Hi Maverick
It's either battery quality, always stick to a reputable brand from a source you can trust....there have been several cases of counterfeit batteries around (can always tell by the weight - quantity of lead) or it is the alternator....not difficult to check with a multimeter. Should be producing around 14.4 - 14.8 volts with lights switched on. Any lower than 13.8 volts it's probably on it's way out.
Also, very importantly, check the battery terminals are clean and tight !
Cheers Trevor