
1. Your iPhone shuts down unexpectedly
If your phone still has some charge left but then unexpectedly shuts down, that’s a problem. It may be a battery issue or the calibration may just be off.
To re-calibrate your iPhone, follow these steps:
1. Use the iPhone until it shuts down on its own.
2. Charge it with a quality iPhone charger.

3. Wait until the phone is fully charged. Then perform a soft reset.
4. Hold the home button and sleep/wake button at the same time until the Apple logo comes up.
When charging your device, always make sure to use a trusted, high-quality charger from a reputable source you can trust.
If this random shutting down issue continues, then it is time to replace your loose iPhone Battery.
2. Your iPhone is getting hot

When you suddenly notice that your phone is unbearably hot, it is time to consider getting a new iPhone battery. Important to consider is the nature of your surroundings and the prevailing weather conditions.
If your phone is hot to the touch, it most likely needs to be replaced. You want to replace it soon before it starts affecting other parts of the phone.
However, if you leave your phone somewhere like a hot car or out on the beach, then it’s probably the surrounding environment that’s making the phone hot.
3. Your iPhone only works plugged in

When your iPhone cannot hold any charge without being plugged into a power source with an iPhone charging cable then it is about time you got your iPhone Battery Replaced.
In essence, this means that you cannot go away from a power source and be confident that you can receive calls, answer messages, or do any tasks that you would normally do on your iPhone. Get your iPhone Battery replaced.
4. Your iPhone 6 or later is slow
If you’re using an iPhone 6 or a later iPhone, and it seems to constantly be slow and sluggish, then it may be time to replace the battery with Battery Repair Tool Kits.

This is because your current battery isn’t healthy enough to properly power your phone.
When the iPhone is doing something that’s CPU intensive, it doesn’t have the necessary amount of voltage to perform like it’s supposed to.
Enter: Settings---Battery---Battery Health--- Peak Performance Capability, if you see this blurb:

It means your phone is being throttled and it would benefit if you replace battery.
How to check your iPhone battery health

Ever since iOS 11.3, Apple added a new feature under the Battery tab called Battery Health. This particular section lets you see the percentage of your battery and performance capability in regard to how your battery health is doing.
Once your maximum capacity drops below 60%, it will no longer be at peak capacity performance. Your device will then be throttled.
This is when Apple purposefully slows down your phone so that it can still function with the weak battery.