The best way to know if you need to replace your iPhone battery is by going to Settings > Battery > Battery Health. On this page, you will see a quick overview of your battery’s health via its “Maximum Capacity” and “Peak Performance Capability”
“Maximum Capacity” is a measure of the battery’s capacity relative to when it was new. A brand new phone should read 100% and will reduce over time; the lower the percentage, the faster your battery will deplete on one charge.
The “Peak Performance Capability” measure is more of a ‘Good’, ‘Not-so-good’ or ‘Bad’ scenario. If your phone reads “Your battery is currently supporting normal peak performance”, you are in the clear – your battery is up to par and there is nothing you need to do. On the other hand, if you see “This iPhone has experienced an unexpected shutdown… Performance management has been applied…” it’s a sign that your battery is experiencing some significant wear. That means, Apple has started to slow down or “throttle” your phone processing speed in an effort to reduce further battery strain. You can disable this throttling feature but if you do, the setting cannot be toggled on again until the device unexpectedly shuts down, so proceed with caution. Finally, there is “Your battery’s healthy is significantly degraded…” message which means that your battery severely depleted and you should strongly considering getting it replaced.
If your iPhone is pre-2014 or running iOS 11.2 or earlier, you will not see battery health in Settings. However, there are a few other ways to tell that your iPhone battery might need to be replaced:
If you’re having to recharge your battery every few hours, even when usage is low, it might be time for a new battery
If your phone unexpectedly dies when the battery is 10% or higher, this could be a sign of a battery worth replacing
If your iPhone only works plugged it, you guessed it – replace the battery
OR if you’re looking for more of a scientific approach, you can download an app like Battery Health which gives you a look into battery capacity, power usage, etc.