User Datapoint: Ambient Temperature vs Charge

[suraj1194]

I have an interesting datapoint on doing the same ~125 mile drive at about the same time, within two minutes of the same drive duration, in successive weeks with significantly different ambient temps.

Total drive duration: 1:55
Distance: 124miCabin temperature: 74
Wheels/tires: 19" with Goodyear Eagle Tesla-foarm Touring, approx 20K miles.

I set AP to 75mph continuously with some places where I boosted it 5mph to overtake the right lane and move over for advancing traffic behind me. Mild freeway traffic overall.

Last week: temps in low 60s during drive
Today: temps 85-90

Before both drives, car was range charged to 249mi. Range mode on.

Rated range after drive:
Last week: 118mi
Today: 131 mile

So with temps in between 60-70, the range for a 124 mile drive dropped from 249 to 118 (131 miles equivalent). Today, the same drive utilize 118 miles, or 6 miles less than the actual distance, despite driving at 75mph most of the while with A/C on. Very nice indeed. I expected more residual range due to higher ambient temps, but not more than a dozen miles more.

 

[Missile Toad]

Was there a difference in the headwinds to the tune of about 6.5 MPH? That will get you an 11% difference in apparent efficiency. Go to wunderground.com and see which way the winds were blowing on each day. I would guess the use of AC to adjust temps about 15-20º F produces at most a 2% change in actual RM used for a given trip.

 

[suraj1194]

Thanks for your response. Very interesting question. The data from a few waypoints suggests winds of ~5-10MPH in the southerly direction. The route is ~50% westbound, and the rest S/SW bound (I-80 followed by I-680). I checked waypoints along both stretches. I've been getting fairly consistent numbers similar to the ones from last week for about a month or more under different conditions, prior to the much higher temperatures in the Bay Area these past few days. I pretty much always end up with 110-120 miles of remaining rated range after this drive, and today's the first time since last summer that I recall seeing 130+ .

 

[Missile Toad]

My 70D gets about 290 wh/mi in no-wind situations at about 70 MPH. I put together a table a while back. Pure crosswind at 7 MPH makes it 293 wh/mi and a 45º cross/headwind is about 311 wh/mi. As you drove half and half, I'm going to split the difference and call it 297 or so. So penalize your recent trip 2.3% because of 'conditions'.
Can you lookup your earlier trip? Did you have tail-winds? Maybe on the order of 4 MPH? One thing that can make a huge difference, regardless of the wind, is if you drive in a herd of other cars, and can benefit from drafting/sheltering effects. Maybe you had more of that in your first drive?

 

[suraj1194]

Thank you for the excellent details! I'm fairly certain I don't have such fine grained data on the two trips My most notable datapoint was the end SoC of >130mi and the rated mile consumption < actual range, which I hadn't seen since late last summer. Since we're currently experiencing the hottest temps since the end of last summer, I assumed the temperature had a major role to play in this. Until now, through fall, winter and spring, I'd always gotten 115-122 or so leftover rated miles on this weekly drive.

Working back from 75kwh and 249mi total rated miles, that 301 wh/mi I was getting somewhere in the 300-320 wh/mi for a long time. The fact that I consumed fewer than actual miles suggests I dipped under that level. Does the A/c blowing slightly cold (75) use less juice than when the heater runs with the thermostat at ~72, as it's been doing for months now ?