Someone else asked this the other day. It seems like the only way would be is to make a voice recording saying "watch battery fully charged", add it to your notification sound folder then select it as your notification sound. Make sure notification categories is enabled and then go to the app settings for the watch manager, select the battery charged notification category and select your new sound for that notification.
To enable notification categories, go to settings/notifications/advanced settings/manage notification categories for all apps.
It really abnoys me that one cannot make routines that apply to the watch.
