Saturday, February 13, 2016
Walking into the wind takes a little extra effort than walking in still air. Walking into a strong wind takes a lot of extra effort. Bicycling into a strong headwind really slows you down, and that’s only at a speed of about 15 to 20 miles-per-hour. So, you can imagine my surprise at how quickly the power depletes from my car’s battery while driving down the freeway into a strong headwind. At first I thought that maybe the tires were a little low and in need of a good pumping up. Then I looked at the tree tops. (The palms are a dead giveaway.) The top branches were swaying in the wind, suggesting a strong wind was working against me.
How big is the difference in energy consumption driving into the wind? Normally, my drive from home to the exit I would usually take for work (and to visit mom, and to drive up to the winery) uses about three ticks of energy on the gauge. Driving into a strong wind, I noticed that the energy used at any moment to maintain speed was slightly higher, but shortly after taking my exit, I noticed the forth tick drop off. I cover a distance of about 13 miles, suggesting that the extra wind load uses about 20% more energy. Considering that the battery performs a little worse in colder weather (such as typically happens with a strong wind), I would attribute about 10% to 15% of the extra 20% power drain to the wind itself.
The impact of this extra power drain is more significant than it seems, especially for freeway trips. Under better conditions, my now four-year-old battery can travel about 58 to 62 miles on a charge, mostly on the freeway. If I am driving into a strong headwind, that range drops to 49 to 56 miles. Even when I was working in Palo Alto, the 49 miles was enough to get me to work and back, and a quick trip for tacos nearby for dinner. But, if I had planned to visit the wine store (an “errand”) in Redwood City during lunch, then that would add another 12 miles to my trip, meaning that I would be at risk of not making it home from work on a windy day. When the battery was newer, the range was closer to 70 miles, and even a 15% drop in efficiency would still get me to the wine shop and home.
Monday, February 1, 2016
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.)