I'm building a 48V e-Bike. I want to use a Predator 212 to spin a 12V car alternator to use as a onboard recharger. How do I connect this?
Apart from the negatives to this approach that have been given, for the sake of experimentation let me suggest the following:

The alternator is not set up to charge LiPo batteries, so you'll need a suitable charger.

The batteries are 48V, so you'll need some means of increasing the voltage. One way is to rewind the alternator to increase the voltage. The current will also decease proportionately. Then you'll need to have a regulator to work at that voltage.


In all, your proposal isn't too unreasonable, but the implementation has a few hurdles, especially if you want it cheap and off the shelf. I'm thinking it would be one of those hard-core projects you see on YouTube where someone has done a lot of thinking and tinkering to come up with a slick result. I don't think we'd be able to give a step-by-step answer here.
Short answer - unless you want to spend an inordinate amount of time or a wad of cash, maybe look at another project.