September 2012 Archives

Quiet Vehicles and Pedestrians

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I've been driving electric vehicles since 2008, logging over 50,000 miles, and have never had an experience where my vehicle's lack of engine noise created an unsafe situation. However, the issue of quiet vehicles and pedestrians is subtle and complex.

I have had a pedestrian walk backwards through the traffic lane in a parking lot while carrying on a conversation with someone across the lot. She didn't notice my vehicle, but I was watching where I was driving and going slow enough to react to her carelessness. I just stopped and waited about 30 seconds for her to see me. She of course made a rude comment blaming me for the unsafe situation she caused.

I've also had many times when driving through a parking garage where pedestrians are walking up the middle of the traffic lane, totally oblivious to my presence. However I find that happens with about the same frequency it did when I was driving gas cars, which I attribute to the echoing in concrete garages making it hard to hear slow-moving vehicles from behind, even when they are close.

I've also had the experience of being in a shopping center parking lot, hearing a car drive up behind me, and before turning around knowing it was my wife, Cathy, because I recognized the distinctive sound of a Tesla Roadster. On the flip side, Cathy has noticed not being able to hear a nearby gas car in a parking lot because of the noise made by a much louder gas car an aisle or two over.

As a responsible driver, I don't depend on pedestrians hearing the roar of my engine so they can scramble out of my way before I mow them over. But the situation is more complex than just my personal experience.

In May of 2011, I participated in a meeting of the United Nations working group that is developing a proposal for an international standard for quiet vehicles. There I learned a great deal about the subject and was able to share my insights as an experienced electric vehicle driver.

The predominant sound made by cars moving above 15 to 20 miles per hour is tire noise. At slower speeds, it's engine idling, fans, and so forth. It's those lower speeds that are of concern.

Hybrid and electric vehicles aren't the only quiet vehicles on the road. Many modern sedans are also virtually silent at low speeds where tire noise is not significant. Therefore, the UN is taking a broader approach to this problem than the US Pedestrian Safety Enhancement Act of 2010, which only considers minimum noise levels for electric and hybrid vehicles.

The sound made by gas cars is actually quite poor for alerting pedestrians to nearby vehicle traffic. Most of the sound made by internal combustion vehicles is low-frequency, which humans have difficulty locating, and carries for long distances, adding to ambient noise levels that can mask out nearby vehicles.

For an artificial car sound to be effective, it has to be localizable and distinguishable as coming from a vehicle. So having an EV rumble like a muscle car or chirp like a bird is a terrible idea. Studies presented at the UN workgroup meeting show that the best sounds are broad spectrum sounds without low frequency content.

The issue is even more complex for blind pedestrians who develop skills in using their senses in ways that are completely outside the experience of the sighted public. The idling sounds made by stationary vehicles are useful not only for detecting the presence of nearby cars, but also for using them as positional markers. Consider walking across a wide, busy street and trying to stay in the crosswalk with your eyes closed. The sound of the idling cars nearest the crosswalk act as navigational beacons, keeping blind pedestrians from drifting out of the crosswalk and into traffic. For this reason, it's important to be able to hear a car in the street even if it is not moving.

It's also important to be able to judge the size of vehicles by their sound. Drivers behind a large, stopped vehicle can get impatient at the hold-up and decide to blast around the unwanted obstruction, only to find that there was a good reason for the large vehicle to be stopped: pedestrians in a crosswalk. For this reason, blind pedestrians may choose to avoid this risk by choosing not to cross when they hear a large vehicle at the head of the line.

As an EV driver, I appreciate the quiet ride of electric cars. The last thing I want is some obnoxious artificial sound added to my car. From what I learned at the working group meeting, a properly designed sound doesn't have to be overly loud, it can be effective even at a sound level that is below the ambient noise level. I believe we can add sound to quiet cars to increase pedestrian safety without compromising the advantage of electric cars in improved driving experience and reduced noise pollution, but doing so demands careful thought and consideration of many complex issues.

Quick Chargers: Ignore The Charge Percent!

