Battery Health Issue & Concern About Parts History

It may have been boosted and that would not appear on your Service History page. Just one of the risks you take when purchasing a used device. There is no indicator that will appear if your device has been previously opened. The Service History page is limited on what it will show as an authorized repair.

Also the analytics data is not useful for a Battery Health reading and it would be rare for it to match what is shown on your device. Those 2 numbers are calculated differently using a different original battery amperage number and current amperage that will vary by voltage. No 2 batteries are going to start with the exact same Battery capacity, but will normalize the results based on the rating of the battery installed on the phone. The same is true for those third party applications/shortcuts that claim to provide details about your battery.

Apple Battery Health algorithm uses a number of measurements such as number of charges, temperatures (both hot and cold), physical age of battery, peak performance capability, current maximum capacity and several other factors. Apple’s algorithm then weighs the importance of each factor and determines an overall health.

The only thing analytics does is measure current maximum capacity and divide maximum capacity when new and that calculation is represented as battery health. Analytics looks at just one factor, while Apple’s battery health number uses about a dozen factors.

Which model of iPhone did you buy?

If it was an iPhone 15 or higher, the number of Charge Cycles that the battery has completed will be displayed. Post back with that number if you can see this reading and we'll do some quick calculations.

I don't suppose that the seller might have offered a money back guarantee.....correct?

As Mac Jim ID suggests, you take your chances when you buy a used device, which you known nothing about it's history. They could have boosted the battery meter reading, but we can't tell you if they did or not. What we can tell you, is when the health dips to 80%, you will then need to pay Apple to replace the battery in your iPhone. And that is likely going to happen in the next few weeks.

iPhone Battery Replacement - Apple Support