Just like everything else, nothing lasts forever; it must die out at some point. The same is the case for phone batteries. Even so, one expects that they will use their battery for a period of not less than 3 years before a necessity of purchasing another one arises. However, you may notice slowdowns in your battery just a few months after buying it. No need to run to the shop, just have a look at the following tricks on how to solve that:
Restart your phone via soft reset
Try charging using a mains adaptor
Close background applications
Clean the charging ports
Avoid huge tasks with your charging phone
Replace the battery