My iPhone 11's battery health have became 99% after 5 months of usage. Is it normal?!
And generally, how should I treat it to maintain it in 99%?
I read somewhere that we should keep the battery percentage always between 60 to 80. Is it correct?
Yes, it is normal.
Check the battery health again in 6-8 months. If it has significantly degraded at that time, contact Apple.
There is no reason to try and micro-manage the charge process.
ATJ ~ Apple says: "Maximum battery capacity measures the device battery capacity relative to when it was new." ...So not relative to 5 months ago. Also, in measuring a chemically dynamic process such as in a battery, it's unlikely that the estimate can be accurate to within 1%. Read more in this Apple Support Article:
iPhone Battery and Performance
...found by searching Apple's Knowledge Base.
"And generally, how should I treat it to maintain it in 99%?"
See this Apple Support Article:
Maximizing Battery Life and Lifespan
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It is normal and the best thing you can do is just use it and don't let the battery get totally depleted. The only way you can keep your battery health at 99% is no never use your iPhone again.
So quit looking at your battery health and check it again 1 month prior to your warranty expiring. If it is at 80% or below use your warranty to get your battery replaced.
Battery
