To securely charge a lithium ion battery, you require to adhere to the appropriate billing procedure, which entails a constant-current phase complied with by a constant-voltage stage. You'll finish up billing the battery faster than it's made to cope with if you simply make use of a constant-voltage source. The second line indicates that under no circumstances I must raise the input charging voltage past 4.2 V, else it could harm the battery. Completely draining the batteries breaks down the crystals to their initial state.