Rated miles vs Wh/mi

[hacer]

I'm a new owner, model S 70D refresh (and thus an actual 75 kWh battery but software limited to 70); I've had the car 12 days now. I notice that every day when I go to charge my car that the rated miles consumed exceeds the actual miles, even though my Wh/mi since last charge always seems quite good. For instance, today I drove a total of 44.8 miles (mostly freeway, TACC set to 60 mph, start and ending at home) with an average of 273 Wh/mi, 12.2 kWh used and went from a starting rated range of 192 miles (80% charge setting) to an ending rated range of 144 miles, or 48 miles of rated range used for ~45 miles of actual travel. Now if you take the 240 mile max rated range of my car (EPA sticker, I recall it was 243 on the website when I originally ordered) and multiply by 80%, that would be 192 miles so the starting reported range makes sense. If the battery capacity if 70 kWh and range is 240 that translates to an average of 291.67 Wh/mi to achieve rated range. My days driving had 7% lower consumption per mile than average rated would have yet the range consumption was 6% higher.

As a cross check, I put (approximately) my day's driving into EV trip planner and it produced startlingly close agreement (better than 2%) to my reported average Wh/mi and kWh used; however it projected that I would have only used 41 rated miles instead of the 48 that actually happened (a 17% difference between projected and "measured" rated range). This has me concerned because I plan to do some long distance travel and use the EV trip planner. it seems if I follow its recommended charging and reserve and the discrepancy persists, my car will die on some of the legs! e.g. one leg has 198 rated miles used ending with 32 rated miles arrival at the supper charger but if the 17% trend holds it would be -1.3 rated miles left when arriving at the supercharger.

Tonight's recharge showed 14.7 kWh consumed from my home charger to replenish the 12.2 kWh used (83% efficient seems a bit low) and ended with 191 miles of rated range.

So is this normal? Is it just some calibration issue that I need to (nearly) fully charge/discharge my battery (so far lowest I've done is 40% and highest 80%) to get it on track?

 

[Topher]

It will take a bit to get your personal average Wh/mile.

Thank you kindly.

 

[hacer]

It will take a bit to get your personal average Wh/mile.

Thank you kindly.

I don't understand how that (long term or lifetime average) is relevant to my question. On any given trip Wh/mile can change but rated range consumption should track total energy consumed regardless. My car keeps showing that 10kWh of energy consumed is ~40 miles of rated range used up, but 10 kWh is 1/7 of my brand new battery capacity, so that should be only 1/7 of the max range or ~34 miles not ~40 miles.

Perhaps the range displayed which I've seen described as "rated range" is not based on the vehicle itself but on the average behavior of the person driving it? If that's the case then I guess it will adapt over time. But if that's true wouldn't everybody have a (potentially quite different) range reading for a given charge percentage? From what I've read that varies only a little and people associate that variation with battery degradation, not driving habits. Maybe the starting recharged value is based on the car, but the rate that it changes is based on some (long term) past history of driving? That would be weird because you'd get totally different range estimates for identical states of charge, one for just charged to that state and another for just discharged to that state.

Also it won't be easy for me to get my personal (lifetime or long-term) average Wh/mile because it seems that the trip meters are gone in the software my car came with. There is only last 30 miles, since last recharge, and trip related to navigation system. At least I can't seem to find the Trip/A, Trip/B meters that people have described being used for very long term averages. Some other threads have suggested they were removed by a recent software update.

 

[jerry33]

Rated range is based on a formula that's close to the EPA estimate, not on usage. Projected range is based on usage (best set to the last 30 miles).

The trip meters are now only in the large display. You should still be able to see them--just not as conveniently.

 

[hacer]

i'm pretty confident now that this is a software bug where it is computing the change in %charge and/or %range as if the car has a 60 kWh battery. Taking 3 points from today's driving, the %charge as displayed on my car's screen can be exactly computed from:

(current % charge) = (% charge at last recharge) - 100 * (kWh used since last charge) / 60

Whereas that calculation would be correct for a 70 kWh battery if the 60 were replaced by a 70.

