Gav you linked to the odyssey, I think you meant this for Optima;
Battery Mart Australia
boxhead I thought there were no-no's around using a deep-cycle battery in a car electrical system? To my understanding a car battery will punch out far greater oomph than a deep cycle battery for a short period, while a marine battery is designed for low draw over a longer period (running lights on a boat for example, while the engine is off). Due to the thin plate design in a car battery it will punch out more grunt for engine starting, but will not provide anywhere near as long a low-current draw as a deep cycle battery with its thicker plate design. Given the Optima's are such a good battery it's probably not a problem anyway.