The batteries for diesel cars are more expensive, generally, than those for petrol. the diesel engine needs a larger capacity battery for the glowplugs and feeding the high torque starter motor needed for turning over a high compression engine such as the diesel.

Battery capacity is measured in Amp hours, for example a 250 AH battery will provide 1Amp draw for 250 hours, or to look at it a different way a 250Amp draw for 1 hour etc.

A lot of modern cars have batts hidden away under covers, I guess it;s for protection but there will be a better reason i'm not picking up on for sure.