Wh/mi values and people on the forums routinely report higher Wh/mi figures for winter driving when they are using heat. For another, Tesla would have to separately meter the power going to the drive train in order to exclude all the accessories etc. In doing so, they would create customer confusion (like mine) regarding why their energy consumption is so low, yet their rated miles consumed is so high, so it doesn't make sense to intentionally exclude accessory power while driving from the trip power meter.
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