The battery is the most important part of a phone and everyone has a question that how to get maximum backup from battery. Its become a big headache if your battery not giving you proper backup. Phone batteries do need to be initialized properly to ensure a full life and best backup. New batteries should always be fully charged before their first use to obtain maximum capacity.
Mostly old batteries are based on Nickel, like nickel cadmium batteries, and they should be charged for a minimum of 16 hours initially and should be run through 2-4 full charge/full discharge cycles.
The mostly new batteries are lithium ion batteries and they required to be charged for about 5-6 hours, even if the phone says that the battery is full, it should be ignored as it may not be accurate unless the battery has been initialized. Lithium ion batteries should never fully discharge as their life is shortened every they are fully discharged. Instead they should be charged when the battery meter shows that there is only one bar left.