Point-of-no-return: Using 18.5 kWh capacity as a gauge.

[studio460]

I'm trying to plan out-of-range round trips (e.g., 40 miles, one-way) for contingencies in case planned charging facilities are unavailable (i.e., occupied or "ice'd") upon arrival. So, I've come to depend more on total kWh used on particular trips as a more useful guide than the battery indicator on the dash. I've quoted an excerpt ftom michael's post from the "70-mile" thread here:

. . . I estimate battery capacity at 18.5 kWh (although others may have slightly different estimates). So to make [a 70-mile round-trip], you would need to maintain 18.5/75 = 246 Wh/mile. This is a reasonably achievable value, even at freeway speeds . . .

Making the 31.6 freeway commute to work at 60-65MPH uses from a minimum of 6.5 kWh when driving conservatively, and 8.7 kWh on the high end. The other day, I burned 3.9 kWh just running a couple of errands around town (there's lots of hills where I live). Here's two samples, one from each extreme (very conservative driving; very spirited driving) of my daily work commute, which includes a large incline up the Sepulveda Pass.

Now, the 8.7 kWh-trip is a bit of an anomaly (my real "high average" tens to be 7-8 kWh), since I was testing the FFE's high-speed acceleration on this particular trip, and having a bit of fun (since I know that I can charge at both ends of the commute, I've been driving a bit less conservatively in recent days). I've also published this "running errands around town" kWh sample to show that short trips can often be less efficient than long commutes.

Last week (before myfordmobile had been activated), I burned up quite a few kWh going up and down a six-level parking structure (after already driving for 40 miles), looking for the charging station. I depleted my already-spent battery well beyond the point-of-no-return, and had I not finally found the charging station (it was in a different parking structure), I wouldn't have had enough energy for the trip home.

Although individual driving habits vary, I'm curious to know what everyone's kWh point-of-no-return number seems to be, basically, your estimation of total battery capacity in kWh, divided in half. Using michael's example of total battery capacity of 18.5 kWh, his point-of-no-return (all else being equal) would be 9.25 kWh.

 

[jmueller065]

I have a couple of range/driving planner web pages. One will attempt to estimate the power consumption for a given trip (you can enter in your average kWh/mile consumption):
http://jamiegeek.myevblog.com/2014/01/13/estimating-power-consumption/

The other will give you a driving circle around a given point for a given set of miles:
http://jamiegeek.myevblog.com/2014/02/19/how-far-can-i-go/

Also note that the car will notify you of the no-go point (I've received the "warning you may not be able to return" message a couple of times).

 

[NightHawk]

I noticed the MyFord Mobile smartphone app's trip planner feature seems to estimate distance to your destination as the crow flys and not actual distance on the real roadways. So its effectively useless for me since it estimates 15miles for a trip that in reality is about 25miles. The onboard navigation system in the car however gets accurate distance at least.

 

[studio460]

According to Google Maps, the drive to my dad's house is only 38.5 miles (at first I thought it was over 40 miles, but Google was using a slightly less efficient route). I just arrived at my dad's house and plugged in my 120V EVSE:

State-of-charge remaining upon arrival (above).

Energy consumed on outbound leg (above).

Mid-trip 240V charge at a public venue (above).

Energy consumed on return leg (above).

The first trip log represents the outbound leg; the second shows the return leg. The outbound leg included 90% freeway driving at 62MPH (ambient temp: 85 degrees), with no significant inclines, other than freeway overpasses. Surface streets were driven at between 35-40MPH. The return freeway leg was cruise-controlled at 65 MPH the entire way, without any braking applied, then 45MPH for most of the surface streets (ambient temp: 70 degrees).

Since the 8.0 kWh outbound usage represents less than half my theoretical 18.5 kWh total capacity, I may have been able to make the entire trip on a single charge. However, the return leg consumed 9.1 kWh (due most likely to the higher freeway speed), bringing my total round-trip energy consumption to 17.1 kWh (driving behavior both ways was very conservative). Note that my SOC at the end of the return leg (not shown) was exactly 50%. I wasn't willing to bet on these numbers so in addition to plugging-in my 120V EVSE while at my dad's house, I also went out to dinner for a 50-minute, city-provided 240V charge.

 

[studio460]

I have a couple of range/driving planner web pages. One will attempt to estimate the power consumption for a given trip (you can enter in your average kWh/mile consumption):
http://jamiegeek.myevblog.com/2014/01/13/estimating-power-consumption/

The other will give you a driving circle around a given point for a given set of miles:
http://jamiegeek.myevblog.com/2014/02/19/how-far-can-i-go/

Wow! Those are really cool! Thanks!

Also note that the car will notify you of the no-go point (I've received the 'warning you may not be able to return' message a couple of times).

Huh! I didn't know that! That's pretty cool. So I guess there is a fail-safe built-in to the car. When I was driving up and down that parking structure, I didn't notice it, but I had stopped twice (turning the car off), so maybe that interrupted the sequence. Or maybe I was just too distressed to notice.

 

[studio460]

I noticed the MyFord Mobile smartphone app's trip planner feature seems to estimate distance to your destination as the crow flys and not actual distance on the real roadways . . .

That's too bad. I've only used Google Maps and the in-car navigation to get mileage estimates.

 

[michael]

When you are running near empty, to use a line from the old Broadway show The Music Man, you gotta know the territory. (I'm dating myself...)


A specific example is my own commute. I live off Sunset, near the 405, and I generally return from the Valley. I need to know exactly how much energy I have left when passing the 101 Southbound, heading over the Sepulveda pass. If I can make it to the top, I can get the remaining miles home on essentially zero energy. But if I were to run short before the summit, I would be in bad shape.

I personally set a limit of 2 kWh remaining at the 101. This will allow me to get home without white knuckles. If less, I stop at one of the numerous public charging spots nearby, add a quick squirt of juice, and continue on.

In other words, there isn't a specific go-nogo value at the turnaround. If I'm tight, I conserve energy, watch my progress.

The "cup of energy" display is very useful in that regard. It shows what kWh/mile you need to maintain to make it. If it's a reasonable value, and especially if it's climbing, you are in good shape.

 

[studio460]

. . . I need to know exactly how much energy I have left when passing the 101 Southbound, heading over the Sepulveda pass. If I can make it to the top, I can get the remaining miles home on essentially zero energy. But if I were to run short before the summit, I would be in bad shape . . . I personally set a limit of 2 kWh remaining at the 101 . . .

The 101? The 405 and Sunset? Now, you're talking my language! It's really great to hear from other owners in your area. Since I take the same route everyday (405N into the Valley; 110S out of the Valley), I'll hopefully be able to catalog similar indicators to guide my energy usage. Thanks for your post!