Of the 60+ teams that have signed a letter of intent to participate in the Progressive Automotive X Prize, about 15 were on hand at the New York Auto Show. With only four cars on stage, it's easy to calculate that some teams arrived sans vehicle. The group from Motive Industries had just a little folded flyer to show what they've been working on for their entry vehicle. The basic idea for the as-yet-unnamed Motive vehicle is an electric car with an on-board ICE for range extension. The real challenge for Motive will be convincing everyone that "filling up" via quick, robot-aided battery exchanges are the way to go. Four-passenger and full size with biomaterials in the body panels and elsewhere, the vehicle is just now moving off of the design pages. Motive's Darren McKeage and Nathan Armstrong were in New York to give the public a first glimpse of this new car. You can view the flyer in the gallery below and listen to the duo by clicking play.
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1. Exchanging battery packs is a non-starter in economic terms. You'd need to stockpile pre-charged replacement units at filling stations, so the total number of very expensive battery packs would have to far exceed the size of the BEV fleet. Also, any such system would require early standardization of pack specifications, sharply reducing the opportunity to innovate in this still very young field.
Some manufacturers of advanced Li-ion batteries now claim rapid recharge to a high SOC - 80-90% - is possible. There are question marks regarding the impact that would have on battery life expectancy, unless the pack can be aggressively cooled during the process by the on-board A/C. The same may be true of the on-board power converter.
Example: the Chevy Volt is spec'd with a 16kWh pack that is operated only between 90% and 40% SOC to ensure long life. Let's say rapid charging is 95% efficient (plug to battery). To obtain 8kWh in 10 minutes, you need to supply 48kW/0.95 = 50.5kW at the plug. The A/C needs to dump 2.5kW of heat flow to the atmosphere while stationary, which is feasible. Together with the fan, the active cooling effort will consume approx 0.2kWh during those 10 minutes.
The charge would be delivered as three-phase AC at e.g. 400Veff and roughly 130amps, which implies a pretty hefty cable and plug. For safe handling, the system would need a standardized handshaking protocol between the car and the recharge station. Power would be ramped up only after positive contact is confirmed and no short circuits or ground faults have been detected. Ramping up over e.g. 30 seconds gives the grid time to adjust to this new large electricity consumer. It goes without saying that a rapid recharge station for EVs would need a large dedicated feed from a utility substation, since multiple vehicles must be served concurrently.
Similarly, power demand needs to be ramped down when the battery approaches its target SOC, something the car needs to communicate to the recharge station. One option is to use a specific standardized pattern of interruptions to the power flow, much like Morse code (Note: power semiconductors interrupt power flow all the time, that's how they work). Another option is to use a separate wireless or fiber optic communications path.
Any attempt to remove the plug during rapid charging would be extremely dangerous, so the car must not respond to pressing the gas pedal while the plug is attached. In addition, the parking brake must be engaged and effective during recharging. In earthquake zones, the cable must be long enough to accommodate any relative ground motion.
Finally, and this one should be obvious: even given the above precautions, high powered electric recharge systems must be sited well away from sources of combustible fuel. For E-REV design, that implies putting the filler cap for hydrocarbon fuels on the opposite side. More importantly, it means you won't be able to fill up on gas and electricity at the same location.
The bottom line is that rapid charging will cost much more per kWh than trickle charging at night, because a lot of additional infrastructure must be installed by a new franchisee at a new site. Moreover, E-REV owners will be infrequent customers and highway-capable BEVs with useful range will remain extremely expensive for many years to come, so there won't be many such recharge stations. Taking a long detour to drive to one would be counterproductive, so you end up with a classic chicken-and-egg problem.
Until battery prices come down far enough to let BEVs capture significant market share, E-REVs look like the more practical option by far.
Posted at 5:23PM on Mar 22nd 2008 by rgseidl