Apparently all that data is included in the ActivityPub protocol. On Kbin, every post has an Activity button that shows every user (even those on other instances) that upvoted/downvoted/saved that post. So if a Lemmy post happens to federate to Kbin, all that info can be seen publicly by anyone.
Alkaline batteries are the crappy ones that leak. Get the more expensive lithium batteries, or go full on rechargeable ones, and you can leave them in without worrying about your device getting ruined.