I've dealt with countless 70's US Market Vespas, and they all had the crazy battery system that Piaggio came up with to comply with US regulations. Needless to say, their solution was half-assed, and basically didn't work well. Mostly, I just removed the battery from the scooter, and re-wired it to run AC. It is actually pretty easy to do, especially so with the trouble-prone 1974 versions. Here is a nice video on how to do the modification from one of my favorite shops, Vespa Motorsport in San Diego.