Battery life

Battery life is the amount of energy in any device that has a battery. Like other portable devices, the battery life of iPhones, iPads and iPod touches are shown at the top right after the warning and health tip.

Battery health
Apple has integrated a measurment that shows how much battery health you have left. Using a device that uses iOS 12 or higher, this can be seen when you go to settings>battery>Battery health. There will be a statement at the top saying, "Phone batteries, like all rechargeable batteries, are consumable products that become less effective as they age." Below that, a maximum percentage measurement that show how much power your device can hold will be shown. The maximum capacity of a device that is newer will be more than of a device that is older. After that, there will be a bar telling you your peak performance capacity. This can either be "Your iOS device is currently supporting normal peak performance", or "Your iOS device's battery health is significantly degraded. An Apple Certified Service provider can replace it for you". You could replace the battery yourself, though. Just be cautious if you will be doing this as batteries will explods/leak/swell/catch fire when provoked (damaging it, poking it, burning it, etc.)