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Electric vehicle drivers are excited to see the first DC Quick Charge stations coming online. Oregon and Washington have done their part to power up the West Coast Electric Highway allowing electric vehicle drivers to travel I-5 from the Canadian border to the Oregon-California border and take advantage of stations that can charge a Nissan LEAF or Mitsubishi iMiEV from empty to 80% in about half an hour. This greatly increases the usable range of electric vehicles for longer trips and also provides a safety net for rare situations when drivers unexpectedly need more than their normal overnight charge.

Unfortunately, there's a problem that is causing a lot of confusion that can result in a driver getting less charge than needed. Even though the stations are working properly, drivers may think something went wrong because of a user interface issue.

AV-DCQC-Screen.pngThe above is the screen from an AeroVironment DC Quick Charge station in Tumwater, WA, as shown while charging our Nissan LEAF in June. The screen shows the driver two pieces of information: the amount of energy delivered to the car and a charge percent.

The problem is the displayed charge percent: it is not the car's state of charge (SOC) and should not be treated as such by a driver to decide when to end the charge.

It's pretty well known that it's difficult to determine the exact SOC of a car's battery. Even the best estimate of the battery's SOC may be off by a few percent. That's not what's going on here. The SOC value reported to the station is completely artificial and differs significantly from the car's estimate of the true SOC.

In addition to showing the invalid SOC value to the driver, Blink quick charge stations also require the user to choose a station-controlled charge limit. This has two big problems. First, the LEAF wants to control the charge and will stop the charge at either 80% or near 100% based on the battery state at the start of the charge, so even if you choose 100% on the station the LEAF will terminate the charge at 80% if the car was at 50% or less when the charge started. Second, the Blink station doesn't know the real state of charge and therefore cannot know when to stop charging at the point it says it will.

Here's an example. I recently used the Blink quick charge station at Harvard Market in Seattle, WA. I arrived with just over a half charge remaining, which means the LEAF will allow me to do a full quick charge up to near full capacity. After plugging in the car, the screen on the Blink station gave me a choice of charge levels, defaulting to 80%, which was the highest level shown. I had to press a "more options" button to be able to choose a 100% charge. The graph below shows data collected from the resulting 52-minute charge, comparing the car's actual SOC with the SOC shown on the station's screen.

Blink_50_to_100_Graph.pngAs you can see, not only is the reported SOC higher than the actual SOC, the reported SOC rises more quickly, increasing the gap as the charge progresses. Throughout the entire charge, the SOC shown on the station consistently overstates the actual charge level and the problem gets worse later in the charge period. As the car gets to about 80% actual SOC, the reported SOC jumps up to plateau near 100% and just sits there for the remainder of the charge, even though the car is far from fully charged.

Had I left the default 80% setting, the charge would have stopped when the reported SOC hit 80%, but the car was really only at 73% at that time. A requested 90% charge would have stopped around 80% actual.

Any driver who sees this behavior and doesn't know that the charge percent value on the station is not the SOC would see it jump up to 97%, perhaps watch it sit there for a few minutes, and likely decide that it would be a waste of time to spend any longer waiting for that last 3%. If the driver ends the charge at that point, the car will be missing perhaps 10% of the potential charge. If that last 10% is needed to finish the journey, this could result in a very unhappy EV driver.

It's not clear where this value comes from, but displaying this invalid SOC on the quick charge stations has created user interface problem with unfortunate consequences for LEAF owners, and perhaps iMiEV owners as well.

So to any EV driver using a CHAdeMO quick charge station that shows an SOC percentage:

1. Ignore what the station shows. Put a sticky over it if you have to. Only look at the car's representation of the SOC.

2. If the station offers you different charge levels, choose 100% charge so that you get the car's best available charge level. If you want to stop the charge early for some reason, do it based on the SOC shown by the car.

I'll contact the quick charge station manufacturers to make sure they are aware of this problem. In the meantime, please help spread the word so EV drivers can get the maximum benefit from these highly valued stations.

For charts of two AeroVironment quick charge sessions, see Cathy Saxton's report. More tips for using quick charge stations are available on our Avoiding Quick Charging Pitfalls page.

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This page is an archive of entries from September 2012 listed from newest to oldest.

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