Basically, it seems that the boot of most electric cars are occasionally empty. (and perhaps even the passenger seats)
What if we can have a large empty battery rack, that we can lift and place in the boot, that would plug into the hybrid via a standardized port for external batteries in the boot.
each battery are prolly very heavy, and different car have different sized booth, a standard sized 'boot batt cell' are about suitcase sized. The boot batt has a handle, and a battery level check individually; though you can check all of them via pressing a button on the 'rack'.
You load each of these batts into the 'rack', and you can now have instant battery boost!
And since these batteries are done in 'cells', you can replace each cell as it dies(to the original manufacturer for refurbishment), rather than the whole rack. (thus spreading out the cost)
To sweeten up the deal, while you can charge it inside the car... you can also take each 'boot batt cell' out, and place it on a 'charging rack at home. This will be very very useful, especially when batteries become cheap enough, to have plenty of extra batteries left charging at home (or at work).
Bonus idea: Since these batteries are standardized, you can have large assed rack of these batteries on 'petrol stations', where by swiping your credit cards, you can exchange a specially marked 'boot batt cell' for a fresh one. (Or borrow a new cell)
In addition, these kinds of batt should have additional ports for larger industrial charging sockets.(It means, its slower to charge at home, compared to the petrol station)
To prevent people from abusing these batteries (as petrol stations needs to charge it for the next customers), you do have to place a deposit on it. In addition it has a internal GPS, to prevent theft of the battery, as well as a battery lifetime cycle monitor for quality control.
[as for worries that people would just borrow, and use it for self charging purpose, and rarely return it. You can make it so that people do have to return these batteries after a certain amount of time. And if you make it really convenient to swap batteries, people will pay for it, instead of losing the right for a few extra batteries.]