get a multimeter and run the engine to 3k revs.
across the battery you should be seeing 13.5 - 14.5 volts
don't know how old the bike is, if you have a mechanical voltage regulator, you can adjust the output.
you need good battery or it won't charge.
check out my rewire in my blog (hint, click on my signature)
further info in the classic section sticky or www.xs400.com