Energy consumption at start of drive is insane.. anyone else?

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#1
Whenever I first get in my car and start driving (whether immediately after unplugging in the morning or leaving work in the evening), the first several hundred feet are usually within the 800-900Wh/mi range (highest I've seen is 963Wh/mi) - and this is with gentle acceleration, I'm not gunning it by any means. Within a couple minutes it drops into the 500-600Wh/mi range, and after about 10-15 minutes my numbers start dropping in the 300-350Wh/mi range. Only after driving for 25-30 minutes does my consumption drop back down into the <250Wh/mi range. Does this happen for anyone else? Any idea why the energy consumption works like this?
 

Pescakl1

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#3
Whenever I first get in my car and start driving (whether immediately after unplugging in the morning or leaving work in the evening), the first several hundred feet are usually within the 800-900Wh/mi range (highest I've seen is 963Wh/mi) - and this is with gentle acceleration, I'm not gunning it by any means. Within a couple minutes it drops into the 500-600Wh/mi range, and after about 10-15 minutes my numbers start dropping in the 300-350Wh/mi range. Only after driving for 25-30 minutes does my consumption drop back down into the <250Wh/mi range. Does this happen for anyone else? Any idea why the energy consumption works like this?
Did you check your fuel consumption on your previous car in the same situation?

Yes, same results... nothing unusual, that is called physics ;)
 

jsmay311

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#5
Is it cold where you live? It may be running inefficiently to generate heat to warm up the battery.
Agreed. That'd be my first guess. That and the cabin heater. And if the battery is really cold, maybe limited regen could be contributing to the low efficiency.

Still, such high numbers seem surprising. Do you sit in the car for awhile after turning it on before beginning your drive?

Did you check your fuel consumption on your previous car in the same situation?

Yes, same results... nothing unusual, that is called physics ;)
That's a really poor analogy. The reasons for high fuel consumption right after starting a ICE vehicle with a "cold" (i.e., ambient temp) engine have little to do with why an EV might have high energy consumption at startup.

ICEVs don't have electric cabin heaters or battery heaters. And EVs don't have to worry about air-fuel ratios or mitigating exhaust air pollutant. And electric motors don't need to be hot to run efficiently.

If an EV's battery is warm enough upon startup and cabin heat or A/C isn't on, there should be little difference between efficiency early in a drive and later on.
 
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KarenRei

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#6
Energy needed to get 1800kg of car+occupant up to 35m/s (78mph, 126 kph) at 94% efficiency = 1/2 m dv² / eff = 1/2 * 1800kg * (35m/s)² / 0,94 = 1.172.872 joules = 326 watt hours. Let's say you take 20 seconds to accelerate to that speed - hardly flooring it! Acceleration is dv / t (change in velocity over time), 35m/s / 20s = 1,75m/s². Distance traveled is 1/2 a t², or 1/2 * 1,75m/s² * (20s)² = 350m = 0,35km. 326Wh / 0,35km = 1253 Wh/km = 2017Wh/mi. Ergo, acceleration yields huge Wh/mi figures.

 
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Prodigal Son

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#8
I've observed this, too. It seems like it in fact has nothing to do with the acceleration as much as it rolls all power consumption while the car was not being driven into the drive statistics for the next time it's driven. I keep meaning to test this theory by running the cabin heater on "hi" via the app for 30 minutes and then putting it back to 67°F, driving 1000 feet in a straight line at a fixed speed, and coming to a hard stop with regen on low. Note the consumption. Park the car, get out, close the door. Get back in. Do a U-turn. Park the car again, get out, get back in. Immediately drive back at the same speed to your starting point where you heated the car, do another hard stop. Note your consumption.

If my theory is correct, the consumption on the first 1000' drive will be much much much much higher. If my theory is wrong, I'm just another jerk on the internet :p
 

Pescakl1

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#9
That's a really poor analogy. The reasons for high fuel consumption right after starting a ICE vehicle with a "cold" (i.e., ambient temp) engine have little to do with why an EV might have high energy consumption at startup.
How is it a poor analogy? Who said the car and/or engine (ICE) was cold or not? I still experience the same behavior whatever the temperature is : only the amount changed.

Even with cars warmed up and ready to go, you will still see these differences: better average efficiencies the further you go (even better if this not stop and go traffic).

