The Ergosphere
Wednesday, March 02, 2005

Checking the shelf

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:  Altair Nanomaterials 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  Their 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:

Battery capacity,
weight, kg
 Electric range, mi
(100% discharge)
  Commuter, 75 kW 7.5 126      $4583 30   
  Sport, 112 kW 11.2   188      $6844
694.44 92.59
  Commuter, 75 kW 10      63.8 $6944
  Sport, 112 kW 14.93 94.3 $10370

Salient points:

What can we expect in the future?

What are the prospects for plug-in hybrid vehicles?
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.
Try biking.
For every problem, there is a simple, wrong answer.
First of all, keep up the good work. Love your stuff.

1) I thought super-capacitors could overcome the power limitations of Li-Ion batteries. Would that change the cost model?

2) Have you considered what parameters (max distance, cell efficiency etc) would it take to make a totally solar powered car feasible?
1.) Supercapacitors would improve instantaneous acceleration and regenerative braking, but the added performance would not be available for e.g. climbing long hills.  Exactly how much of what kind of performance drivers want and are willing to pay for is an open question.  On the other hand, falling device prices will make more and more performance per dollar regardless of the technology selected.

2.) There are two possibilities for solar-powered cars:  ones which receive energy from off-board solar collectors, and those which have integral collectors.  The second has far more difficult constraints of weight, durability and form factor than the first.  I blogged a bit about the first scenario last year (how time flies).  As for the second, if the producers of plastic PV cells can deliver on the claim of 30% efficiency, a car with a PV skin could harvest ten or so miles of "free" range each day just by parking in the sun.

How much range?  Assuming a car 1.8 meters wide by 5 meters long, an average of 800 W/m^2 of sun falling on it for 6 hours/day (7.2 kWh total energy), 30% conversion efficiency (2.16 kWh output) and 250 Wh/mile, you'd get about 8.5 miles/day.  If you can boost the conversion efficiency to 50% (quantum dots may be able to reach 60%) and cut energy demand to 200 Wh/mile, you'd get 18 miles.

IIRC, the average commuter drives 22 miles or less to work and back.
take a look at E-One Moli Energy and A123 Systems Li-ion batteries. They are being used in Miliwaukee and Dewalt power tools. They have much much higher power densities than previous Li-ion batteries and they can be recharged much quicker, thus allowing for higher eff during regen.
You'll find them mentioned in a more recent entry.
Question your pricing for NI-MH batteries. I saw a price for bulk purchases D size NIMH batteries that runs about $300/kwH. ALso other internet sources list the cost of NIMH as around $350/kwh Check the link on ebay for low cost NiMH batteries
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