Question about powering an electric car?

If an electric car has an altenator and a stored power sorce that needs to be recharged couldn't you just add a second power sorce and altenator. A sensor could switch the first power source off when its at like ten % power at which point the second altenator would then start recharging it while the other source takes over and just keep switching out. i dont know much about electric cars or how big their power cells are but this seems like a good idea you could go much farther for longer periods with cleaner energy. Im just a regular dude with no experience in this maybe some one could explain why it would or would not work