Probably because you updated iOS 13. A new feature in iOS13 is that your phone can only be charged to 80%, thus increasing the battery life of your phone (this feature has a switch, you can choose to turn off). It seems to say that the lithium battery charged to 80% life cycle is 0.36 to 0.96, in fact, there are many mobile phone software reminders or Settings to protect the battery, for example, when our usual mobile phone is about 20%, the mobile phone will remind you to charge, this is because you charge below 20% will affect your battery life (the impact is particularly slight). If you are not a heavy user, turning on this setting is actually very helpful for the battery.