Apple's iPhone stores valuable information about the health of its battery: logged inside your phone is information about the battery cycle counts, or put simply, how many times you have charged your phone to 100%.
Keep in mind, that Apple counts 1 battery cycle as a full 100% recharge run: you can charge up your phone 25% and then 75%, and combined together these two charges will count for 1 100% cycle.
This is important information: different Apple devices use different batteries, and they do not last forever. Here are the battery counts that different Apple products are designed for:
iPhone is designed to retain 80% charge until 500 cycles
Apple Watch is designed to retain 80% charge until 1,000 cycles
iPad is designed to retain 80% charge until 1,000 cycles
iPod is designed to retain 80% charge until 400 cycles
MacBook is designed to retain 80% charge until 1,000 cycles
So if you own an older Apple device, you definitely want to know how many counts your battery is on.
Unfortunately, Apple does not allow iOS apps to report battery cycle counts, so you will need to connect your iPhone to a computer - running Mac or Windows - to find out.
How to do it
On Mac, you can use the free coconut Battery app. The app shows detailed battery stats for all sorts of Apple devices: it shows current battery charge, the full charge capacity, the design capacity, the model name of the device, the manufacture date, cycle count and battery temperature.
On Windows, you can use the free trial iBackupBot. This ancient application still works, and if you need to quickly check up your iOS device battery count, you can get the free trial, or if you need to use it for longer, you can purchase the $35 full version. In iBackupBot, you need to select your device, click on More Information, and look for "CycleCount".
So... did you try any of these? How much battery cycles is your Apple device currently on? Keep in mind that changing the battery is not too costly and could be a worthy upgrade for an old iPhone that you still want to use or pass to relatives or friends.
How to check how many battery cycle clocks is your Apple iPhone on