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

By David Herron

What electric car charging rate do we need at home, at the office, on road trips, at airports, or elsewhere?

Since gasoline car drivers enjoy a 5 minute recharging time, many think electric car drivers should be demanding the same. But the "fastest possible" recharge time isn't always what we need. That five minute recharge time will require a power level - several hundred kiloWatts - that can only be supplied at a commercial electric car charging station, for a significant fee. Charging at home at a more sedate 1 kiloWatt or 3 kiloWatts or 6 kiloWatts rate can save the electric car owner a ton of money on fuel costs.

A rational analysis of our real needs shows the optimum charging rate depends on the situation at any given time.

Generally speaking, at home we want the least expensive charging equipment possible and we can afford to wait several hours for a recharge. It's when we're away from home that higher speed charging is needed, but not always.

Consider these scenarios:

  • At office You'll be at work for 8 hours (in a typical job), and 8 hours at a 3 kiloWatts will recharge over 80 miles of driving range. 80 miles of range should be enough to return home.
  • Shopping center The person running around town often needs to quickly top up the car while zipping in and out of stores, unless they're instead lingering inside a shopping mall for a few hours. In the first case a fast charge is desired to quickly top up the car, but if the person is lingering in the shopping center for hours then their car should be connected to a normal speed charging station.
  • At home Typically we're at home for 8 or 10 or 12 hours a day. Typical daily driving, vehicle miles traveled, is 40 miles. Recharging 40 miles of range is easy with low power charging equipment. Recharging at home gives the lowest cost of electricity, and therefore gives you the lowest fuel cost of all recharging scenarios.
  • Road trip Here is where fast charging is an absolute requirement, because the effective trip speed of a long trip is dominated by the refueling time.
  • At Airport long-term parking Those using long term parking have taken an airplane somewhere and are likely to be gone for a few days or even a few weeks. With a 6 kiloWatt charging station the car will be fully recharged within a few hours - making it really convenient when you return from the trip, but it means that for most of your trip your car has been hogging a charging station. This is a Major FAIL that breaks etiquette recommendations. Airports should focus on either low power charging, or fast charging.

In other words, fast charging isn't always required. Excruciatingly slow charging is perfect for some cases.

The following chart demonstrates these scenarios, and is derived from one presented at the Plug-In 2010 conference, by Mr. Anagawa of TEPCO (Tokyo Electric Power Company). Japan was the first country to deploy electric car fast charging stations, the CHAdeMO network first deployed in 2009. The results were derived from that research.

The vertical axis is charging speed - Fast Charging at the top, Slow Charging at the bottom.

The horizontal axis is location, ranging from private charging at the left to public charging at the right.

The chart asks you to think of the acceptable "waiting time" to recharge an electric car in each scenario. A long wait to recharge is suitable in some cases/scenarios, and is unacceptable in other cases/scenarios. That's because each location lends itself to particular needs, and not to others.

The question "How long does it take to charge an electric car?" leads us to think we must always have fast charging. If we change the question to "What is the best type of electric car charging at each location?" we can start to see the value of the different types of charging equipment.

The chart explains why.

Optimum charging rate for home or the office (workplace)

At home or at the workplace "waiting time" is long, and therefore a long charging time is acceptable.

At home, low power charging equipment gives the benefit of a low cost for electricity at home, and the low power charging equipment is less expensive to buy and install. Because you're typically parked at home for 10+ hours, even a slow charging rate will be enough for the next days driving needs. In practice many people do fine at home with a 120 volt outlet and the line cord charger that came with the car. Bumping up to the 3 kiloWatt charging rate will satisfy most of the need for speed, at a low cost.

At the office we have a similar consideration. We'll be parked at the office for several hours, possibly punctuated by a drive to a nearby restaurant for lunch. Even with low power charging equipment, the car will recharge enough to handle driving home.

In both cases low powered charging equipment has three advantages:

  • Reduces the cost of the electrical service to power the charging station. The 40 amp circuit to handle a 6 kiloWatt charging rate costs more than the 20 amp circuit to handle a 16 amp charging rate.
  • Using 20 amp circuits for charging stations means more total charging stations for a given service panel.
  • Lower cost for the charging station.

A workplace might find it necessary to host a few fast charging stations. Occasionally employees will have a sudden emergency - spouse suddenly in a hospital 100+ miles away - and need to quickly refuel to get there. Or the workplace might have delivery trucks where fast charging means they can be on the road for more hours.

Optimum charging rate for public charging - but not road trips

When we're out and about the acceptable "waiting time" is often short, and therefore we need a shorter charging time. But that's not always the case, since some of our public locations involve a long waiting time.

Generally speaking fast charging is preferable for public charging because we just don’t want to wait very long. There are many “running errands” scenarios that don’t allow for a 3.5 hour delay to charge the car. We have things to do, places to go, people to see, and ain’t nobody got time to wait for charging to finish.

A fast food restaurant, a coffee shop, and similar places obviously need fast charging. The waiting time is short at these locations, because those businesses want people to come and go quickly.

Some stores are designed for a longer waiting time. In fact, store owners often want their customers to stay longer so they'll spend more money at the store. Such a store may prefer AC Level 2 stations rather than fast charging.

Optimum charging rate for road trips

The road trip is a different animal, because we're looking to cover as many miles as possible. Therefore "waiting time" must be kept low, and therefore fast charging is absolutely required. There is talk of 350 kiloWatt charging in the not too distant future, which would mean a 5-10 minute recharge time, which will be a boon to electric car road tripping.

In "Understanding charging rates and effective trip speed", we went over calculating the effective trip speed for each charging rate. The higher the charging rate the faster the effective trip speed because we'll be spending less time at the charging station. Basically, a 6 kW charging station provides 20-25 miles range per hour of charging, a 50 kW DC fast charging station provides provides about 120-130 miles range per hour of charging, and a 120 kW Tesla Supercharger station provides about 300 miles range per hour of charging.

Suitable waiting times for various scenarios

  • Home base – 10 hours or more overnight
  • Office – 8 hour work day
  • Church – 3 hours
  • Shopping – 2-3 hours (or less)
  • Visits to doctors and the like – 1 hour
  • Hospital visit - 1 hour up to several days
  • Movie theater, Opera, etc - 2-3 hours
  • Parking garages – depends on the trip, could be all day or several days if at an airport
  • Rest areas along highways – quick, fast, up to an hour for a meal

Range Confidence is Copyright © 2016-17 by David Herron

What electric car charging rate do we need at home, at the office, on road trips, at airports, or elsewhere?

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