Saturday, February 13, 2016

Gone with the wind

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.

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