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

By David Herron

Understanding charging rates and effective trip speed

Every powered vehicle carries an energy storage unit. Whether the energy stored is gasoline, or hydrogen, or natural gas, or electricity, it is the stored energy and the engine or electric motor which distinguishes powered vehicles from human or animal powered vehicles. Powered vehicles can only travel as far as the energy storage unit lasts, and then it must be recharged. The effective trip speed is then limited by the vehicle speed, the range per charge, and the charging time.

The big difference between gasoline and electric cars is the method to recharge the energy storage unit. With gasoline (or diesel) powered vehicles you recharge the storage unit with liquid diesel (or gasoline) from a pump at an appropriate station. With electricity the storage unit is recharged with electricity that comes through specially designed electric cables attached to an electric vehicle service equipment (EVSE) unit.

Typical gasoline/diesel cars drive 300-400 miles per charge. Because it’s a liquid fuel, the storage unit can be recharged as quickly as the liquid can be pumped. Or, about five minutes for a complete recharge.

Typical (affordable) electric cars of early 2015 can drive 80-100 mile per charge, with the Tesla Model S (luxury car at luxury car prices) having 265 miles range per charge. Not that it is 2017, we have an affordable 200+ mile range electric car (Chevy Bolt EV), and others (such as the Tesla Model 3) promised soon from several manufacturers. Because it is electricity, the charging rate is limited by the charging equipment. The typical has “level 2” AC charging equipment running at a 6 kiloWatt charging rate. Some electric cars have fast charging capability, with the fastest supporting a full recharge in about an hour.

It’s the recharging time which is the biggest practical difference between the liquid fuel and electric fuel cars.

It’s best to think how long the charging session takes, but the amount of range gained per hour of charging. The purpose of recharging the car is to drive further, therefore the measurable benefit is miles of range gained per hour of charging.

Gasoline car Affordable electric car Affordable EV, Fast charge Tesla Model S, Supercharger
350 miles range gained per 5 minutes 80 miles range gained per 3.5 hours charging 64 miles range gained per 30 minutes charging about 300 miles range gained per hour charging
or 4200 miles range gained per hour of charging or 23 miles range gained per hour of charging or 128 miles range gained per hour of charging

At the moment, the charging rate on even the fastest charging electric car, the Tesla Model S, is less than 1/10th that of gasoline powered vehicles. The typical affordable electric car of early 2015 has a charging rate .5% that of gasoline.

That’s the practical problem we face driving an electric car – the much slower charging rate. The good news is the effective charging time is incredibly short. Usually recharging electric cars takes only the 1 minute (or less) it takes to plug in the charging station cord. We walk away and do other things while the car is recharging, such as sleep or work. We normally do not experience the 3.5 hours for full recharge, since our time is doing other things.

The scenario where we WOULD experience the time to recharge is when taking a long trip. In that case we don't have much choice but to stay near the car and wait for it to finish charging. We might bring something with us (laptop, tablet computer, book, musical instrument, etc) to do while waiting. We might go to a nearby restaurant. Anything we can find to productively occupy ourselves while recharging the car mitigates the fact we must wait for the charging session to end.

For longer trips the charging time limits the effective speed of the trip. The recharging time for gasoline cars is so fast we don’t notice its cost on effective speed, but for electric cars the charging time is so slow that long trips would be excruciatingly slow.

Electric vehicle owners should do their best to have a home charging station. The luxury of refueling your car at home is an advantage we have over gasoline car drivers. Often a simple low power 120 volt outlet is enough, depending on your real range requirements.

Electric vehicle owners should join ChargePoint, Blink and other charging networks to have access to public charging stations. When should one seek out fast charging, or level 2 charging, and why?

HINT: Gauge your electric car by its effective trip speed.

Virtual Experiment — what’s the effective trip speed?

As a virtual experiment, let’s take a 600 mile trip. To simplify the experience, lets assume the ground is flat, the weather is warm, there are AC level 2 charging stations every 70 miles and frequent gasoline stations, allowing us to drive either an electric or gasoline/diesel car.