Now the question remains will it actually stop me from driving when this number reaches 0% ?

If so, then I have a 60 kWh car even though I bought and paid for a 70 kWh car.

Anybody else with a 70D want to check their displayed values against this formula?

 

[hacer]

Rated range is based on a formula that's close to the EPA estimate, not on usage. Projected range is based on usage (best set to the last 30 miles).

The trip meters are now only in the large display. You should still be able to see them--just not as conveniently.

Thanks for the tip about the trip meters - I found them!

 

[Ben W]

The "70" is not really 70 usable kWh; some amount of the theoretical capacity is reserved for safety on both the low and high end. So the actual usable capacity is probably closer to 66kWh. Divide this by 240 and you get 275Wh/mi for the actual EPA efficiency. There will be some extra vampire losses during the day, which could account for the extra three lost miles you observed. Charging efficiency is typically around 90% I believe, but it could vary based on all sorts of factors: temperature, charging rate, and so forth. Tesla's estimates will of course be closer to the optimistic end of the range, so it's not surprising that you're seeing a bit less than that in real-world use.

Still, it's one heck of a lot more efficient than an ICE!

 

[Redmiata98]

Don't get overly concerned about your mileage around town, you will be able to fill it up every night. I was overly concerned about it when both my S and X were new. First of all, you need to bed in your tires. You should notice better numbers once you have passed about 1000 miles. Second, take it on a longer highway ride somewhere in excess of about 100 miles and load your energy app to the touchscreen setting it for last 30 miles average. You will soon see that the projected numbers are quite realistic and achievable. Be cautious, you should know what YOUR car is capable of with YOUR driving style. Overall, you will soon find that the numbers are spot on for longer range driving. I spent countless hours trying to develop my own logarithms and they were all wasted time. Unless you are travelling in the rare places that are not Supercharger deserts (e.g. southwest Texas or North Dakota) you will have enough juice to safely travel between recharge stations.

My experience was with fully loaded 2014 S85P+ and XP90DL.

 

[tnt1971]

I have an 85D and I have noticed the same thing. To get the rated miles, I am supposed to get a Wh/mile number around 290-292. I routinely get lower than that, and I have seen 240 in warm weather driving, particularly when not using the AC. Yet, the miles since last charge when subtracted from the starting mileage do not seem to add up to expected.

I believe there are two things happening. First, I do not think that calculator factors in the vampire loss, which there are many threads on. Second, when you pre-condition the car (run the AC or the heat), and it is not plugged in, I do not think the energy you used to do that is factored into the calculation.

I believe these items would be much less of a factor in a long trip and you (and I) should get our rated range at the right Wh/mile. I have a 240 mile trip coming up in a month when I plan on doing my first range charge. There are 2 SCs along the way for security, but I am going to pay close attention to actual versus projected usage, and try and make it in one shot without SCing.

 

[hacer]

The "70" is not really 70 usable kWh; some amount of the theoretical capacity is reserved for safety on both the low and high end. So the actual usable capacity is probably closer to 66kWh. Divide this by 240 and you get 275Wh/mi for the actual EPA efficiency. ...

So here is my car-reported data. Note the 240 mile range at 100% charge point is made-up; I have never range charged my car yet, but that is the EPA sticker rated range for my car. All other data points are from the display.

From this chart I think it is possible to determine the reserve capacity and the rated Wh/mi. First notice that the data is very linear, the trend lines are from Excel's linear curve fit. We can see that the slope of the kWh consumed / % change is 59.6 kWh/100% charge, thus my available for driving capacity is 59.6 kWh. Since I have a 70D, that means there is 10.4 kWh reserve capacity to the virtual 70 kWh battery or 15.4 kWh reserve for the physical 75 kWh battery. Next we can divide the slopes of the two curves to figure out rated Wh/mi. 596 Wh/% divided by 2.4328 mi/% gives 245 Wh/mi. Which explains why I can barely achieve rated miles at constant 60 mph with AC off. This still leaves me wondering if my car's firmware isn't somehow partly entangled with the 60D software limit.

