If you build one like his, go get a deep cycle marine battery, like the one you use for a trolling motor. About 115 Amp hrs, around $67 from Wally World. Regular car batteries are not made to go from full charge to full discharge and back- they will quit real soon if you do that. A 5 hp motor could turn two of those alternators at once, doubling your charge rate- but he did a decent job for a "what junk do I have in the garage?" project- and those can be fun.
What we have here is... failure- to communicate.