To safely bill a lithium ion battery, you require to follow the proper billing treatment, which involves a constant-current phase followed by a constant-voltage stage. If you simply use a constant-voltage source, you'll end up charging the battery quicker than it's made to deal with. The 2nd line suggests that under no conditions I ought to boost the input charging voltage past 4.2 V, else it could harm the battery. Entirely draining the batteries breaks down the crystals to their original state.