Start with a vehicle design similar to a battery electric vehicle, then eliminate the on board battery pack.
Add in a high voltage power connection in the rear of the vehicle, and a trailer hitch.
Put the main power supply (either a battery pack or a genset) onto a trailer, and attach it to the rear of the vehicle. The vehicle still has a 12-volt battery for accessories, but no on-board motive power.
The power supply trailer has, on it's front, an electrical connection and a trailer hitch connection to go to the vehicle, and on it's rear, a power connection and trailer hitch similar to the one on the rear of the vehicle. This allows the vehicle to use either a battery, or a genset, or both, or more than one of either or both.
The mechanical connection has both a yaw sensor and a tension/compression sensor. The trailer's wheels have both electrical power steering, and traction motors and brakes. The steering of the trailer is automatically controlled to keep the yaw angle very close to zero, and the traction motors and (when necessary) brakes are used to keep the measured tension/compression very close to zero. This allows the car to be driven almost as if the trailer were a weightless, rigid attachment.
The suspension of the trailer can be stiff (and therefor energy efficient), with just enough dampening to protect the cargo from being damaged by bumps in the road, while the main vehicle's suspension can be as soft or as firm as the driver chooses.
When a battery carrying trailer reaches the end of it's life, it's easy to replace it, just by unplugging it electrically and disconnecting the tow hitch.
To make moving these power supplying trailers easier, each trailer would have two fully retractable caster wheels: one front and one rear. When a key is put into an "ignition" switch on the trailer and turned, these wheels extend, and the power steering straightens the left and right wheels; a human-operated joystick then controls the traction motors in a manner similar to a skid-steer vehicle. Basically, the trailer transforms into a miniature (non-roadworthy) vehicle.
To make charging the battery pack super simple, if the vehicle has a battery trailer attached, and is turned off near a charging station, the battery trailer extends it's caster wheels, disconnects from the vehicle, and robotically drives itself to the charger, and plugs in. It would move back to the car in response to a timer, or a when it's fully charged, or when the driver presses a button inside of the vehicle to recall the battery pack.
The fuel tank, or tanks, of the genset trailer would be designed so that the center of mass of the fuel always remains over the center of the axle (both left/right and front/rear), regardless of whether full of fuel, or nearly empty. Obviously the other components (whether with the genset, or the battery) can be statically arranged to be balanced.
If the vehicle is being for relatively short distances, a battery pack trailer would be used. If the vehicle is being driven hundreds of miles, both a battery pack and a genset trailer would be used, much like a hybrid car. The genset alone might be used, if the battery pack reaches the end of it's life, and one is waiting for a replacement for it... this last mode would of course be less fuel efficient.
The same vehicle would be used regardless of whether the genset is gasoline or diesel, or whether the battery pack chemistry is lead, nickle, lithium, vanadium, etc.
Both types of power supplying trailer can (and should) be made aerodynamic; and even if they were air catching cubes (which they shouldn't be), they get to ride in the vehicle's slipstream.
Downsides:
Cabin heating: This needs to be done through electrical resistance, or by using the air conditioner as a heat pump. Either is less efficient than using waste heat from an engine.
Heat management: If the genset's engine can't take advantage of the high pressure air in front of the vehicle, cooling is less effective. It would be possible to put a radiator in the front vehicle, and connect and disconnect plumbing for coolant between the vehicle and trailer, but it might be impractical.
The battery pack trailer also needs cooling, though not as much as the genset. It might be nice to share a cooling system with the genset, but this might be impractical.
Collisions: Measures need to be taken to prevent the trailer(s) from flying through the vehicle, if the vehicle crashes headlong into something.-- goldbb, Nov 08 2011 Paragraph 1-6: ho-hum, it's been discussed to death. Paragraph 7: an awful lot of complexity just to make it easy to move. But paragraph 8: very half-baked. Just imagine the mahem of batteyr packs justling for position around the charging station if this idea was widely used. +-- scad mientist, Nov 09 2011 Why would you want to always have a trailer hitched ?
I like the modular idea to a degree, but you'd want the vehicle to always have enough batteries for neighbourhood driving and (short distance) commuting, and a small gas/diesel engine (or turbine) onboard in case the batteries run out. This makes it a plug-in neighbourhood EV with limp-home capabilities. And for the driving mode which involves the most turning, parking and stop'n'go it relieves the driver of worrying about a trailer.
Then you'd want a large battery pack for intermediate distances (removable or trailered).
For long distance, a genset... or a direct-drive engine, utilizing the onboard electrics for regen and assist.-- FlyingToaster, Nov 09 2011 The fact that we need to think of solutions like this makes electric cars in general a terrible idea. What could be less efficient than dragging around a trailer, with seperate wheels and a drivetrain of its own, stuffed with batteries and a generator all the time?-- DIYMatt, Nov 09 2011 I agree, [DIYmatt]. To me, the pro-electric/hybrid arguments come across like this:
A: I kno! Let's make houses out of wet cardboard and mercury!
B: But those materials are really crap for houses!
A: Yes, yes, but ... with improving wet cardboard technology ... and ...
B: But they're crap right now, and you're trying to say things will be better right now if we use them right now. That's illogical.
A: Ah ... I've got it! We'll make hybrid houses! They'll be made out of wet cardboard and mercury for light use, like when there's no wind or rain or furniture or occupants, but have a steel and wood frame for other times! Yeah, kewl!-- spidermother, Nov 10 2011 random, halfbakery