a second set of batteries is not really practical, heres how the setup goes
these batteries are in an electric vehicle, powering an electric motor, the idea is to use the batteries to drive the car and then for a fuel cell to charge the batteries at night time ready for the following day.
volvo are working on a similar concept, it uses batteries, but when they become low on power an engine starts that will charge the batteries and run the motors until the batteries are charged again.
i plan on using a fuel cell though instead of a petrol powered engine though.
for this idea to practically work i think it would need a recharge time of around 4 hours, faster if possible, this means that you could use the car to go to work, whilst at work the car charges itself, then you go home and it charges itself overnight, you might go to friends house, while your there it charges itself, etc etc.
i estimate that the bank of batteries to last for about 100 miles on a full charge, i think that if you can do 100 miles every 4 hours without actually plugging the car into the mains this would be progress