Monday, October 25, 2010

EV Range Isn't set in Stone

Electric cars like the MINI-E are often gauged by how far they can go on a single charge. It seems that most of the auto manufacturers are focusing on 100 miles per charge as the minimum acceptable single charge range. The MINI-E has been tested to get 156 miles on a single charge using the EPA LA-4 cycle, but this doesn't mean much to those of us driving these cars because we have come to realize the real-world driving range is around 100 miles. Some get more, some less, it all depends on your driving habits.

The slower you drive, the further you can go. That is the simplest way to explain the how your range can vary. This isn't much different from a gasoline powered car. The slower you drive, the better your mpg.  My range varies from day to day, depending on how aggressive I drive. The MINI-E is a lot of fun to drive. The instant torque of the electric drive and great handling that all MINI Coopers have make it easy to want to drive the car spiritedly. Since I record all the data from my daily driving I can clearly see the difference when I drive normally, aggressively and economically. The other day I forgot that I was going to need to drive about 115 miles before I could plug in and was driving pretty fast on the highway, around 75 mph for a while. When I was halfway through my day I realized that I was already down to about 40% SOC and now needed to be very efficient for the rest of my journey.

When I was about 85 miles into the day, my estimated range was at only 10 miles and I still was 32 miles from plugging in. I now needed to be very efficient. I had two choices, get off the highway and drive a route that I knew. This was all secondary roads and I could drive 35mph or so and I would be able to squeeze out the 32 miles for sure. The other choice was much more risky: Stay on the route 80, find a tractor trailer and hypermile by drafting behind the 18 wheeler. Being the risk taker that I am, I decided to go the dangerous route. It didn't take me long to hook up with a tractor trailer, pull up right behind him and begin draft.

This is very dangerous and I don't advocate doing this. You need to be very alert and ready to brake immediately if the truck does. This hyper-miling technique is commonly used by people in gasoline powered cars and hybrids that try to get the highest mpg that they can. They even have competitions on who can go the furthest using the lease amount of gas, so this isn't an "electric car thing". It works because by drafting behind the large truck, your car doesn't have to use much energy to push the air away from its path. This air resistance is the main reason why going faster reduces efficiency because the faster you go, the wind resistance increases exponentially. As I started moving along behind the truck I could see the range indicator actually go up for a while. A few miles and I now had 15 miles of range remaining, up from 10. I followed the truck the entire time I was on route 80 which was about 28 miles. When I got off the highway I still had 6 miles of estimated range. Therefore, I drove 28 miles and only lost 4 miles of range!

When I arrived at work I had driven 117 total miles and had 0 miles of estimated range left. Had I driven the whole day more efficiently, I'm sure I could have done the 117 miles and had 5 -10 miles of range remaining without having the need to dangerously draft behind a truck. I want you to know I do not regularly do this, I don't find it necessary and I really wouldn't want to take the risk of an accident. This was as much a test for the car as it was a product of me having the inexplicable need to live dangerously every now and then. I don't want anyone getting the impression that this would be necessary if you had an electric car because it in no way is. In fact most of the people that do this drive gasoline powered cars. If you look around while your driving on a major highway it won't take you too long to see a car following a big rig a little closer than you would normally do, they are probably doing it consciously to increase their fuel efficiency. 

I've talked to a lot of electric car drivers and many of them have their own way to extend the range. Some hyper-mile, some put the car in neutral and coast downhill, others have ways to use the regenerative braking more than others, and some practice all kinds of hyper-miling techniques. However we all agree the simplest way to go as far as possible whatever EV you own is to just drive a little slower.


  1. Interesting post Tom, you forgot to mention you can probably get 20 more miles after the MINI E reaches 0.
    There are a number of companies working on systems that allow you to follow anouther car very closely in automatic mode, where only the first car in the line is being driven, that will help fuel economy and driver fatigue.
    Personally, I being tailgated and always follow the 2 second rule.

  2. Hi Robert,

    Yeah, that was my insurance policy just in case my drafting plan didn't work out!
    I have read about these systems that allow cars to form a train-like convoy on freeways, interesting stuff. I don't like tailgaters either, but it's a little different with a big rig as they can't see you and don't even know you are there so it doesn't bother them. However the fact that they can't see you makes doing it even more dangerous. Like I said, I don't make a habit out of this, I was just seeking some thrills and some info for a blog post!

    On another note, email me directly when you have the time, I have some Chevy Volt information that you will probably be interested in.

  3. I used to do this all the time in my geo metro. I could get an additional 5-10mpg just by drafting as you describe. You do have to worry about the truck throwing up a stone though, I have had friends get cracked windshields from this