EV Road Trips - Why 100-0% Range Doesn't Matter

Range tests at highway speeds do not give a complete picture of the road trip worthiness of the vehicle.  Not if your trip is longer than the single-charge range.  At highway speeds.  In the winter.  Let's use a recent EV market entry as an example for the good ol' American road trip?

PlugShare Trip Planner: 
Single Charge Ideal Road Trip
A December 14, 2020 article from Green Car Reports tested the soon-to-be-released Ford Mustang Mach-E at highway speeds.  The manufacturer's overall range estimate of 270 miles sounds impressive, but does not hold up under continuous highway speed use.  According to the article, the expected highway range of the Mach-E would be closer to 219 miles.  

The average reader might take away that the car can comfortably travel about 219 miles between charges.  Divide by a 70 MPH average highway speed and we get a cruising time of just over 3 hours before the car needs to be charged again.

That scenario will likely never happen.  Road trips longer than the single-charge range of the car don't work that way.  As such, highway range tests are deceptive.

Anatomy of an EV Road Trip

I regularly make a 450-mile trip with my 2015 Tesla Model S with a rated range of about 240 miles.  Based on that distance, a single charge at the Maumee, OH Supercharger would suffice according to the PlugShare Trip Planner.  

The Supercharger Network makes Teslas
capable of comfortable road trips.
However, that first 226 mile leg would mean 3 hours and 20 minutes of driving followed by a 1+ hour charge to get the battery back to 100% state of charge (SOC).  Assuming the car even is as efficient as the range guess-o-meter suggests and I had no detours or other pit stops, I would arrive with an anxiety-inducing 14 miles of range or less than 6% SOC.  The second leg isn't much different.  

If you plot this route using Tesla's built-in navigation, the algorithm will find the 6% arrival SOC unacceptably low, adding additional charging stops.  It doesn't bode well for my human need for coffee or a pit stop either.  

A much more palatable solution in terms of driving time and charging time is likely a 3-stop itinerary with about 100-120 miles between stops and 20-30 minutes to charge.  

Let's analyze this in more detail.

The Gas Car Road Trip

PlugShare Trip Planner: 
3 Charging Stops
Think back to the days of a road trip in a gas car.  If you're a Myers-Briggs INTJ like me, you've planned out the trip in detail, topped up the tire pressure, filled the windshield wiper fluid reservoir, and, of course, topped up your fuel tank to 100%.  

Let's say my rented minivan has a fuel capacity of 20 gallons and a worst-case summer A/C fuel economy of around 20 miles/gallon (MPG) giving us a range of 400 miles.  My wife starts to get nervous when the fuel tank drops below 3/8 bringing the real range between stops to 250 miles.  

EV Road Trip:  The First Leg

Similarly, EVs are not deliberately driven to empty.  Discharging the battery below 10-20% SOC is not a good idea.  In addition to having long-term battery degradation effects, even seasoned EV drivers start getting range anxiety at this point.  An unexpected detour, hill climb or headwind can leave you stranded with a dead battery.  Hence the Tesla algorithm parameters.

If we expect to arrive at the first fast charger with 20% SOC, the first leg range for that Mach-E just dropped to 175 miles.  

Tesla Supercharger Power vs. State of Charge

EV Road Trip:  The Subsequent Leg

DC fast charging speeds start high, but the charge rate drops off as the SOC increases.  As an example, the typical Supercharging curve of Tesla 90-100 kWh battery packs is shown on the left (source:  Tesla Tap).  

Notable from this curve is that at the charging rate drops from a peak of about 110 KW to just over 40 KW at 80% SOC and keeps decreasing.  That means squeezing in those last few kWh between 80 - 100% SOC takes a really long time.  

Most EV road trippers recommend charging the battery to about 80% SOC and then heading out to the next DC fast charger, obviously making sure you have enough of a charge to get to your next destination with about 20% SOC.  That places the ideal road trip SOC range is between 20-80% or 60% of the total battery capacity.  

For the Mach-E, 60% of 219 miles is about 131 miles.  That's a far cry from the advertised 270 miles.  That range at 70 MPH translates to under 2 hours of continuous driving time.  The question for the road tripping driver is whether this a reasonable amount of time between stops.  For me personally, 2 hours is more than enough time before I want a pit stop and at least a coffee or a meal.  Obviously this is a personal preference.  

Driving time between charges is one of two main considerations for EV road trips.  The other is charging time.


Charging Power and Time

Audi E-Tron Charging Curve
Source: Green Car Reports
While I do not have access to the Mach-E charging curve, this Car and Driver article claims a 10-80% charge time of 38 minutes.  Using the peak power of 115 KW and a battery size of 68 kWh, the 10-20% SOC charge time should be 4 minutes, so that leaves a 20-80% charge time of about 34 minutes.

As a side note, there is one vehicle currently on the market that claims longer sustained high-power fast charging:  The Audi E-Tron (Green Car Reports article).

EV Road Trip Cadence

Putting above information together, the road trip driving and charging cadence of the Mach E is 112 minutes of driving and 34 minutes of charging for a driving: charging ratio of 3.3.  That is for every 3 hours and 17 minutes of driving, expect to spend 1 hour charging.  While a shorter charging time would be more appealing, it seems quite reasonable to me.  

I posit that 60% of highway range and the time to recover that charge is what really matters on road trips.  That is, how far can you go on 60% of the highway range and how long does it take to recover that charge from 20-80%?

Ultimately the question is whether you have the patience for that cadence on a road trip.  

Conclusions

I don't mean to single out the Ford Mustang Mach-E.  This vehicle, along with many other recently announced EVs, has decent highway range and decent charging speed on par with older Teslas like mine.  However, these values are no longer competitive with newer Teslas that have better range as well as higher sustained charging power that make road tripping nearly as convenient as gas-and-go ICEVs.  Legacy automakers still have their work cut out for them in producing EVs that are as convenient for road trips.  Obviously, the battery and charging technology will continue to improve.

0-60 times for EVs may be great for impressing your friends and posting YouTube launch reaction videos.  Before you fork over your hard-earned money on a new EV intended for road trips, make sure you look at not only the highway range, but the DC Fast Charging (DCFC) specifications like the charging curve. 
Stay Charged Up!


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