All diagrams using stock alternator parts need a battery. The battery supplies the current to the rotor to get the alternator to create electricity. So any diagram that has the stock parts needs a battery.
A smaller battery is ok if you run kick start only. If you want to run the e-start you will need a battery at least as big as the stock battery, electrically speaking. That's 12 to 14 amp hours and around 200 cca's. In a lead acid wet or AGM type battery this means one as physically large and heavy as a stock battery. These can be gotten for $40 to $50. They sell Lithium Iron batteries that have the same electrical power but are much smaller and weigh practically nothing. Shoria or Ballistic are good examples. The start around $150.
I have an AGM style battery in the 81 XS650 as well as the Harley and Honda. I'm very pleased with them. Being AGM batteries they don't require much care, they don't lose water.
I have a Ballistic battery in my 75. I had a 8 cell till I killed it, I left the key on and completely drained it. I left it in the bike and the 20 below was a bit more than it could handle. I just replaced it with one of Ballistic's EVX14L-12 batteries. This has a case size that is the same length and width as the stock battery and about 1/2 shorter. It is a 12 cell instead of the old ones 8 cell. It cost more, $229. but it has more cca's 360 VS 240. It cranks the 750 kitted engine very well.
I'll take better care of this one.
Leo