I love this car and for my daily driving there's no worries at all. Heck, I could do a normal week of driving without recharging. But I do admit I'm worried about a trip to Canada I'm planning this summer. I'm confident I can make all the range between charging stations, but it seems I will need to charge significantly more and longer than EV Trip Planner suggests. Fortunately, long trips will be a tiny percentage of my driving.

As for vampire consumption, it appears to me that that gets accounted for at the start of each trip. When the first 0.1 miles pops up I always have a huge initial power consumption and resulting huge Wh/mi reading. I think that is where all the energy drain since the previous trip gets put in.

 

[jerry33]

I have an 85D and I have noticed the same thing. To get the rated miles, I am supposed to get a Wh/mile number around 290-292. I routinely get lower than that, and I have seen 240 in warm weather driving, particularly when not using the AC. Yet, the miles since last charge when subtracted from the starting mileage do not seem to add up to expected.

Lifetime average 71K miles = 248 Wh/mi

 

[msnow]

Lifetime average 71K miles = 248 Wh/mi

But that's all downhill. -

 

[cjt3]

As for vampire consumption, it appears to me that that gets accounted for at the start of each trip. When the first 0.1 miles pops up I always have a huge initial power consumption and resulting huge Wh/mi reading. I think that is where all the energy drain since the previous trip gets put in.

Vampire consumption does not get accounted for in the Wh/mi readings. The large initial consumption at the beginning of the trip is probably from the car powering up and HVAC starting to work. I have left my car sitting at the airport for a week and the initial startup Wh/mi is no different than if I left it for a few hours - even though the vampire consumption amount is significantly different.

 

[DCGOO]

Tonight's recharge showed 14.7 kWh consumed from my home charger to replenish the 12.2 kWh used (83% efficient seems a bit low) and ended with 191 miles of rated range.

So is this normal? Is it just some calibration issue that I need to (nearly) fully charge/discharge my battery (so far lowest I've done is 40% and highest 80%) to get it on track?

i think anything above 80% is pretty good actually. Don't forget, there are systems in the vehicle that powered and consuming energy that does not go in to the battery, but are powered by the chargers in the car. That will be included in your 14.7 consumed number.

 

[tnt1971]

This morning I charged my car up to 169 miles and it finished charging about 30 minutes before I left. I had several trips that totaled almost exactly 100 miles. The wh/mile since last charge was 289, right around where it should be to get me my rated miles. Yet, I arrived back at my work charging station with 58 miles left, around 10% more usage than it should have been. I ran the AC for about 3 minutes prior to leaving one location, and it all took about 6 hours meaning the vampire drain should be negligible. The accounting here does not seem to add up.

 

[Boatguy]

So is this normal? Is it just some calibration issue that I need to (nearly) fully charge/discharge my battery (so far lowest I've done is 40% and highest 80%) to get it on track?

You're making this too difficult. There are no software bugs, just really poor UI design and data presentation by Tesla. Here is how this works.

1) Rated Miles (RM) = usable battery capacity (kWh) / Rated Wh/mi

Rated Wh/mi is a constant, so RM is just a proxy for the estimated (see comments at the bottom) available battery capacity, nothing more; it has nothing to do with driving style. The Rated Wh/mi for my S90D, it is 290. Your Rated Wh/mi is indicated by the dotted line across the graphs in the energy display; that dotted line never moves, but unfortunately is also not labeled.

To compute your Rated Wh/mi, look at the trip meter for any averaging period (use 30 if in doubt) and then:

Rated Wh/mi = (Projected Miles / Rated Miles) x Average Wh/mi.

