There are several types of numbers:
1. Those related to the fuel tank size, or capacity:
in an ICE, that is the 12 gallon tank. For an EV, that
is the battery capacity, or its usable capacity, usually
in Amp-Hours (a measure of the number of electrons
the battery will hold) or kilowatt-hours (a measure
of the energy that the battery can hold). In all cases,
it is the usable capacity that is of more interest
than the total capacity. For example, a half-gallon
sludge sump at the bottom of a gas tank would contain
gas (dirty) but the car might be designed to not use
that gas, to better protect the car.
2. Those related to the fullness of the tank, like the
typical gas gauge, usually from full (or "1") to empty
(or "0"). Often the gauge lies, trying to save us from
ourselves, having a below-empty, sometimes "hidden",
usable gas reserve. In the EV, this is often displayed as
the tank-fullness bars, or an SOC percentage, or the
unidentified percentage shown by the 2013 LEAF.
Usually the story stops here, but some EVs, like the LEAF,
have a variable size (capacity) fuel tank. An analogy in the
ICE would be a rubber tank that shrinks substantially
with use, with temperature, and with age. Then, knowing
that the tank is full does not tell us how much usable
fuel there is on board. If the tank shrunk to 50% of the
original "new" capacity, the "full" indication might mean
6 gallons instead of the original 12 gallons.
3. So, a third class of fuel-related numbers represent
the actual amount of fuel remaining in the tank. Since
the car runs on energy, measuring that fuel in energy
units, like watt-hours, or kilowatt-hours, or "therms"
is usually the most helpful. However, some "volume"
measurements are also used, like amp-hours, which
indicated the volume of electrons, but not their energy.
The ICE analogy is that applies is that the
gallon represents a volume of fuel, but not the
usable energy content. "Gas" might contain a
substantial amount of filler liquid, alcohol,
synthetic fuel, etc. that might have noticeably
less energy per gallon. When using such mixtures,
we get fewer miles on a tank. However, we mostly
ignore that aspect of the fuel and car performance.
Without knowing what number you are dealing with
makes the number difficult to compare with anything
else. The units are very important. For example,
apply 12 to a baby. What does it tell you?
12 pounds, 12 days, 12 fingers, 12 teeth, 12 weeks,...
you get the idea.
4. Last, is the estimated range, always based on
a guess of the future. You can guess your own
driving future better than the car can. But, a well
done Guess-o-Meter (GOM) could tell you something
useful: "If you keep driving just like you are driving
now, under the same conditions, you might expect
to achieve N more miles. " However, very few GOMs
do that. Most average your past driving over several
to many minutes or miles, and use that performance
as the basis of their GUESS.
Real life analogy: Someone tells you that the store
you are looking for is 2 km further down the street.
We generally take that to mean some estimate of
distance more than a quarter of a mile, and less
than 5 miles, not 2000.0 meters.
Hopefully this helps.
Happy November to all, Ho, Ho, Ho.
Gary
