Do an amps drain test - remove the negative terminal of the battery and connect a multi meter between it, then read out how many amps you get (should be around 0.05amps) Before you do this make sure all accessorys are off doors closed etc. If its excessive start pulling fuses to identify which circuit is draining. Id start off with any aftermarket electrical items first, as there usually the first to fail or cause problems. If thats ok, check your alternator charge rate. It should charge the battery to around the 14v mark.....