Dell Inspiron 13 5000's losing battery quick last 3 months

So I have a inspiron 13 5000 that I got last year. My brother and my cousin both have the same laptop as me and they also got it last year. For some reason these last few months my laptop has been running really really low, like 3hrs on a full charge, it use to normally be 7hrs! Does anyone of the same problem? Does anyone know what is happening or how to fix it, please please say you do?