Rechargeable lithium-ion batteries power iPhones. They are lighter than ever before batteries and charge quickly. Older iPhones are more vulnerable to battery problems than newer models, which are designed with cutting-edge features to make up for a weak battery. But with time, all batteries deteriorate. 

You probably have a battery issue if your phone keeps switching off unexpectedly and you can't go a day between charges. Slow-loading apps and a lower-than-normal speaker volume are additional warning indicators. Make sure your phone is charged before going to Settings > Battery and selecting Battery Health to check the condition of your battery. 

You can learn more about the iPhone battery's maximum capacity, performance capabilities, and essential battery messages here. Let's examine what you might discover in more detail. You require a device running iOS 11.3 or later and an iPhone 6 or later. If your iPhone won't upgrade, ask a professional for assistance.

 This will give you a definite indicator of whether your battery needs to be replaced. This percentage indicates how much more battery power your phone has compared to when it was new. You get less hours between charges as the battery's capacity declines as it ages from 100% (or just under) at startup. According to Apple, you have a healthy battery if the percentage you see under Maximum Capacity is greater than 80%. 

When your battery is fully charged and operating normally, the following message will appear in your Battery Health screen: 'Round-the-clock peak performance is currently supported by your battery.' You might need to upgrade your battery if it's creating slower load times or other performance problems. You'll see the following message in this instance:

This iPhone unexpectedly shut down because the battery couldn't provide the required peak power. This has been avoided through the use of performance management.' This notice will show up if you off applied performance management by tapping Disable: 'This iPhone has unexpectedly shut down because the battery was unable to produce the required peak power. 

You manually turned off performance management safeguards. Remember that once you switch off performance management, it cannot be turned back on. But in the event that your phone suddenly switches off, it will restart itself. You may also decide to turn it off.