The only way...
The only way I can see this working is to two batteries in each vehicle. (and maybe a third 5 mile emergency reserve)
One built in, yours to own and charge, polish and stroke, and enough for 50 miles or so. This is your main reserve, or enough to get you to work and home each day.(dont lug a pallat bat around when you dont need it)
second rentabat, pick one up when you want to go on a long journey, and change it on route. run it till its flat, and you swap back to your pack, and go to get a another swapper.
the swappers need to be a minimum standard and need to contain a certain amount of charge which can be paid for. if the pack will not hold enough it goes for recyling. maybe two options are available to buy at different prices, max charge (100%) and a full charge (80%) each a specific amount of charge, this way older batteries can still be used. and it leaves the way open for new technology the 200% super charge lithium iron phosphate technology or whatever.
each time you rent-a-bat you pay a deposit (£50?) and an amounrt for the power, and each time you return one you get your deposit back if you swap you only pay for the power.
by using your own battery aswell you can do the daily short journies from home to work/supermarket without worry and when you need the distance an add on batt pack will help! and keep you going.. and because you pay for a fixed amount of charge and are able to use it all (without getting stranded) it is cost effective. Whatsmore, because you have your own battery you can start your journey before you need to get the addon.
I would say that the addon needs to have at least twice the power and probably 4 times that of the internal battery. The addon should also replenish your internal pack, so when the addon is flat you have full charge to find a swaper shop. so say internal = 50 miles then the addon needs to ideally be 200 miles.
so if you drove 50 miles on internal bat then added a swapper it would refill your internal and enable you to do 150 miles before the pack was flat you then have 50 mile from your own pack again to find nother swapper shop.
if you put an addon on at the start you could drive 200 miles before swapping to internal and looking for a swapper shop.
of course if you are driving a 4 ton 4x4 truck/tractor then you might only get 100 miles out of an addon pack.
how can this recharging thing work? if the internal battery is lower voltage than the addon then the addon can fully charge the internal, it also means that the internal cannot provide maximum power to the car on its own so without an addon it may only be suitable for city driving, (short commutes) with an addon it is capable of full motorway driving.
However I still say that the main develeopment for electric cars needs to be Electromagnetic regenerative braking. this will make a vast difference to all electric car mileage.