I like this. It's not the end-all statistic but it could be very useful,
at least until we have ubiquitous fast charging and huge kWh batteries.
Perhaps a company like Edmunds will take note and start publishing this
kind of metric.
Peri
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------ Original Message ------
From: "Mark Laity-Snyder via EV" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Cc: "Mark Laity-Snyder" <[email protected]>
Sent: 28-Sep-20 5:57:19 AM
Subject: [EVDL] Drivability again
I did include the link but it didn't show up so here is the pasted article.
There are endless factors used to compare electric vehicles (EVs) from various
manufacturers. There are 0-60 times. All-electric range (AER). Usable
kilowatt-hour (kWh) in the battery. Luggage space. Number of seatbelts. Charge
rates for DC fast charging. Efficiency.
But something is missing. Something important. For the lack of a better term,
I’ll call it “drivability.” Drivability is related to efficiency, but it isn’t
exactly that. It’s also related to charge rate, but it isn’t exactly that,
either.
I would define drivability simply as how long I have to charge my car in order
to get back on the road. In terms of basic arithmetic, it could be defined as
the ratio of how quickly I can add energy to the car, to how quickly energy is
used while driving the car.
To make drivability a useful metric to compare EVs, some standardizing
assumptions are required. So let’s assume that we drive the car for one hour at
62 miles-per-hour (MPH). How long will it take to replenish the energy used?
The less time it takes to charge the car to replenish the energy used, the more
drivable the car becomes.
From personal experience…
My first EV was a Miata conversion. While I loved driving that car, it had a
horrible drivability factor. At 62 miles per hour (MPH), the Miata traveled
approximately 2.2 miles for every kWh consumed. Worse, its maximum charge rate
was merely 2.5 kilowatts (kW). Expressed mathematically, that is 2.5*2.2 = 5.5,
which has units of MPH. Taking it a step further, and since the 2.2 mi/kWh
efficiency assumed 62 MPH, for every hour charging, I could drive for only
5.5/62 = 0.09 hours! As I’m defining it, my e.Miata had a drivability of 0.09.
Not good for long trips.
Now I drive a Jaguar I-Pace. Its efficiency isn’t much better at 2.4 mi/kWh at
a constant 62 MPH, but its maximum charge rate is far superior at 100 kW from a
capable DC fast charger. Running through the same arithmetic gives a
driveability factor of 3.8. What this means is that for every hour I charge my
I-Pace, I can drive for 3.8 hours at 62 MPH. Those with an eye for numbers will
cry foul about now because most EVs will not sustain the maximum charge rate on
a DC fast charge for very long. But we’re going to ignore this reality for a
moment.
Let’s look at one more example. I convinced my younger brother to buy a Tesla
Model 3, which has an efficiency at 62 MPH of 4.5 mi/kWh, and will charge at
120 kW. Running through the same arithmetic gives it a driveability of 9. Ergo,
for every hour of charging one could drive for nine hours, assuming the car’s
battery and the driver’s bladder have that type of capacity. This can also be
interpreted to mean that every minute charged enables nine minutes of driving.
The numbers discussed above reflect a maximum drivability using the maximum DC
fast charging rate for a specific model. Understanding that no EV can sustain
maximum charge rate when using a DC fast charger, it makes sense to be more
focused on the average drivability. Over the course of a full charge, the
I-Pace average charge rate at up to 90% state of charge is about 70 kW. The
Model 3 is about 100 kW at up to 90% state of charge. This yields an average
drivability factor of 2.7 for the I-Pace and 7.1 for the Model 3.
So, what should we do with this information?
I am suggesting that manufacturers start using a standardized metric similar to
drivability, enabling consumers to compare different models among various
brands. Until that happy day, I’d like to see EV enthusiasts use the
drivability factor that they’ve calculated themselves while comparing EVs.
How to do this? Simply determine the efficiency of an EV in miles/kWh at 62
MPH--the results should be between two and five. Most EVs will express this
number either in a connected app or in the driver’s user interface display.
Some manufacturers express this number in watt hours per mile, or even in
kWh/100 mi. To go from watt hours per mile, simply divide into 1,000. For
example, an EV that has an efficiency of 333 watt hours per mile is 1,000/333 =
3.33 mi/kWh. To go from kWh/100 mi, just flip it around. For example, an EV
that uses 33.3 kWh/100 miles is 100/33.3 = 3.0 mi/kWh.
Once you have efficiency in miles/kWh, find out the average charging speed for
a DC fast charge session in kW. This can be easily determined by charging from
a near 0% state of charge for an hour. The DC fast charger will tell the driver
how many kWh have been added to the car. Once you have efficiency in mi/kWh and
the average charge rate in kW, simply multiply the two numbers; 62 divided by
that result yields the drivability factor. Stated more plainly, for a
drivability factor of five, charging for one minute yields five minutes of
driving. Charging for five minutes yields 25 minutes of driving. Charging for
one hour yields five hours of driving.
What is the drivability factor of your electric car?
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