As always, Karen explained it better than anyone can, and certainly way better that I did.

After that, you can still believe what you want at the end...
 

jsmay311

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#10
Energy needed to get 1800kg of car+occupant up to 35m/s (78mph, 126 kph) at 94% efficiency = 1/2 m dv² / eff = 1/2 * 1800kg * (35m/s)² / 0,94 = 1.172.872 joules = 326 watt hours. Let's say you take 20 seconds to accelerate to that speed - hardly flooring it! Acceleration is dv / t (change in velocity over time), 35m/s / 20s = 1,75m/s². Distance traveled is 1/2 a t², or 1/2 * 1,75m/s² * (20s)² = 350m = 0,35km. 326Wh / 0,35km = 1253 Wh/km = 2017Wh/mi. Ergo, acceleration yields huge Wh/mi figures.
But presumably his driveway isn't a highway on-ramp, so he'll probably have multiple stops early in his drive, and there's nothing unique about the basic physics of that first acceleration/cruise/deceleration/stop cycle that would be any different from any subsequent acceleration/cruise/deceleration/stop cycle. And yet the OP says that the Wh/mile figures remain 20-40% higher than the trip averages for 10-15 minutes. Which is why I suspect something else must be contributing, like the cabin and/or "battery" heater and/or letting the car sit there with the heater(s) running for awhile before starting the drive.

And, sure, the physics of accelerating from a stop for the first time can explain high Wh/mile figures for the first seconds/minute or the few hundred meters or so. But after much longer than that, that effect on the average diminishes quickly.

Even in your relatively extreme example where his driveway IS a highway on-ramp and he averages 1500 Wh/mile over the first 350m (not 2017 Wh/mile... double check 326Wh/0.35km) while accelerating to 78mph, if he then averages 250 Wh/mile for the next 10 miles, he'd already be down to a trip average of 277 Wh/mile.
 

KarenRei

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#11
But presumably his driveway isn't a highway on-ramp, so he'll probably have multiple stops early in his drive, and there's nothing unique about the basic physics of that first acceleration/cruise/deceleration/stop cycle that would be any different from any subsequent acceleration/cruise/deceleration/stop cycle. And yet the OP says that the Wh/mile figures remain 20-40% higher than the trip averages for 10-15 minutes. Which is why I suspect something else must be contributing, like the cabin and/or "battery" heater and/or letting the car sit there with the heater(s) running for awhile before starting the drive.

And, sure, the physics of accelerating from a stop for the first time can explain high Wh/mile figures for the first seconds/minute or the few hundred meters or so. But after much longer than that, that effect on the average diminishes quickly.

Even in your relatively extreme example where his driveway IS a highway on-ramp and he averages 1500 Wh/mile over the first 350m (not 2017 Wh/mile... double check 326Wh/0.35km) while accelerating to 78mph, if he then averages 250 Wh/mile for the next 10 miles, he'd already be down to a trip average of 277 Wh/mile.
It of course depends on the circumstances - including, I should add, how long of a period of time the Wh/mi is spread over. Unless it's instantaneous that is (is it? Does it go negative when they decelerate? If so, how are they determining the average consumption - manually?). Any averaged energy consumption will reflect driving conditions, and driving conditions where you're generally going faster and faster with time mean you have to pay for that very large energy difference between being stopped and being at full speed. Note also that "multiple stops early in his drive" would be a bad thing from an efficiency perspective. Also note that I've been assuming 94% battery+motor efficiency!

I was simply addressing the first part of what they described. They stated, "First several hundred feet are usually within the 800-900Wh/mi range (highest I've seen is 963Wh/mi)". My calculations, for acceleration to 35m/s (78mph, 126 kph) in the first 350 metes, are - 2017Wh/mi. So you're right - they certainly *weren't* going immediately up to highway speed, because their energy consumption would have been much higher if they had.

But lets go further. They next wrote, "within a couple minutes it drops into the 500-600Wh/mi range". So let's say they need 300Wh to get up to their final speed, and they average over the first couple minutes 30 mph - some random acceleration / deceleration and then culminating with getting onto a high speed road. Then those - let's say 2,5 - minutes travels 1,25mi. So the acceleration portion of their energy consumption adds 240 Wh/mi on top of whatever energy they would have otherwise consumed. And it's worth reiterating that if they are doing start-and-stop driving, cornering, all of that sort of stuff, that's wasting power over whatever their baseline would have otherwise been.

