To safely bill a lithium ion battery, you require to follow the proper charging treatment, which involves a constant-current phase complied with by a constant-voltage stage. You'll finish up billing the battery much faster than it's developed to deal with if you simply utilize a constant-voltage source. The 2nd line implies that under no scenarios I should raise the input billing voltage beyond 4.2 V, else it may harm the battery. Entirely draining pipes the batteries breaks down the crystals to their original state.