One of the things I like to do every so often is look at various
commercial offerings and announcements and see what they imply for
certain trends. Something I haven't looked at in a while (and not
blogged about here) is advances in battery technology and the
implications for the best immediate prospect for slashing oil
consumption: plug-in hybrids.
Batteries have been the weak link of electric vehicles for well over
a century, so any development is of great interest. One bit of
recent news was very exciting:
announced a new anode material for lithium-ion
(Li-ion) batteries which triples their current capacity and drastically
shortens the necessary charging time. The implications for EV and
HEV use is obvious: more power and better regenerative braking
from the same battery pack.
According to certain authorities, the average commuter travels 22
miles a day or less; this means that a car which can travel 22 miles on
electric power alone can eliminate these people's fuel requirements for
work travel, and cut total fuel needs by as much as 80%. Longer
all-electric range translates to less fuel use.
According to EPRI
, a compact electric vehicle would
require about 250 watt-hours per mile of range (it is not clear if this
is measured at the charger input, the charger output, or between the
batteries and the motor). Others differ; AC Propulsion
claims 205 Wh/mile
for their tzero (presumably as output from the batteries), while Commuter Cars
says a Tango would
need about 180 Wh/mile. For a slightly larger vehicle, the EPRI
figure 250 Wh/mile seems to be reasonable for a BOTE analysis.
Range is the other figure. 30 miles is well over the average
commute, and would certainly capture the 80% reduction in fuel
requirements projected by analyses which find 22 miles is
sufficient. To obtain 30 miles range at 250 Wh/mile and 80%
discharge, a battery would require a capacity of 30 * 0.250 / .8 =
9.375 KWh; call it 10 KWh even, for simplicity's sake.
The last element is power. To meet consumer demands, a car
will probably need at least 100 horsepower, perhaps 150
horsepower. This means that the battery must be able to supply 75
to 112 kilowatts of power for acceleration.
For batteries, I like to look at batteryspace.com
best Li-ion offering at the moment is a pack of 50 cells in the 18650
configuration (18 mm diameter by 65 mm long), which store 2000
milliamp-hours (2 amp-hours) at 3.6 volts nominal; for this they're
asking roughly $5.00/cell. The 50-pack is specified at 81 ounces,
or roughly 45.6 grams/cell. The specifications say that they are
limited to a 2.5 C (5 amp) discharge rate. Suppose that Altair's
electrode technology can triple this to 7.5 C; at that rate, a 10 kWh
battery would be able to supply 75 kWpeak
, nearly as much as
a typical NiMH battery.
For NiMH, the cost leader is a 10-pack of C cells, 4500 mAH at 1.2
volts nominal for $3.30/cell. Assuming a 10 C discharge rate, a
10 kWh pack would be able to supply 100 kWpeak
I chose to assume two different configurations: a commuter car
with 75 kW (100 HP) of power, and a sport model with 112 kW (150 HP) of
power, with 30 miles minimum all-electric range at 100% discharge. My
calculations came out like this:
| Electric range, mi
|| Commuter, 75 kW
| Sport, 112 kW
|| Commuter, 75 kW
| Sport, 112 kW
- Li-ion batteries are getting very close to NiMH in cost per unit
energy. (This is new in my experience.)
- The commuter configuration with the NiMH battery sits right
at the "sweet spot"; it has neither excess power nor excess capacity
for the range requirement. This is partly due to the cells
chosen; some NiMH cells have much higher discharge rates (up to 20 C)
and could provide very high performance for similar weight and only
slightly greater cost.
- Cars using Li-ion batteries are power-limited and require greater
battery capacity to meet performance specifications. This adds
- The Li-ion batteries make up for this with a substantially
greater all-electric range.
- The Li-ion batteries are also substantially lighter, by as much
as an adult passenger's worth for the sport configuration.
- Either Li-ion car would probably be able to run entirely on
electricity for a large majority of most user's driving.
What can we expect in the future?
- Cost of Li-ion cells will continue to fall. If they follow
the standard experience curve of 20% cost reduction for every 2x
increase in cumulative production, an 8x production increase will see
the cost of the commuter battery close to $3500.
- Cost of NiMH will also fall, but probably not as fast.
- Li-ion will probably be the cost leader for both energy/$ and
power/$ in a few years.
What are the prospects for plug-in hybrid vehicles?
- Cost of a Li-ion battery will approximate the cost of a gasoline
drivetrain when it hits the $2000-3000 range.
- This requires about a 2.5-4x decrease in cost, depending on
- We can expect this at a 16x to 64x increase in cumulative
- Use of cells for traction batteries will consume far greater
volume than portable electronic gear, and would increase production
much more rapidly than the current trend.
California once tried to force battery technology with the ZEV mandate.
Unfortunately, the initiative was ahead of the technology; it was
too much, too soon, and the few ZEV's which hit the roads cost up to $1
million apiece. But times have changed, and the technology is
ripening. If California tries again with a PIH mandate, the cost
curve is ready to meet us.