Last year, Apple admitted that it had throttled the CPU on some older iPhone models. These units contained worn out batteries that were not upto providing handset with enough power to perform certain complex tasks. Apple included the software required to throttle these older models through the update to iOS 10.2.1 and never told the public what was going on till last December.
The company added certain tools in iOS 11.3 that allows iPhone users to monitor the health of the battery in the Handsets. The update allows owners of these models to block Apple from throttling the CPU on their phones, no matter the condition of battery inside it.
Apple is offering these tools to those who own the apple iPhone 6 and newer versions of the device. It allows a different performance management system that more precisely allows iOS to anticipate and avoid an unexpected shutdown.
Apple iPhone 5s received iOS 11.3, the oldest iPhone model to do so, it does not come with the battery health features.
This is because apple says it does not officially throttle the CPU in that device. The company says that the issues that forced them to slow down the processor inside older models are first found in the apple iPhone 6.
The older model is built differently and can even squeeze enough power out of an elderly battery to run complicated tasks without shutting down the phone.