the convertables are suppoed to have a special battery designed to withstand the extra vibration from the convertable top. I'm not sure if the Z-series cars the same, but you should get a battery from a dealer.
"Driving is like a narcotic, an addiction that can take over lives. It ruins people, breaks families apart, and even kills. Itís habit that needs to be fueled. Time, money, rationality, sweat, knuckle skin, and anyone who disapproves are of little importance to a driver, and are often overlooked. Hi, Iím Nelson, and Iím a drivaholic."