1.) Have the charging system checked at AutoZone, or one of the major auto Parts supply stores. Yhey will do it for free. This will help spot charging system problems like an alternator with low output or a bad diode.
2.) Check for parasitic drains on the battery. Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.
The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.
Using the negative side prevents nasty accidents like shorting the wrench to ground while disconnecting the battery cable. Electrically, the test will work with either positive or negative battery cable.
2.) Check for parasitic drains on the battery. Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.
The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.
Using the negative side prevents nasty accidents like shorting the wrench to ground while disconnecting the battery cable. Electrically, the test will work with either positive or negative battery cable.
Any battery will go dead given enough time. Even a clock will kill it. You said it -probably the alarm. Even the computer needs a little juice to stay alive.
Put a $20 ChinaMart 1.5V MC trickle charger maintainer on the battery permanently while it sits for extended periods of time. Minor inconvenience of an extension cord run to the car. If you live where it ever freezes the battery will freeze and swell if the electrolyte (acid) gets too weak. Then you are out an expensive battery. A maintenance free battery reportedly will lose 10% of it's output everytime you run it dead. So that's a bad thing.
The cheapo alternative if you don't need the alarm is to simply disconnect the negative post when you're not using the car and it will be fully charged when you reconnect it. This will also remove all codes from the computer if you have any.