I agree with what user tarquin.ss5 said, perhaps urahara's bankai's power depends on his well being, maybe something to do with if his health is in really poor shape or dangerously low, then his bankai's power is skyrocketing and at its maximum level, but if he's at full health, then it wont do much damage at all which would explain why he had renji train chad instead of himself, but again also a theory