Monday, February 1, 2016

Charging atop a mountain

After losing my job due to a company layoff, and while searching for work (a full-time dedication for an older engineer), I found myself missing the moral support of coworkers (and a paycheck).  So, I decided to apply for work at a winery that was only open to the public on weekends.  This would mean that I would have the option to keep the job even after I resume my career, allowing me to build my experience working in the wine-hospitality industry.  The winery is atop the mountains in Cupertino at an elevation of 2,200 feet.  (Home is a mere 100 feet above sea level.)  The winery also has a level-2 EV charger available to employees.  Having driven to the winery twice before, I knew that the EV charger would be a comfort more than a necessity for getting up and back on one charge.

My prior experience driving up the mountain suggested I would use one-half of the total charge to drive up the mountain.  I would also regenerate about 10% coasting back down the hill, allowing me more than enough power to make it home (and to the store).  So, on my third trip up the mountain for work, I learned about charging atop the mountain.  I had only driven about eight miles the day before and decided not to recharge before driving to work.  When I got to work, the power level was lower than I am usually comfortable with, so I plugged in the car (just towards the end of my shift).  By the time I had disconnected, there was just one bar remaining (out of 16) to be filled, and I knew that the remaining energy would be generated while braking on the trip down the mountain.

Apparently, the engineers at Mitsubishi had anticipated my very situation and had to prioritize the survival of the charging system.  Once the car’s battery is nearly full, most EV chargers will send less current to the car (as observed using the ChargePoint web site dashboard).  It would seem that this is at the request of the car, not the charging station.  While driving downhill, this time I noticed that the amount of regeneration happening was a fraction of its usual potential.  It dawned on me then that the same battery-protection logic had kicked in and the battery was accepting less charge from the brakes.  That meant that I had to work harder using the friction brakes to slow down the vehicle.  As the charge level approached full-charge, the regeneration brakes were almost completely ineffective and I was relying almost entirely on the friction brakes.  While this is only a minor problem, the biggest concern is having to take the turns more slowly because the friction brakes act on all four wheels, while the regeneration affects only the rear wheels, allowing the front wheels to dedicate all friction to traction in turns.  (Only the Tesla and BMW share this rear-wheel drive configuration with my Mitsubishi – all others are front-wheel drive.)   

So, what have I learned from this experience?  I have to time my charging carefully when atop the mountain.  I estimate the time needed to attain a full charge (say about four hours), then I deduct an hour from the charge time and start a timer on my phone.  When the timer’s alarm goes off, I dash out quickly to unplug the car.  This leaves me with just enough empty battery capacity to run the regenerative brakes nearly all the way down the mountain.  (In the winter months, I soon learned that I could drive down the mountain in toasty comfort by channeling excess power generated by the brakes into the cabin heater.)

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