bad battery on new pre-owned iphone

What’s the phone’s battery health at?

10% within 10 hours isn’t too outrageous for a phone dependent on what you define as normal usage.

99.99% certain it’s a non-genuine battery. They don’t hesitate to install non-genuine display. So, I won't be surprised if they install non-genuine battery. It’s been proven many times in several posts here.   

what is the battery health? 

Reddit Post: Battery in a "certified pre-owned" phone?

Screen on time for an iphone 12 pro is lower than on the 11. Reviews for when it came out said screen on time was 6 hours or less. You can check the actual battery health, but it’s extremely subjective for use cases. You can go through 100 reviews and their usage patterns for “real life” will all vary. You’d have to be far more descriptive of your usage during this 10 hour period. I can burn 10% an hour easily with browsing on any phone. Gaming is even higher.

OP does mention it’s under “normal use” . I would assume no gaming.

Can you confirm the battery health on your iPhone? Have you been using it outside in extremely cold weather? https://support.apple.com/en-ca/guide/iphone/iphd453d043a/ios

They have no problem putting off brand parts into these phones and pretending they didn’t. I received a phone with a shot mother board and notifications on my phone saying that it couldn’t recognize an un genuine part or something.