The gasoline powered car will complete the trip in about 10 hours of driving time, adding about an hour human for needs like toiletry, drinking and eating.

driving time: 600 miles / 60 miles/hr = 10 hours
refueling time: 2 refuelings, 10 minutes each
human time: 30 minutes per refueling? Depends on the driver
effective speed: 600 miles / 11 hrs 20 minutes = 53 miles/hr

The electric car will require about 9 charging sessions, each requiring 3.5 hours to complete plus the 10 hours of driving. It adds up to over 40 hours total between charging and driving.

driving time: 600 miles / 60 miles/hr = 10 hours
recharging time: 9 charging sessions, 3-4 hours apiece, 27-36 hours total
human time: at least one hotel stay, meals etc can be part of charging time
effective speed: 600 miles / 50 hours = 12 miles/hr

If instead we had an affordable 200 mile range electric car, let's see how this changes. It only requires 3 charging sessions (plus a short one, probably), but seach session takes longer time.

driving time: 600 miles / 60 miles/hr = 10 hours
recharging time: 3 charging sessions, 10 hours apiece, 30 hours total
human time: at least one hotel stay, meals etc can be part of charging time
effective speed: 600 miles / 50 hours = 12 miles/hr

It comes out to about the same total trip time. Why? It's because of the 6 kiloWatt charge rate. If instead that 200 mile range EV were charged at fast charging stations, the effective trip time is much lower.

driving time: 600 miles / 60 miles/hr = 10 hours
recharging time: 3 charging sessions, 1.5 hours apiece
human time: accomplished during charging sessions
effective speed: 600 miles / 14.5 hours = 41 miles/hr

The charging time requirement is based on early specifications for the 2017 Chevy Bolt. It's supposed to get 90 miles of range per 30 minutes of charging. The 1.5 hour estimate is the added time to bring the car to a 100% full battery pack.

The Tesla Model S would require 2 charging sessions, each less than an hour, for 10 hours of driving time, and about 2 1/2 hours of charging time, or nearly the same speed as the gasoline powered car.

driving time: 600 miles / 60 miles/hr = 10 hours
recharging time: 3 charging sessions, one short, 2 1/2 hours total
human time: accomplished during charging sessions
effective speed: 600 miles / 12 1/2 hours = 48 miles/hr

That’s why the conventional wisdom says the affordable electric cars are only good for “around town driving”. The number of miles gained per hour of charging is too low for long road trips. EV’s with only slow charging can be taken on medium length or even long trips, if you’re patient enough, at the risk of cranky kids or spouse.

However, some of the affordable electric cars have fast charging support. While it’s not as fast as Tesla’s fast charging, it’s better than the level 2 charging we just outlined:

driving time: 600 miles / 60 miles/hr = 10 hours
recharging time: 10 charging sessions, 40 minutes apiece, 6 hours 40 minutes total
human time: during charging sessions, might require 1 hotel night
effective trip speed: 600 miles / 17 hours = 35 miles/hr

Therefore, while DC Fast Charging makes the affordable electric cars better for long range trips than those limited to level 2 charging, it’s not quite as good as the Tesla Supercharger system. But, hey, not all of us can afford a $100,000 car.

Rule of Thumb

These numbers are courtesy Tesla Motors, but are more-or-less true for any other electric car:

  • 120 volt 12 amps: 3 miles range gained per hour of charging (typical line cord charger)
  • 120 volt 15 amps: 4 miles range gained per hour of charging
  • 240 volt 16 amps: 12 miles range gained per hour of charging (older model 3.3 kiloWatt on-board chargers)
  • 240 volt 30 amps: 25 miles range gained per hour of charging (newer model 6.6 kiloWatt on-board chargers)
  • 240 volt 40 amps: 29 miles range gained per hour of charging (Tesla Model S mobile connector)
  • 20 kiloWatt: 58 miles range gained per hour of charging (Tesla Model S high powered wall connector)
  • Tesla Supercharger: 170 miles range gained per half-hour of charging

Take this rule of thumb with a grain of salt. The Tesla Model S has lower energy efficiency than do the affordable electric cars. (see Driving habits, energy consumption, and driving range) That means other electric cars, like the BMW i3 (the most efficient of current electric cars), will get slightly better miles of range gained per hour of charging.

The bigger consideration is that, at public charging stations, you’re unlikely to see a full 240 volts. In the U.S. commercial electrical service runs at 208 volts, and often sags below that. Where a 240 volt 30 amp supply delivers 7.2 kiloWatts to the car, at 208 volts it’s only 6.240 kiloWatts. The lower voltage lowers the effective charging rate.

The actual determiner of range gained, is the kiloWatts delivered to the car. The typical equipment is set up to deliver at most 30 amps to the car, with the typical on-board charger in the affordable electric cars supporting up to 6.6 kiloWatts. The actual charge rate, in kiloWatts, depends on the voltage at the station at that moment, the capacity of the station, and the capacity of the on-board charger. Upon plugging the car into the charging station, the charging rate is negotiated between car and charging station.


Range Confidence is Copyright © 2016-17 by David Herron

Understanding charging rates and effective trip speed

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