If you can't easily remove the battery cap, just use some fine screwdrivers to remove the whole bottom plate. Then you can apply solvents and/or penetrating lubricant to the threads more easily. Don't use too much force - allow time for the lube get through. It's very likely that the reason it's stuck is a corroded battery. Use white vinegar and contact cleaning solution to clean out the compartment. But it's not uncommon for the corrosion to have migrated to the solder joint and wire connecting to the battery compartment. It's possible to repair such damage, but it's easier to use an external meter or sunny f/16 rule.