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

By David Herron

What charging rate do we need? “As fast as possible” isn’t always what’s needed

You might think we always need as high a charging rate as possible. Gasoline car drivers enjoy a 5 minute recharge time, so why shouldn't electric car drivers enjoy the same? The fastest-possible recharge time is sometimes not what we honestly require, and the expense of high powered charging equipment precludes it being installed in every garage. 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 the fastest charge possible is needed, but not always.

Consider the long-term parking lot at your nearby airport. Each car will be parked for several days at a stretch. With a 6 kiloWatt charging station, car will be charged within 3-4 hours, meaning the charging station will be blocked for several days because the cars owner is away on a trip. Major FAIL that breaks etiquette recommendations. Therefore a bank of 6 kiloWatt charging stations is not required for long-term airport parking.

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

This chart is derived from one presented at the Plug-In 2010 conference, by Mr. Anagawa of TEPCO. He presented results based on Japan's experience with fast charging installation in Japan. The CHAdeMO fast charge protocol was deployed there in 2008, and immediately there was a major increase in electric vehicle utilization as the drivers discovered the fast charging system gave them greater autonomy.

Derived from that research he developed these four quadrants:

The chart asks you to think of the acceptable “waiting time” in one scenario or another. A long wait 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 airport long-term parking lot charging station just mentioned, will tend to see cars connected for several days at a time, and need to serve thousands of cars. Expensive charging equipment at each stall won't be economical, and the cars don't need a long charging session. By contrast, a fast food restaurant wants its customers to come and go pretty quickly, because they thrive on high customer turnover. Therefore fast charging is required at fast food restaurants.

Slow home charging is acceptable because you’re typically parked for 10+ hours at a time. Charging equipment cost is important, because who doesn't want to save a buck if they can? In practice many people do fine at home with a 120 volt outlet and the line cord charger that came with the car.

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.

Does workplace charging truly need Level 2 charging speed? An 8 hour workday can put back over 80 miles of range at a 3 kiloWatt charging rate, and 35 miles at a 1.5 kiloWatt charging rate. That's enough to handle most commutes. Further, the cost for lower power charging equipment means the employer can afford to install more charging outlets. The employer could install stations supporting different charging levels, for those times an employee can't wait for a slow charging rate. The instructions could go out for PHEV owners to use the low power charging stations, leaving the higher power stations for BEV owners.

Stores will want to install a mix of AC Level 2 and DC Fast Charging stations. Some of their customers will need the fast charge, while others will be satisfied with Level 2. The stores where customers naturally spend many hours should limit fast charging stations, and instead use more Level 2 stations.

Electric vehicle charging scenarios goes over the typical usage patterns. An electric car owner can base their driving range on their home charging station, or base it on the trip between their home and office charging stations, or using charging stations scattered around town for opportunity charging. Additionally fast chargers can be located in critical spots to to support longer distance driving.

Understanding charging rates and effective trip speed, went over the effective trip speed for each charging rate. 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. Therefore it’s best to place the best charging station for a given location based on the typical needs in that location.

Suitable recharge times

First, the Electric Vehicle Charging And Usage Scenarios gives us some typical locations for recharging and the typical time expectation at each:-

  • 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
  • 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 charging rate do we need? “As fast as possible” isn’t always what’s needed

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