Alancito wrote:
sabeehah225 wrote: "I bought an iPhone 8 from the Istore pre-owned in October 2021 and the battery health was perfect! It was on 100%
."
That implies the battery isn't the original one, and fast draining is possible if that Istore didn't use a genuine iPhone battery for replacement:
About genuine iPhone batteries
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That doesn't necessarily mean that. They might be selling "pre-owned" devices with the original battery that hasn't been replaced. I see a lot of different retailers selling their own "certified" devices. Amazon's "Renewed" devices are all sold with at least "90%" original battery health, but for iPhones that's likely based on what iOS is reading out.
The whole iOS battery health number isn't really something I rely on to indicate that a battery is at a particular state. It only provides limited guidance. I've seen it stuck at 100% for months, where I'm sure it dropped some. Some have claimed that they saw it stay on 100% for a year, even when the battery was fairly heavily used. It only tells me that it can stay stuck at a value for a while. I wouldn't rely on it to guide me on whether or not I should buy a device or if I should request a discount to help defray the costs of a new battery.
If I were going to really check out that battery health, I'd bring my Mac and check via Coconut Battery, which has access to the cycle count as well as what they call a "defensive" battery capacity reading.