Electric Vehicle Ownership: Costs, Environment, Climate, Politics

By David Herron

Calculating effective trip speed – Fast charging cars more valuable

The Range Anxiety bugaboo is partly about effective trip speed over a long distance.  While it's presented as the fear of getting stranded with a dead battery pack, it's also the fear of being unable any kind of trip you want to make.  Gasoline car owners are accustomed to driving wherever they want, whenever they want, relying on a ubiquitous network of refueling stations offering a 5 minute full recharge, along with convenience services like coffee, drinks, snacks and restrooms.

Given sufficient charging infrastructure, it's possible to take trips of any length with an electric car.  But, a slow charging rate will make the trip impractical, as we'll see below.  The charging speed (range gained per hour of charging) is what determines the feasibility of making a long-range trip with a given electric car.

Earlier we said electric cars with longer driving range are more valuable than ones with a short driving range.  That's clearly true for trips around town, but what about long trips, proper road trips?  Electric cars with fast charging are, as we'll see below, more practical on long trips and therefore are more valuable than those with level 2 charging.  Well, as long as your region has a network of compatible fast charging stations.

It boils down to a measure, range gained per hour of charging.  Some places publicize a rating like 100 RPH.  RPH is Range Per Hour (of charging), hence 100 RPH is 100 miles range per hour of charging.

To start this off, rules of thumb with a typical electric car:

  • 6 kiloWatt level 2 charging: gains 20-25 miles range per hour of charging
  • 24 kiloWatt DC fast charge: gains 80-100 miles range per hour of charging
  • 50 kiloWatt DC fast charge: gains 160-200 miles range per hour of charging
  • 120 kiloWatt DC fast charge: gains 300+ miles range per hour of charging
  • Gasoline: 300 miles in five minutes, or 3,600 miles range per hour of charging
Let's ponder a trip requiring 600 miles of charging with each of these charging rates.
  • 6 kiloWatt: 30 hours charging time - 10 hours driving time
  • 24 kiloWatt: 7.5 hours charging time - 10 hours driving time
  • 50 kiloWatt: 3.75 hours charging time - 10 hours driving time
  • 120 kiloWatt: 2 hours charging time - 10 hours driving time
  • Gasoline: 10 minutes charging time - 10 hours driving time
Clearly the Tesla Model S is nearly equal to gasoline road tripping, because face it with 10 hours driving time you're going to stop for a couple hours along the way to eat meals, and take human needs breaks.    The 50 kiloWatt fast charge options (CHAdeMO and ComboCharging System) are almost good enough, and both are expected to support higher speed charging in the coming years.  Anything slower is simply not suitable for road trips.

Why do I say a fast charging electric car is more valuable?  It's that the faster the charging rate, the more flexibility you have about when and where you can drive your car.

We can also take from this the factoid that, over a long trip, the effective trip speed is dominated by the charging rate.

  • 6 kiloWatt: 40 hrs driving & charging time equals 15 miles/hr
  • 24 kiloWatt: 17.5 hrs driving & charging time equals 34 miles/hr
  • 50 kiloWatt: 13.75 hrs driving & charging time equals 43.64 miles/hr
  • 120 kiloWatt: 12 hrs driving & charging time equals 50 miles/hr
  • gasoline: 10.1 hrs driving & charging time equals 59.4 miles/hr
By the way, I'm making a couple simplifying assumptions - 60 miles/hr driving speed - no time spent getting in and out of refueling areas - all driving on flat ground - no time spent on human needs like food, potty breaks, sleep breaks, leg stretching breaks, etc.

To understand this a little more deeply, let's take a look at how the rules of thumb above were calculated.

The EPA ratings label for electric cars shows the kiloWatt-hours consumed per 100 miles.  The BMW i3, for example, consumes 27 kWh per 100 miles or 270 Watt-hours per mile, which is a bit less than 4 miles per kiloWatt-hour of energy.  As a simplification let's say a typical electric car delivers 3.8 miles per kiloWatt hour of energy, and has a 24 kiloWatt-hour pack.

  • 6 kiloWatts: 6 kWh gained per hour, or 22.8 miles range per hour
  • 24 kiloWatts: 24 kWh gained per hour, or 91.2 miles range per hour
  • 50 kiloWatts: 50 kWh gained per hour, or 190 miles range per hour
  • 120 kiloWatts: 120 kWh gained per hour, or 456 miles range per hour
These numbers are suffering from the simplifications we've made.  For example, at a 120 kiloWatt charge rate the Model S fills so quickly that the charging rate quickly starts tapering off, and therefore the car doesn't spend the entire hour at a 120 kiloWatt charging rate.  The same holds true of vehicles like the Nissan Leaf where the 24 kWh peak means it simply cannot charge at 50 kiloWatts for an entire hour, and in any case the charging rate starts tapering off quickly.

The point is that the range gained per hour of charging is a simple equation: kiloWatt-hours gained per hour of charging, multiplied by the range per kWh of energy consumed.

Calculating effective trip speed – Fast charging cars more valuable

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