In the JLA cartoon, it is revealed that Batman carries around a lump of Kryptonite, for 'insurance'
Amazo: [upon scanning Batman] You don't have any powers.
Batman: I have this. [Pulls out piece of kryptonite. Amazo begins to suffer its effects.] It's a package deal. You get our powers, but you get our weaknesses too. [Amazo jumps off the building into the sewer.]
Hawkgirl: You always carry a piece of kryptonite with you?
Batman: Call it insurance.
Hawkgirl: And they say I'm scary.
How long has that rock been in his utility belt? Is it there in the comics? And does it have any negative effect on batman?