Definitely something I'll be looking into and I've mentioned to Graeme, here's a cut and paste to save the re-write:
lipo with removable cells, take a look at e-flite for excellent implementation of this method, I would guess the 160mah single cell range would be right for the cars, they package micro helis and planes with simple low cost chargers (fixed to specific battery settings rather than hobby grade adjustable chargers). Their 'msr' heli comes with a charger that can handle 4 x single cells at once, extra batteries are relatively cheap so you can easily have 6 x cells, 4 on charge 2 in the cars, race all day.
A cheaper/simpler option is the usb chargers very common now with the cheap micro quads coming out of China, with a single usb charger I can have 4-5 batteries charged then pretty much fly continuously by cycling through the batteries, if anything it's motor temp that would force a break before ever having to worry about the next battery being ready.
All stuff that already exists, relatively cheap. I mentioned 160mah but you could definitely go quite a bit higher while still fitting I'm sure. I'll be posting here what I come up with.
Can't wait for my Pro Racing Set Ultimate + Pro-Racing Track Upgrade...
Yep 2 x flyovers