Apple iPhone, when to replace the battery. When battery healthy says “Service”, unable to determine battery health

Apple iPhone, when to change the battery. When battery healthy says “Service”

On my iPhone 6S by going in Settings -> Battery, it does not show anything, just a line(—)

If your iPhone uses iOS 10.2.1 through iOS 11.2.6, this notice might appear in Settings > Battery: “Your iPhone battery may need to be serviced.”

If your iPhone uses iOS 11.3 or later, refer to iPhone battery and performance to learn more about how to check your battery’s health.

That is the question When to replace the battery for your iPhone?
1- When the battery healthy says, “Your iPhone battery may need to be serviced.”
2- When the battery healthy says less than a certain amount of percentage low than 50% dependent on the use of the iPhone. If you use the iPhone to call and receive a call, maybe do not need to replace the battery, for high use the iPhone is recommended to replace the battery.
3- When checking in Settings ->Battery Health, the Maximum capacity says nothing(–) and Peak Performance Capacity says:
“This iPhone is unable to determine battery health. An Apple Authorised Service Provider can service the battery.” You should definitely change the Battery or sell the iPhone, UKgoodbye, second life for stuff, buy and sell old iPhones.