You may have a "parasitic battery drain". (Hint: google "finding parasitic battery drains") Basically, there is something "on" that is draining the life of the battery even while the key is off. Common sources are courtesy lights under the hood and in the trunk; even in the sun visors. Also, anything that is hooked into the cigaretter lighter (or "power port") circuit. These items could be aftermarket car alarms, inverters for operating portable electronics, phone chargers etc. If you eliminate all these and still have the battery draining down, you may have a low amperage short somewhere. To find the circuit responsible, you will have to connect an ammeter in series with the battery negative cable. Say for example that it still shows a .1 amp drain--even with everything turned off. Start pulling fuses one at a time until the amp drain reads zero. The problem will be on the circuit of the last fuse you pulled. From there, you'll have to try and disconnect each item on that circuit until you find the culprit. It sounds tedious and it is.