Anything that looks like a billy club has long been illegal in Cal., as Seth correctly points out. If you want to carry an impact weapon, consider a rattan cane like the ones Cold Steel sells for about $20. They are light enough to use one-handed, like a baton, or you can wield them like a two-handed sword.
I have friend who carries a T-ball bat in his daypack/gym bag. It is a lot shorter/thinner/lighter than a regular bat. If you do this, you need to have a glove and ball in the bag with the bat. Of course, there's still no guarantee you won't be prosecuted if you use it as a weapon.
DPD.