Range Confidence: Charge Fast, Drive Far, with your Electric Car

By David Herron

Total driving range in electric cars: plug-in hybrid versus fast charging

Over the past hundred years or so humanity has driven cars, we've collectively learned the expectation of driving hundreds of miles a day. It's become normal to hop in the car, racking up hundreds of miles, stopping only for quick gas stops and eating junk food while driving. The story about electric cars is they cannot do this, and they're only good for around town driving. The plug-in hybrid and fast charging are two approaches to driving long distance on electricity.

The key to a road trip isn't the size of the battery pack or the gas tank. The key is how quickly energy can be added to the car. That energy can be gasoline, or diesel, or electricity, but the point is the car cannot move without adding energy to the energy storage tank.

The best measurement is the total driving range in a day. This is how far you can drive accounting for recharging sessions throughout the day. One definition is "The theoretical maximum distance drivable in 24 hours with optimal driving and recharging."

Even though that number - distance in 24 hours - is not applicable to most of us (who drives for 24 hours straight?) it is a way to compare different vehicles.

What you need to know is which electric car handles your long trips, your weekend trips to the mountains, or over the hills and through the woods to Grandma's house, or wherever it is you're going.

Procedure to calculate total driving range for any car

To understand this let's think about how to calculate the number. It's defined as

  • 24 hours
  • Normal highway speed (65 miles/hr)
  • Stopping to refuel and take care of human needs

To simplify we'll assume flat land, no traffic delays, and you can maintain 65 miles/hr while driving.

You'll have a series of N phases of driving, each of which consists of

  • Driveable distance per tank of energy
  • Time required to refuel energy storage tank
  • Taking care of human needs while stopped (pee breaks, eating, etc)

I don't know how to turn this into a simple equation. Instead it'll require building a timeline on paper to arrive at the estimate.

For example consider 240 miles of driving per charge. That's a bit less than 4 hours of driving per charge. I arrived at this number by assuming a 300 mile range car. Charging it to 90% for each charging session, you'll have 270 miles range to work with. Then leaving 30 miles as a safety buffer you arrive at 240 miles per segment.

Hence each segment is:

  • 3 hrs 42 mins driving
  • About 40 minutes charging (at 120 kW Supercharger rate)
  • Human needs should be achievable within those 40 minutes

Therefore we have 4 hrs 22 minutes per segment. That gives us 5 full segments of driving, plus another half of a segment.

Therefore that combination gives us about 1320 miles of driving in 24 hours.

The belief that gasoline is the only way to drive long distances

Our collective 100+ years experience with driving cars has taught us that gasoline is an all-powerful fuel with which we can drive anywhere we like.

We just saw a rough estimate that a pure electric car can drive about 1300 miles in a 24 hour period. That's pretty impressive, is it not? The legions of Tesla car owners who drive long distances using the Supercharger network demonstrates that electric vehicles can be driven long distances if by using fast charging.

There have been two competing theories about driving long distances with electric cars:

  • Relying on gasoline with a Plug-in Hybrid car
  • Relying on fast charging with a pure BEV

The primary example of the Plug-in Hybrid approach is the Chevy Volt. Volt owners have been immensely happy with their cars, and routinely take long distance trips relying on gasoline to go the distance. The Volt is designed to convert itself into a normal Hybrid car when the battery pack state of charge goes low enough. Therefore taking a long trip simply requires topping up at a gas station every so often.

There are other PHEV's on the market such as the BMW i3 REX, the Ford Energi cars, and a few others. With all of them we have the same sales pitch - that gasoline gives you all the range you need.

For the pure battery electric vehicles our only solution to long distance driving is to use fast charging. Of course Tesla Motors developed the best implementation of DC fast charging. The Supercharger network was designed to support long distance travel, and Tesla's cars carry a large enough battery pack to support driving long distances.

With the other electric cars it is possible to take long distance trips so long as fast charging is available. The non-Tesla cars are hampered by usually having a shorter driving range, and the non-Tesla fast charging network runs at a lower power level and is not as well deployed. Despite those limitations, many people do take long range trips with these cars.

The non-Tesla cars are starting to have enough range - e.g. the Chevy Bolt has 240 miles range, and other 200-300 mile range EV's are on the market from Nissan, Hyundai, and others. Plus the non-Tesla charging networks are starting to improve. Therefore owners of non-Tesla cars now can take long distance trips just as easily as owners of Tesla cars.

Bottom line is that we should collectively challenge the belief that gasoline is the only way to implement long distance driving.

