Monday, November 3, 2014

Living on the edge

Today we’ll start with an uber-nerd post about the complications of driving a Tesla cross-country.  (Not to be confused with an Uber-nerd post, which would describe how to get into ridesharing.)  Thanks to Joe for asking the following question,” Would you explain the data on the circular dial?”  Yes, I would love to explain the data.

The first screenshot below shows what the dial looks like during typical highway driving conditions (at least on our trip).  The left side of the dial is the standard speedometer that you’d find in any car.  The blue line matches to the value shown in the center of the dial, indicating that we were driving 75 mph at the time this photo was taken.  The little triangle at the far left is the cruise control setting, indicating that LJ is cruising at 75 mph as noted by “Cruise On” at the top.  If LJ clicks on the cruise control, the triangle would move to the current speed, where it would remain even if he clicks out of cruise control. 

Day 2: "Normal" speed

The right side of the dial is power, where 3 o’ clock on the dial is the zero point.  Above that point, power is going from the battery to the drivetrain; in other words, we are using electricity.  Below that point power is being regenerated from the drivetrain into the battery.  In the first photo, we were driving at the 25-30 kW typical of highway cruising speeds on a flat road.  In the second photo, LJ is accelerating pretty rapidly as shown by the orange bar up near 160 kW.  Note that it’s on a log scale.

Day 2: Acceleration
The smaller number at 6 o’ clock in the center of the dial (107 in this photo) shows the Rated Range.  This number assumes power consumption 300 Wh/mi (watt-hours per mile) as a standard rate of consumption and calculates how far you can go based on how much energy is left in the battery.  This rate is about what would be consumed at 60 mph on a flat, dry road.  Our trip average for the first day was 365 Wh/mi.  The majority of this higher consumption is due to driving faster (more like 70-75 mph rather than 60 mph) as the Tesla Model S efficiency curve starts to get pretty steep at those speeds.
However, two other factors have a pretty strong impact on the energy consumption: weather conditions and geography.  The 10-15 mph winds with gusts to 30 mph from the front that moved through yesterday definitely impacted our driving efficiency.  Given that we’re averaging 380-400 Wh/mi again today, the winds are clearly still strong.  The most important number is the effective airspeed, which means that driving at 65 mph in a 20-mph headwind is effectively the same as driving 85 mph in dead calm air.

In terms of hills and mountains, for the trip so far, we climbed about 250 feet from RDU (435 ft) to Toledo (683 feet).  While we get back some of the energy through the regen, LJ guesstimates that we lose about 5% by driving up and down hills with zero overall elevation gain as compared to a flat road.  We can get back roughly 10-15% if we draft behind an 18-wheeler or a Greyhound bus. 

The third photo shows what happens when you put power back into battery as shown by the green line below the zero point.  In this photo, the green line shows that LJ is instantaneously putting almost 60 kW back into the battery.  Any number of things contributes to this regen: going downhill, braking, decelerating, etc. 

Day 2: Regen

And this last photo shows living on the edge in the Tesla as we get close to an empty battery.  At low State of Charge (SoC), the voltage in the cell is low, which limits the power that can be pulled from the battery.  The dotted yellow line in the upper right quadrant shows that we can only access about 160 kW instead of the > 320 mph usually available.

Day 2: Living on the edge

The one photo I don’t have here is what happens when the car battery is very cold or very full.  A yellow dotted line pops up in the lower right quadrant indicating that full regeneration cannot be achieved due to the limitations of the battery chemistry when it’s very cold or very full.

That’s probably enough science nerding for now.  If any of this super-cool super-car science interests you, check out for much more in-depth debates on discussions on all things Tesla.

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