No need for a seperate alternator. the elecric motor can be configured as a generator. of course as hs been pointed out you can only get back out what you have put in, so if you could extract all the eergy stored in a vehicles movement you could only bring it to a standstill. Asimilar system called"Regenerative Braking"is used on electric railway(in Europe ).which is viable when traffic is dense. but not on national routes where vehicles are well seperated