Saturday, May 23, 2015

Measuring Battery Degradation

One thing that I "miss" about driving my gasoline powered car is being able to calculate the fuel efficiency for any fill-up.  The math was straight-forward:  divide the miles driven by the fuel used to top off the tank and presto!  MPG.  Home EV chargers don't typically display the amount of electricity consumed during a charge-up cycle, so it's nearly impossible to compute the energy efficiency accurately.  There were a number of reasons for doing this with the gas car.  Most importantly was to understand how my driving habits impacted my fuel economy.  Secondarily, by watching for a drop in fuel economy I could detect problems in the engine that might otherwise have gone unnoticed.  A misfiring spark plug, a problem with the emissions equipment, or low (or burning) oil could all have a small impact on fuel economy (though newer cars have sensors for these problem areas).  So, why would I want to do this with an electric car?

Anyone who has used the same cellular phone for more than two years has noticed that the "talk time" starts to drop off after a few years.  Suddenly, sending a few text messages drains the battery faster than a phone call, and forget about doing anything beyond an e-mail scan using the internet, because the power will drain quickly.  Electric cars use a very similar battery technology, so it is logical to expect that the driving range of an electric car will start to drop off after a few years.  The manufacturers even admit to this storage decay in their warranty for the car's battery.  In my case, the warranty assures me that 80% of the original charge capability will be available after ten years.  So, how do you know if your battery charge capacity is dropping?
The charge gauge on my Mitsubishi i-MiEV is informative but lacks precision

To check your battery's capability, you have two options.  For those who are math averse, you can drive the car until the battery is depleted (or indicates about one mile of range remaining).  The problems with this approach are that it tends to leave you stranded somewhere, and completely draining the battery hastens its end-of-life, making the problem worse.  Enter mathematics ...  Instead of looking at the range, look at the distance traveled per kilowatt hour.  To measure this, you have to get an accurate reading of how much electricity was used to top off the battery, and how many miles were driven.  Then just track the miles-per-kilowatt-hour (or MPkWh) over time.  This way you don't negatively impact the battery's longevity (or strand yourself) in order to measure its performance.

It would seem that driving to work during Spring Break and March Madness on a Friday is the ideal time to find a vacant charger spot at work.  When I arrived at work, I had already driven an extra 16 miles from the day prior, so the range I could drive after work was already reduced.  Checking the ChargePoint network app, I found an empty charger spot right near the entrance to my office, so I parked there and hooked up.  One nice feature that public charging stations offer is a tally of the energy used to charge your battery.  (Often times this is used to calculate your billing.)  On this day I drove 36.5 miles and needed 7.589 kWh to recharge.  This yielded an efficiency of 4.81 MPkWh.  When the car was less than a year old, I was able to measure the same way and usually got about 4.95 MPkWh, which is within 3% of the other measurement.  This suggests that I have had no appreciable battery degradation in three years, which is comforting.  (This winter I was starting to think the range had dropped off more dramatically, but I lacked any solid evidence or numbers.  I guess it was just colder weather and stronger winds.)

Monday, May 11, 2015

A little traffic, please?

When I would drive my gasoline powered cars, I would dread having to drive through congestion on the freeway.  Not only was I slowed by the traffic mess, but the car ran less efficiently, it was a little jerky on the frequent stop-starts, it was irritating listening to the engine idling or running at low speeds, and there were the exhaust fumes, from my car and the others around me.  And, in the summer, all those hot engines running seemed to make the freeway steam in the sun.  I always looked forward to the traffic break when I could again cut through the air at high speeds and the car was running near its peak efficiency.  So, why would I want to endure highway traffic congestion now?

Electric cars today lack a multi-gear transmission, which allows a gasoline engine to operate at both low and high speeds, while optimizing for the latter.  Generally, gasoline engines are rather inefficient at lower speeds, and they require a clutch mechanism in order to stop and resume, robbing the engine of its abundant power.  The electric car has just one speed, directly connected to the wheels because it can start turning from a stop.  Lacking other gears, the electric motor is less efficient at typical freeway speeds.  In short, the electric car can drive much further on the same amount of electricity at lower speeds (such as found in most highway traffic congestion).
My commute - 12 interstate miles and 7 expressway miles

Today was a case in point.  My drive to work consists of 12 miles driven at 55 to 60 MPH on the freeway, followed by 7 miles driven at 45 MPH on an expressway.  Lately, by the time I return home at the end of the day (38 miles late), the ninth (of sixteen) power bars in the charge gauge has disappeared.  Because of my work hours and meetings, I typically commute after much of the heavy traffic has subsided.  Today, however, was different.  I had to leave earlier for work and dealt with congestion along half of the freeway route to work.  I also drove home early and had to deal with a brush fire and two accidents along the way, which backed up traffic for nearly the entire stretch of the freeway I travel.  I also ran a few errands after arriving at home.  By the time that the charge gauge had dropped its ninth bar, I had driven 44 miles (or 15% farther).  So, I saved nearly 1.5KWh of charge by driving in congested traffic.

So, the efficiency is greatly improved in the slower moving traffic.  What about the other issues I face in congestion?  The interior noise in my car is dramatically reduced because there is no roaring engine (especially while idling).  Every start from a stop is delivered with silky smoothness thanks to the low-end torque available in an electric motor.  I am able to save a little time by using the carpool lane instead of being stuck with the rest of traffic.  And, of course, my car is not producing any exhaust gas fumes (and its electric charging is offset by solar power, both at home and at work).  If there were many more commuters also using electricity instead of gasoline, we’d all enjoy the drive home a lot better.