Battery faults

I'm extremely dissatisfied with the Samsung S4 battery that I purchased just over 6 months ago. The battery has a 6 month warranty but why does is only work properly for those 6 month?? Because now I have to charge it at least twice a day and it sometimes won't even get to 100% within 6 hours of charging! It worked well when I first got it but now it seems to also have a 6 month guarantee!! Please fix your batteries, I'm not willing to purchase a new battery every 6 month