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.)