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.
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 teslamotorsclub.com for much more in-depth debates
on discussions on all things Tesla.
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