What is the reason your 1985 Honda Shadow is not charging the battery?

There may be several reasons for this. First, is your battery in good shape? If it is, then the problem is most likely a faulty voltage regulator, also called a rectifier. This is a recurring problem with older bikes. The rectifier cannot be repaired in any easy way and you should try to get a new one, or a second hand one from a motorcycle used spare parts dealer. I recommend getting a new one. When you have installed it, which is very easy, you should have someone in the know how measure the various electrical circuits of the bike with a multimeter to make sure there is no shorting to ground of any of the wire harness or connectors and repair any such if found faulty.