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Plug In America’s Tesla Roadster Battery Study

by Tom Saxton
, Chief Science Office
r
Released
:
July 13, 2013
Introduction
As the first full

production all

electric vehicle of the modern era, the Tesla Roadster
represents a unique opportunity to study batter
y pack longevity. Deliveries of the vehicles
began
in 2008
with a total of 2,500 Roadster
s
produced through
the
end of production in
January, 2012
1
.
As of July 4, 2013, Tesla Motors reports that “2
,
100+ Roadsters” have
been driven over 35 million miles
2
.
F
or this study, Tesla Roadster owners were contacted via social media including the
Tesla Motors Club and various other online groups and asked to submit data for their
vehicles.
As of
July 5, 2013
, 126
vehicles totaling 3,1
98,749
miles have contributed to
the survey, amounting to approximately 5% of the user base and nearly 10% of the miles
driven as reported by Tesla Mo
tors. In addition, data from 106
Roadsters was collected
anonymously through the Open Vehicle Monitoring System
3
(OVMS), with considerable
overlap expected between the two data sets.
In 2006, Tesla Motors projected
4
that the Roadster battery pack would have
more than
100,000 miles of driving range and more than 5 years of useful life. Over this period, the
capacity of the battery pack would b
e expected to decline. As an example, a Roadster
driven 10,000 miles per year for 5 years would be expected to have 70% of the original
battery pack capacity.
The purpose of this study is to determine how the battery packs are holding up and how
real world
performance compares to the expectations set by Tesla Motors in the
timeframe when the Roadster was first announced and described in 2006.
Summary
of Results
Of the factors considered

miles d
riven, vehicle age, and climate

only mileage showed
a signif
icant correlation with battery pack capacity.
D
ata collected
suggests that, on average, a Roadster battery pack will have between 80%
and 85% of original capacity after 100,000 miles.
1
Tesla Motors, Inc, Form 10

Q for the quarter
ended September 30, 2012, filed with the
SEC on November 7, 2012.
2
http://www.teslamotors.com/enthusiasts/millions

of

miles
3
https://www.openvehicles.com/
4
Martin Eberhard,
JB Straubel,
“A Bit About Batteries” November 30, 2006
http://www.teslamotors.c
om/blog/bit

about

batteries
Plug In America’s Tesla R
oadster
Battery
Study
page
2
Unlike results from the Plug In America LEAF Battery Survey
5
, no signifi
cant
correlation
was found
between climate and battery longevity.
There is significant variation in battery capacity reported
;
the difference in capacity
be
tween vehicles with similar mileage
can be as large as the projected loss over 100,000
miles of use
.
Individual owners should therefore expect variation between their
experience and the projected average performance.
Because of the variation in battery pack longevity experienced by owners, especially
where such variation may be due to factors beyond the
owners’ control, it would seem
desirable for the manufacturers of electric vehicles to guarantee not only the life of the
battery pack, but also the capacity performance over time and miles. Nissan Motors
responded to the climate
issues reported by LEAF ow
ners (
and confirm
ed by the Plug In
America study)
by amending their battery warranty to cover capacity. With the
performance of the Roadster battery packs exceeding early expectations, it’s curious that
Tesla Motors doesn’t offer any capacity warranty, eve
n on the 85 kWh Model S
,
which
has a warranty good for 8 years and unlimited miles.
Methodology
Sample Distribution
The survey was promoted through various social media services including the
Tesla
Motors Club
forum (
TMC
), various
online Tesla
owner groups
in the United States and
internationally
, an
d Plug In America’s newsletter. Owners who participated are self

selected and may be biased toward better (or worse) outcomes.
Survey participants
reported their location and information about their driving and
charging habits.
The OVMS data was collected anonymously from
106
owners who have installed
the
OVMS device and chosen not to encrypt their data. These owners represent a subset of
owners interested in remote monitoring their vehicles and are thus perhaps
more
technically savvy, which might bias the data set in other ways.
Measuring Battery Capacity
Full Charge Energy in Ideal Miles
Tesla Motors’ state

of

charge instrumentation is the most precise and objective of any
mass

production electric vehicle in the
modern era, providing the owner with the option
to view the car’s estimate of the battery pack’s current stored energy in an energy unit
referred to as an ideal range mile, or more briefly as an ideal mile (IM)
6
. This unit is both
fairly high resolution a
nd easily comparable between vehicles, drivers, and environments.
Other production electric vehicles
on the road today report state of
charge
in
coarse
charge bars
relative to the pack’s constantly varying estimated
capaci
ty
and/or estimated
5
Tom Saxton, “Plug In America’s LEAF Battery Survey”
http://www.pluginamerica.org/surveys/batteries/leaf/Leaf

Battery

Survey.pdf
6
Owners may also choose to see state of charge in ideal kilometers.
Plug In America’s Tesla R
oadster
Battery
Study
page
3
miles of range
based
on recent driving. Both of these
values
depend on many factors and
thus cannot be compared across time
,
between vehicles
,
or used to measure capacity
variation.
By providing state

of

charge information
in absolute energy units
, Tesla Motors enables
owners to better understand both how much charge the pack has at any moment and also
how their battery pack capacity varies over time. This also
facilitates more meaningful
results from
studies such as this.
To measure battery capacity, the owner
just need
s to observe the state of
charge in ideal
miles at the end of a charge. Several factors make this more complex than it sounds.
The Tesla Roadster has two primary charge modes. Range mode charges the car to full
usa
ble capacity and displays state of
charge
relative to the lowest charge level allowed.
This mode
allows
maximum
charge and
discharge
,
and thus maximum driving range. In
typical use, the full driving range of the battery pack is rarely needed, so to increase the
health and longevity of the battery
pack, a second mode is provided:
Standard mode
charges to about 90% of the pack’s usable level and
also hides the bottom 10%. C
harging
and driving within Standard mode protects the battery from both high charge and
discharge levels.
Many owners use only
the default Standard mode and some don’t fully understand Range
mode, making collecting data on the full capacity of the battery pack challenging for
those owners.
Further complicating matters are technical issues making it difficul
t to accurately
determin
e state of
charge while the battery pack is in use, either while driving or
charging. After a charge or drive session, the Roadster’s firmware will conduct additional
measurements on the battery pack
and provide a more accurate estimate of charge energy
st
ored
, updating the estimate
after about 10 minutes
.
This update can be significant,
perhaps a few ideal miles in typical situations, but over 20 ideal miles in some unusual
circumstances.
After that update, the car continues to use
power
, which depletes th
e
stored energy
ov
er
time. In most circumstances
after a Standard mode charge, this energy use
is small and
the reported state of
charge generally remains unchanged for 10 to 30 hours. However,
after a Range mode charge, the car’
s coolant pump continues to
run, using
considerable
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