I see no weirdness here. Of course one can't make specific judgments without knowing the exact details of the drive and the instantaneous power consumption figures. But it's worth reiterating that accelerating ~1800kg of passenger + vehicle takes a lot of energy. A vehicle moving at speed contains a large amount of kinetic energy, which can be clearly seen when a car comes to a sudden unplanned stop in an accident!
 
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#13
I've observed this, too. It seems like it in fact has nothing to do with the acceleration as much as it rolls all power consumption while the car was not being driven into the drive statistics for the next time it's driven. I keep meaning to test this theory by running the cabin heater on "hi" via the app for 30 minutes and then putting it back to 67°F, driving 1000 feet in a straight line at a fixed speed, and coming to a hard stop with regen on low. Note the consumption. Park the car, get out, close the door. Get back in. Do a U-turn. Park the car again, get out, get back in. Immediately drive back at the same speed to your starting point where you heated the car, do another hard stop. Note your consumption.

If my theory is correct, the consumption on the first 1000' drive will be much much much much higher. If my theory is wrong, I'm just another jerk on the internet :p
I would be interested to hear your results!
 
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#15
Finally got around to recording a video of my energy consumption. The temperature this morning was 68F, I charged to 70% overnight, and this is immediately after pulling out of my garage - car had been on for about 30 seconds when I started recording. Notice the speedometer, I'm accelerating as gently as possible and my consumption starts off at 1200Wh/mi. It took 7 minutes of driving (video is only 5 due to cutting out time stopped at lights) and 1.8 miles to finally get below 300Wh/mi. My 15 mile commute which is about 30% street and 70% highway ended up at 280Wh/mi while driving very gently. I have the 18" aeros.
 

jsmay311

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#16
Finally got around to recording a video of my energy consumption. The temperature this morning was 68F, I charged to 70% overnight, and this is immediately after pulling out of my garage - car had been on for about 30 seconds when I started recording. Notice the speedometer, I'm accelerating as gently as possible and my consumption starts off at 1200Wh/mi. It took 7 minutes of driving (video is only 5 due to cutting out time stopped at lights) and 1.8 miles to finally get below 300Wh/mi. My 15 mile commute which is about 30% street and 70% highway ended up at 280Wh/mi while driving very gently. I have the 18" aeros.
My back-of-the-napkin math can't figure out how you could've possibly used 119.4 Wh over the first 0.1 miles while accelerating as slowly as you did (~10 seconds to get up to 20mph) unless significant power was going towards things other than propulsion (e.g., cabin and/or battery heaters).

What were your HVAC settings?

And 280 Wh/mi over a longer drive is quite a bit higher than most people are reporting, despite driving conservatively and having aero wheels.
 
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#17
My back-of-the-napkin math can't figure out how you could've possibly used 119.4 Wh over the first 0.1 miles while accelerating as slowly as you did (~10 seconds to get up to 20mph) unless significant power was going towards things other than propulsion (e.g., cabin and/or battery heaters).

What were your HVAC settings?

And 280 Wh/mi over a longer drive is quite a bit higher than most people are reporting, despite driving conservatively and having aero wheels.
It was set to 72F and auto, it was 68F outside so not working too hard. These numbers are kind of ridiculous right? My lifetime energy consumption is 265Wh/mi over 1,500+ miles.
 
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#18
Is it possible that your are warming up the front wheel bearing grease and the fluid in the rear differential?
That said my hybrid always gets the worst mileage just getting out of my neighborhood.
 
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#19
Normal for me. I figure it’s just bc I haven’t moved anywhere. One time I turned on and did an immediate hard acceleration. It was about 1900 wh/mi. But went down quickly as I kept driving.
 
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#20
Acceleration to 22mph (when the odometer shows first consumption number at 0.1 miles) should consume 53 Wh when counting only the linear movement of 1900kg. The missing energy averages to about 13kW, of which about 6kW could possibly be the cabin heater.

I'm too lazy to exactly compute the energy stored in rotating parts, but it could be about 2Wh for all the wheels and with the motor gear ration 9:1 (see it squared for appropriate comparison of energy), one can see that the rest of the missing energy can easily be hidden in motor's quickly rotating, heavy rotor.