Will,
S Type is renown for battery discharge. If you are certain the battery is good then it will probably be as has been suggested in previous submission. The battery can be checked with a Multimeter and you can buy very cheap, plug into lighter socket, USB unit that also has a V reader that displays the battery charge.
In the long run you would do well to invest in a CTEK battery charger/conditioner to keep your S Type up to the job on battery charge. I have mine on a CTEK MXS 7.0 at least once a month and they are very reliable and viable pieces of equipment.
Good Luck, Best Wishes and Regards, John