In the previous section we saw a rough estimate that a pure BEV could be driven about 1300 miles in a 24 hour period. Surely that's enough? And the possibilities are only getting better as range and charging speed and fast charging availability all improve.

Marketing phrase - Extended Range EV

At the beginning of the current phase of electric vehicles, GM's marketing department coined a phrase for the Chevy Volt: "Extended Range EV". The Volt is billed as being more than an EV, because it has "extended range". Meaning, the gasoline engine extends the vehicle range.

This marketing phrase was part of the effort to get us to continue relying on gasoline for long distance driving.

The Volt, and other PHEV's, are fine cars and the gasoline engine is a pragmatic solution to driving road trips. The gasoline engine range extender means it's not as "pure" an electric vehicle as one that only runs on battery power. But, it also demonstrates the value of quickly replenishing the on-board energy storage for longer total driving range.

Range Extension

What does the phrase Extended Range or Range Extension mean? Doesn't it just mean quickly recharging the energy stored in the car?

Any battery electric vehicle can have its range extended by recharging the battery pack. The only difference is whether the vehicle is recharged with gasoline or with electricity, and the fact that recharging a battery pack is currently a slow process.

There's two general ways to extend total driving range with an electric car

  • Extending the range while driving - somehow the battery pack recharges while driving
  • Extending the range by stopping to recharge - plugging in and charging

Let's expend a few brain cells thinking about all the possible methods to extend the range of a vehicle.

Extending the range while driving

Portable towable battery packs: This is a design idea some have floated, to have a battery pack on a small trailer towed behind a car. One could easily rent the trailer for long trips, connecting it to a special charging port that allows the car to drive while the addon pack is plugged in. Battery trailer exchange stations could be located at stops along highways, similar to the propane tank kiosks seen at gasoline stations today.

Portable towable gasoline generators: A similar idea is to, instead of a towed battery pack, to have a towed gasoline generator. A big issue is this arrangement stops being a battery EV, and starts being a Plug-in Hybrid, and legally it stops being a ZEV. There are government emissions rules with ZEV meaning a vehicle that has zero tailpipe emissions. But if the vehicle has a part-time gasoline engine, it would fall into a classification that does not yet exist.

A problem with the towable battery pack or gasoline generator is that electric cars are explicitly designed to not charge through the charging port while the car is driving. That was a safety feature to prevent drive-off's. The impact is needing to bypass that design feature when using a towed recharging gadget.

Fuel Cell: Fuel cell vehicles do have an all electric drive train, and a small battery pack to hold electricity. Hence, it’s not that much of a stretch of terminology to call the fuel cell a range extender. There’s even some work to design fuel cell vehicles with larger battery packs where the fuel cell is used as a portable range supplement.

Gasoline Engine range extender: These vehicles are what we normally use the phrase “range extender” to describe. The Chevy Volt, Fisker Karma, Via Motors VTRUX line all have explicitly used the phrase “extended range electric vehicle.” BMW does not use this phrase to describe the BMW i3 REX, instead REX is an acronym Range EXtension.

Solar panels on the roof: Many time this is asked, why don't "they" put solar panels on the roof? The problem with that is there's simply not enough space on a car roof to carry enough solar panels to generate enough electricity to make any significant impact. See (greentransportation.info) The number of solar panels required to power an electric car for a deeper discussion.

Extending range by stopping to charge

DC Fast Charging: CHAdeMO or Combo Charging System or Supercharger can all quickly add range to an electric car. DC Fast Charging adds between 150-300 miles of range per hour of charging. That means about an hour of recharging after 4-5 hours of driving. It is rather healthy to stretch your legs for awhile after that much driving. See When will electric cars charge as fast as a gasoline car? (5 minutes to recharge)

The car makers are starting to talk about 250 kiloWatt and 350 kiloWatt electric car charging systems. At that rate a 15-20 minute recharging time is possible. That's pretty fast even if it's not the same as the 5 minutes to refill the gas tank on a gas car.

Battery swapping range extender: This concept is way older than Better Place and Tesla Motors, both of whom have implemented modern roboticized battery swapping systems. A manual battery swap system was used for electric taxicabs in New York City around 1900. With a well designed station the battery exchange can add range at a rate similar to refilling a gasoline tank. However, modern battery swapping schemes have all failed in the marketplace. See Wouldn't battery swapping be preferable over waiting to recharge?


Range Confidence is Copyright © 2016-17 by David Herron

Total driving range in electric cars: plug-in hybrid versus fast charging

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