Gods I'm bored being home-bound. Bored enough to look on Farnell and RS at both power relays and automotive - to be honest there's not an awful lot in it - which looks to be more around the switching capacity (not really surprising) and what you could imagine the associated switch gear and return springs, and with most of the 50A relays around the 90R coil resistance (around 130mA). Automotive are likely more suited to the environment (high vibration, humidity and corrosion resistance) as well as providing economic switching of the high capacity PSUs we all tend to have knocking about.