2) My 100% charge RM was 292 when my car was new, implying a usable battery capacity of 292RM x 290Wh/mi = 84.7kWh. As a cross check, my 90% RM when new was 264 so 264 x 290 / .9 = 85kWh. There is a little rounding error in the RM numbers, but for all practical purposes, when new, my S90D had 85kWh available for driving. Usable battery capacity decreases over time, but the actual amount seems to vary considerably and the causes are a source of continual debate.

3) Your driving style, road conditions, and weather conditions over any period of time cause your actual Wh/mi to be more or less than your rated Wh/mi, which in turn causes your projected range to be more or less than your RM. You can easily see this just by choosing different averaging periods in the Energy display and watching your projected miles change, based on your average consumption over different periods.

In general, just like an ICE, extended freeway driving at a moderate speed (70-75mph) will consume less power than around town start/stop driving. And just like an ICE, aerodynamic drag ultimately rules and power consumption rises with the cube of the speed.

4) Charging the battery is in the range of 90% - 95% efficient. I have a separate electric meter for my EVs so I'm able to measure this very accurately. I actually measured 95.6% for the MS.

5) Vampire loss is not reported anywhere by Tesla, so none of the numbers quite add up or cross check. Vampire loss varies depending on a variety of factors; my vampire consumption is about 2-3RM / day.

You postulated that vampire loss is saved up and then expressed when you first drive the car. But if that was true, the Trip meters would always show consumption that matched the change in RM x Average Wh/mi, which they clearly do not.

The car's measurement of the SOC/RM is amazingly good, but it's digital electronics trying to quantify chemistry by making inferences based on measurements, as opposed to really being able to measure what we want to know. The only way to actually know how much energy is in a battery is to take the energy out and see how much you get.

My lifetime average consumption is 310 Wh/mi, but on my last road trip (Yosemite round trip from SF) it was only 270. My actual consumption is consistently lower than that projected by the nav system.

Enjoy your car!

 

[msnow]

@Boatguy - can you explain what you mean by your charging efficiency % in #4? I don't understand why it wouldn't be 100% all the time.

 

[gravity]

I have done 2 roadtrips in our x. Supercharger distances are more than adequate to get from oone to the other. I do recommend staying at a hotel with a destination charger. Its so much easier getting to your destination and charging to a full battery overnight compared to hassling with either getting to a supercharger or using a slow public charger and planning time around that.

 

[Boatguy]

@Boatguy - can you explain what you mean by your charging efficiency % in #4? I don't understand why it wouldn't be 100% all the time.

No energy transformation, with the possible exception of a nuclear bomb, is 100% efficient due to mechanical resistance, electrical resistance and various other factors.

When we charge our cars, the charger has to convert AC into DC, usually using a rectifier and then some capacitors to smooth things out. The circuits have some resistance and some of the energy into the charger is dissipated as heat. And then the chemical process going on inside the batteries is also imperfect and some energy is loss in pushing around the molecules in the battery to change their state.

So the simplistic answer would be that 4-5% of the energy we put into the charger is converted to heat, rather than stored in the battery. The same thing happens when energy comes out of the battery, and again when we charge the battery with regenerative braking. Energy is lost in every transformation and transmission (whether it be an electrical transmission wire, or the gearing in the car).

Overall, an ICE converts about 20% of the energy in the fuel into moving the vehicle and an EV converts about 60%, hence the reason EVs MPGe is usually about 3x the MPG of an equivalent size ICE.

This is a layman's explanation, I'm not a chemist, or even an EE, so I'm sure that someone can chime in with a more eloquent answer.

 

[jerry33]

@Boatguy - can you explain what you mean by your charging efficiency % in #4? I don't understand why it wouldn't be 100% all the time.

Charging efficiency is never 100%:

1. There are line losses over the wires. How much depends on the size of the wire and the length of the run.

2. Anytime energy is converted from one form to another (In this case electrical power, to electrical storage) there are losses due to heat. Of course, this is far better than gas which typically loses 80% of it's energy to heat when burnt and the best theoretical case is 50% loss.