Sunday, September 14, 2014

The range gauge

When your car carries the equivalent of two or three gallons of gasoline, it is very important to have a good idea how far you can drive without recharging.  Electric car manufacturers understand this and include a driving range gauge.  In the Mitsubishi i-MiEV, the range is displayed by a multi-function gauge that shows eight different measurements, including the outdoor temperature and two different trip odometers.  When I first started driving the i-MiEV, I displayed the range gauge all the time while driving.  This gave me a pretty good idea of how it works.  The gauge reads the battery charge level, then estimates your range based upon the last 5 to 10 miles of driving.  If I finished my day with some city driving, the fully charged car will show 80 to 90 miles of range after charging the battery.  If instead I ended my driving with freeway miles, it will show only 65 to 75 miles or range.  This experience was a little unnerving to watch as the range dropped by 20 miles after having driven only 10 to 12 miles.  This rapid drop happens because the car acclimates to driving on the freeway and lowers its range estimate.  After exiting the freeway, the range stops dropping for a distance and sometimes even increases.

This ever changing impression of how far I can drive lead me to stop using the range gauge.  Instead, I rely mostly on the trip odometer, which I reset after every full charge.  I have learned (and the EPA has confirmed) that the car travels about 4 miles per “tick” (out of 16 ticks) on the charge-level gauge.  The freeway goes a little less per tick, and around town goes a little farther.  To estimate my driving range, I consider where I plan to be driving (city or freeway) and “do the math”.  If I have 9 ticks remaining on the charge, then I figure I can drive about 32 miles on the freeway to 45 miles around town.  Whenever the charge level drops below 4 ticks I switch to the range gauge.  With only 3 or 4 ticks left, I trust the car to know better how far I can drive.
My full-charge range after yesterday's freeway and city driving
So far I have not “turtled” the car or run the battery to empty.  “Turtling” happens when you drain all but a portion of the last remaining charge-level tick.  The car goes into an ultra-conservative energy management mode where the speed is limited to about 25 MPH.  This is indicated on the display using a small turtle graphic.  Because the i-MiEV allows its entire battery capacity to be depleted, turtling the car is not good for the longevity of the battery, though the occasional turtling should not be too harmful.  After about 27,000 miles, the batteries still seem to support the same driving range, so I am guessing that my driving and charging style is good for the car.  (On a similar note, I read that too frequent charging can also be hard on the battery – recharging while in the 25% to 75% charge range is ideal.)

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