LG G3 Cat.6 — is something wrong with my stock battery?

My LG G3 D850 battery is going down pretty fast and always gets about only 18 hrs of standby time (less than 4 hrs on screen) no matter what I did. In such a case, is the extended battery a good option? I already found a mpj 6000 mah extended battery, or is something wrong with my stock battery? I don't really know much about the smart phones, but I really like this phone so if anyone could help me fix the battery issue thank you so much!